Some nematodes from fishes of the Lacantún River in the Lacandon rain forest of Chiapas State, southern Mexico, including Cucullanus potamarii sp. n. (Cucullanidae) from the endemic catfish Potamarius usumacintae (Ariidae)

Abstract

During November of 2007, parasitic nematodes were collected from 161 specimens of 32 freshwater fish species in the Lacantún River of the Lacandon rain forest of Chiapas, southern Mexico. Recent detailed morphological and taxonomic studies of this material revealed the presence of 17 nematode species (12 adult and 5 larval) of nine families, including Cucullanus potamarii sp. n. from the intestine of the ariid catfish Potamarius usumacintae Betancur-R et Willink, which is endemic to the Usumacinta River basin to which the Lacantún River belongs. This new species is mainly characterised by deirids and the excretory pore located far posterior to the level of the oesophago-intestinal junction, which is an almost unique feature among its congeners. Most parasites are briefly described and illustrated, and problems concerning their morphology, taxonomy, hosts and geographical distribution are discussed. Some findings represent new host and geographical records. Cucullanus oaxaquensis Caspeta-Mandujano, Salgado-Maldonado et Martinez-Ramirez, 2010 is considered a junior synonym of Cucullanus angeli Cabañas-Carranza et Caspeta-Mandujano, 2007, and Neocucullanus marcelae Ramallo, 2012 a species inquirenda. Based on absence of relevant morphological differences, Rhabdochona ictaluri Aguilar-Aguilar, Rosas-Valdez et Pérez-Ponce de León, 2010 1s considered a synonym of Rhabdochona kidderi Pearse, 1936.

Headnote

Abstract: During November of 2007, parasitic nematodes were collected from 161 specimens of 32 freshwater fish species in the Lacantún River of the Lacandon rain forest of Chiapas, southern Mexico. Recent detailed morphological and taxonomic studies of this material revealed the presence of 17 nematode species (12 adult and 5 larval) of nine families, including Cucullanus potamarii sp. n. from the intestine of the ariid catfish Potamarius usumacintae Betancur-R et Willink, which is endemic to the Usumacinta River basin to which the Lacantún River belongs. This new species is mainly characterised by deirids and the excretory pore located far posterior to the level of the oesophago-intestinal junction, which is an almost unique feature among its congeners. Most parasites are briefly described and illustrated, and problems concerning their morphology, taxonomy, hosts and geographical distribution are discussed. Some findings represent new host and geographical records. Cucullanus oaxaquensis Caspeta-Mandujano, Salgado-Maldonado et Martinez-Ramirez, 2010 is considered a junior synonym of Cucullanus angeli Cabañas-Carranza et Caspeta-Mandujano, 2007, and Neocucullanus marcelae Ramallo, 2012 a species inquirenda. Based on absence of relevant morphological differences, Rhabdochona ictaluri Aguilar-Aguilar, Rosas-Valdez et Pérez-Ponce de León, 2010 1s considered a synonym of Rhabdochona kidderi Pearse, 1936.

Keywords: nematode parasite, taxonomy, morphology, zoogeography, Teleoste1, Neotropics

The territory of Mexico is very interesting from the zoogeographical ogeographical point of view and many papers dealing with helminth parasites of freshwater fishes in this country have been published, with emphasis on those carried out in central and southern Mexico. However, considering the diversity of aquatic ecosystems and the large number and diversity of fish species in this country, the present knowledge of the Mexican nematode fauna of inland fishes remains insufficient (Moravec 2000a, Caspeta-Mandujano 2005, Salgado-Maldonado 2006).

During studies on the helminth fauna of fishes of the Lacantún River (Usumacinta River basin) in the famous Lacandon rain forest, Chiapas State, southern Mexico, carried out during November 2007, numerous parasitic nematodes were collected. Preliminary results of their examination were published within broader studies on the helminth fauna of fishes of Chiapas (Salgado-Maldonado et al. 2011a,b). Subsequently, results of more detailed studies of these nematodes assigned to Rhabdochona Railliet, 1916 and Spinitectus Fourment, 1884, including the evaluation of their morphology, taxonomy and host-parasite relationships, were also published (Moravec et al. 2009, 2012). The remaining nematodes were recently examined in detail, mostly including the use of scanning electron microscopy (SEM), and evaluated. Results of studies of this interesting material are presented herein.

MATERIALS AND METHODS

Fishes were collected using gill nets at the Lacantún River (station El Remolino, 16.238611 N, -90.85 W) (16 fish specimens also from three nearby localities Arroyo El Venado, Lago El Oaxaco and Rio Trendales), all Usumacinta River drainage system, Chiapas State, southern Mexico, during November of 2007. A total of 161 fish specimens belonging to 32 species, 13 families and 9 orders were examined for the presence of helminth parasites (numbers of specimens examined in parentheses):

Lepisosteiformes: Lepisosteidae - Atractosteus tropicus Gill (4).

Clupeiformes: Dorosomatidae - Dorosoma anale Meek (18), D. petenense (Günther) (9).

Characiformes: Bryconidae - Brycon guatemalensis Regan (9); Characidae - Astyanax aeneus (Günther)(2).

Siluriformes: Artidae - Ariopsis sp. (5), Cathorops aguadulce (Meek) (16), Potamarius usumacintae Betancur-R et Willink (2), Potamarius nelsoni (Evermann et Goldsborough) (4); Ictaluridae - Ictalurus furcatus (Valenciennes) (12); Heptapteridae - Rhamdia guatemalensis (Gunther) (2), Rhamdia laticauda (Kner) (5); Lacantuniidae - Lacantunia enigmatica Rodiles-Hernandez, Hendrickson et Lundberg (2).

Cyprinodontiformes: Poeciliidae - Poecilia reticulata Peters (1), Poecilia sphenops Valenciennes (8).

Beloniformes: Belonidae - Strongylura hubbsi Collette (4).

Perciformes, Eupercaria/misc.: Gerreidae - Eugerres mexicanus (Steindachner) (4).

Cichliformes: Cichlidae - Chuco intermedium (Günther) (6), "Cichlasoma" sp. (1), Cincelichthys pearsei (Hubbs) (6), Maskaheros argenteus (Allgayer) (2), Mayaheros urophthalmus (Günther) (6), Oreochromis mossambicus (Peters) (1), Parachromis friedrichsthalii (Heckel) (1), Parachromis managuensis (Gunther) (2), Petenia splendida Gunther (4), Rheoheros lentiginosus (Steindachner) (1), Theraps irregularis Gunther (1), Thorichthys helleri (Steindachner) (4), Thorichthys meeki Brind (3), Thorichthys sp. (5), Vieja melanurus (Gunther) (3), Vieja sp. (1), Wajpamheros nourissati (Allgayer) (6).

Gobiiformes: Eleotridae - Gobiomorus dormitor Lacepede (1).

Nematodes recovered from the digestive tract were fixed in hot 4% formalin. For light microscopy (LM), they were cleared in glycerine. Drawings were made with the aid of a Zeiss microscope drawing attachment. Specimens used for SEM were postfixed in 1% osmium tetroxide (in phosphate buffer), dehydrated through a graded acetone series, critical-point-dried and sputter-coated with gold; they were examined using a JEOL JSM-7401F scanning electron microscope at an accelerating voltage of 4 kV (GB low mode). All measurements are given in micrometres unless otherwise stated. The specimens were deposited in the Helminthological Collection of the Institute of Parasitology, Biology Centre of the Czech Academy of Sciences, Ceské Budéjovice, Czech Republic (IPCAS).

In addition to the newly collected nematodes, two paratypes (male and female) of Cucullanus mexicanus Caspeta-Mandujano, Moravec et Aguilar-Aguilar, 2000 deposited in the Helminthological Collection of the Institute of Parasitology, BC CAS, Ceské Budéjovice (IPCAS N-767) were studied for comparative purposes. Fish nomenclature follows FishBase (Froese and Pauly 2024).

RESULTS

Adult nematodes

Family Atractidae Railliet, 1917

Atractis vidali Gonzalez-Solis et Moravec, 2002 Figs. 1,2

Description: Medium-sized, whitish nematodes with finely transversely striated cuticle (Fig. 2 A,D). Lateral alae in form of very narrow cordons present, initiating at region of deirids and extending posteriorly somewhat behind level of cloacal aperture in males and somewhat behind anus in females (Fig. 2E,F). Oral aperture triangular, surrounded by 3 small flat lips (Figs. 1D, 2A-C,F). Cephalic papillae arranged in 2 circles: outer circle formed by 4 small spherical papillae, inner circle by 4 small digital papillae; 2 lateral elongate amphids present (Figs. 1D, 2A-C). Oesophagus consisting of dilated oesophageal corpus, isthmus and bulb containing sclerotised apparatus (Fig. 1A,B). Anterior end of oesophagus triradiate, forming short pharynx (Fig. 1 A-D). Nerve ring slightly posterior to posterior end of corpus; excretory pore at some distance posterior to oesophagus. Deirids small, rounded (Figs. 1B,I, 2D), somewhat anterior to level of nerve ring. Tail of both sexes conical, elongate, slender and sharply pointed.

Male (based on 5 specimens from Chuco intermedium; measurements of 5 specimens from Cincelichthys pearsei in parentheses): Length of body 5.92-6.69 (6.03-7.60) mm, maximum width 258-299 (272-394). Length of entire oesophagus 912-943 (844-919); length of pharynx 15-18 (15-21), width 72-81 (72-81), of corpus 503 (476- 517), width 95-109 (95-109), length of isthmus 231-272 (190-231), width 82-95 (68-82); bulb 150-163 (136-163) long and 136-150 (136-150) wide. Deirids, nerve ring and excretory pore 530-558 (517-544), 558-585 (544-612) and 1.17-1.32 (1.18-1.33) mm, respectively, from anterior extremity. Ten pairs of caudal papillae: 3 preanal, 1 adanal and 6 postanal; all preanal and adanal pairs subventral; second, fourth and fifth postanal pairs subventral; first, third and sixth pairs lateral (Figs. 17, 2.F,G). Ventral region anterior to preanal papillae with inconspicuous oblique muscle bands (Fig. 1J). Spicules unequal and dissimilar, well sclerotised. Large (left) spicule 489-597 (453-592) long, its proximal end somewhat expanded, 27-33 (24-39) wide; middle part of this spicule finely transversely striated, 15 (15-18) wide; distal end very narrow, sharply pointed (Fig. 1G,J). Small (right) spicule 168-171 (171-186) long, funnel-shaped, with proximal end widely expanded and distal end narrowed (Fig. 1F,J). Length ratio of both spicules 1 : 2.86-3.55 (1 : 2.65-3.46). Gubernaculum well sclerotised, 150-165 (153-162) long, its maximum width at proximal end 18-21 (18-21) in lateral view; distal end of gubernaculum somewhat narrower, ventrally curved, with rounded tip (Fig. 1E,J). Length ratio of gubernaculum and large spicule 1 : 3.04-3.98 (1 : 2.90-3.72). Tail conical, 707-816 (680-816) long, ending in sharp point.

Female (based on 5 gravid specimens from Ch. intermedium; measurements of 5 gravid specimens from C. pearsei in parentheses): Length of body 6.99-7.62 (6.94-7.82) mm, maximum width 394-422 (490-558). Length of entire oesophagus 935-963 (935-984); length of pharynx 18-24 (18-24), width 75-84 (75-81), length of corpus 530-544 (517-544), width 109-122 (95-109), length of isthmus 218-245 (231-245), width 82-109 (82-95); bulb 150-163 (163-177) long and 150 (163) wide. Deirids, nerve ring and excretory pore 503-571 (530-558), 612-625 (598- 639) and 1.33-1.38 (1.27-1.36) mm, respectively, from anterior extremity. Monodelphic. Vulva at short distance, 54-68 (54-68), anterior to anus; vagina short, directed anteriorly from vulva (Fig. 1H). Uterus containing a few developing eggs, size 136 x 82-95, and/or fully formed, 2-3 (3-5) coiled larvae approximately (450-570) long and (54-82) wide. Tail conical, 952-1,006 (925-1,074) long, with sharply pointed tip (Fig. 1H).

Hosts: Chuco intermedium (Günther), Northern checkmark cichlid, total body length (TBL) 12-23 cm, and Cincelichthys pearsei (Hubbs), Pantano cichlid, TBL 29 ст (both Cichlidae).

Site of infection: Intestine.

Locality: Lacantún River (Usumacinta River basin), Chiapas State, southern Mexico (collected 18 and 19 November 2007).

Prevalence and intensity: Ch. intermedium: 1 fish infected/6 fish examined; ca. 4,000 nematodes. C. pearsei: 1 fish infected/6 fish examined; ca. 2,000 nematodes.

Deposition of voucher specimens: IPCAS N-784.

Comments: Atractis Dujardin, 1845 15 one of six genera of the cosmocercoid family Atractidae which contains species parasitising fishes. Other atractid genera with repre- sentatives in fishes are Klossinemella Costa, 1961, Labeonema Puylaert, 1970, Monhysterides Baylis et Daubney, 1922, Orientatractis Petter, 1966 and Rondonia Travassos, 1920 (see Gonzalez-Solis and Moravec 2002, 2004, Anderson et al. 2009).

The genus Atractis includes many species parasitic in reptiles (turtles and lizards) and one is known from amphibians (toads), whereas only two species were described from freshwater fishes in southern Mexico: Atractis bravoae (Osorio-Sarabia, 1984) from Amphilophus istlanus (Jordan et Snyder) (Cichlidae) in Michoacán and Atractis vidali Gonzalez-Solis et Moravec, 2002 from Ch. intermedium (type host) and C. pearsei (both Cichlidae) in Campeche and Chiapas (Osorio-Sarabia 1984, González-Solís and Moravec 2002).

The former species (4. bravoae) was originally inadequately described by Osorio-Sarabia (1984) as Laurotravassoxyuris bravoae, being reported from the cichlid Am. istlanus and the atherinopsid Atherinella balsana (Meek). Subsequently, its type specimens from Ат. istlanus were re-examined by Moravec (2000b), who had transferred L. bravoae to Atractis; nevertheless, some female paratypes of the type series proved to belong to a different family, Cosmocercidae, being probably identical to Raillietnema kritscheri Moravec, Salgado-Maldonado et Pineda-López, 1993. Specimens of A. bravoae from Ath. balsana were not deposited and could not be re-examined. Without any reference, Caspeta-Mandujano (2005) reported the ictalurid catfish Ictalurus balsanus (Jordan et Snyder) as a host of 4. bravoae in its type locality, but this information seems to be doubtful, because 7. balsanus 1s endemic to Morelos State (Froese and Pauly 2024).

Although both A. bravoae and A. vidali are parasites of cichlids, they clearly differ in the characteristics of spicules, which are similar in shape and almost equal in length in A. bravoae and markedly dissimilar in shape and length in 4. vidali. The morphology and measurements of the present specimens, as well as their host species from the same river basin show that they belong to A. vidali. Nevertheless, the present SEM study of 4. vidali shows that, in contrast to the original description, the cuticle of this species is not smooth but finely transversely striated and the body of these nematodes has narrow lateral alae described here for the first time.

The morphology of Atractis spp. somewhat resembles that of Rondonia rondoni Travassos, 1920, a parasite of freshwater characiform, siluriform and some other fishes in South America (e.g., Moravec et al. 1992a, Moravec 1998, Cavalcante et al. 2017), but representatives of these two genera differ substantially from each other based on the vaginal opening (into vulva and separate from anus in Atractis vs into rectum forming cloaca in Rondonia) (Anderson et al. 2009).

Atractis spp. as well as other atractid nematodes are ovoviviparous and their third-stage larvae develop within the uterus of the female; these are freed in the digestive tract of the host, where they complete the life cycle without passing out into the external environment (Petter 1966). Apparently, this is the reason why the atractid nematodes are often found in fish hosts in huge numbers, as confirmed by the present findings of A. vidali in Ch. intermedium and C. pearsei, in which the intensity of infection reached about 4,000 and 2,000 nematodes per fish, respectively. Mass infections in fish hosts were previously recorded for R. rondoni in Brazil and Argentina, where thousands (maximum 120,000) of these nematodes in one fish individual were reported by different authors (Moravec 1998).

Orientatractis chiapasensis Gonzalez-Solis et Moravec, 2004 Figs. 3, 4

Description: Small-sized, whitish nematodes with finely transversely striated cuticle (Fig. 4D). Cephalic end blunt, posterior end with conspicuously slender, long, pointed tail. Narrow lateral alae present, initiating near mouth and extending posteriorly to about mid-length of tail (Fig. 4E,F). Oral aperture of irregular shape, from roughly triangular to almost circular, surrounded by 4 lip-like formations and bearing 4 (2 dorsal and 2 subventral) large oval submedian cephalic papillae of outer circle, 4 (2 dorsal and 2 subventral) very small oval papillae of inner circle and 2 lateral amphids; each of 4 submedian lip-like formations armed with 1 Y-shaped, well-sclerotised piece whose distal portion consists of 2 horns extending outward and downward (Figs. 3A-D, 4A-C); cuticle immediately posterior to each pair of sclerotised horns forms another, nonsclerotised single horn. Deirids small, knob-like, situated somewhat anterior to end of oesophageal corpus (Fig. 3B). Oesophagus consisting of almost cylindrical corpus with weakly outlined short pharynx, isthmus and bulb containing sclerotised apparatus. Nerve ring encircling isthmus near its anterior end. Excretory pore slightly posterior to level of mid-way between nerve ring and posterior end of oesophagus (Fig. 3A). Intestine straight, narrow. Tail of both sexes slender, long, sharply pointed.

Male (based on 2 specimens): Length of body 3.05- 3.13 mm, maximum width 109-136. Length of lip-like formations 9-12. Size of buccal cavity 9-12 x 9-12. Length of sclerotised horns 9. Length of entire oesophagus 393; length of pharynx 15, width 18-24, of corpus 153-162, width 18-24, length of isthmus 147-165, width 24-30; bulb 60-69 long and 54-60 wide. Deirids, nerve ring and excretory pore 125-189, 108-225 and 225-270, respectively, from anterior extremity. Caudal papillae 9 pairs, of which 4 pairs preanal (all subventral) and 5 pairs postanal (3 pairs subventral and 2 pairs lateral); in addition, 1 small median unpaired papilla present on anterior cloacal lip (Fig. 3E). Spicules unequal and dissimilar, well sclerotised. Large (left) spicule 264-300 long, small (right) spicule 63- 87 long; length ratio of spicules 1 : 3.0-4.8; proximal ends of spicules somewhat expanded, cup-shaped, distal ends sharply pointed (Fig. 3F,G). Gubernaculum simple, rod-like in lateral view, well sclerotised, 54-60 long, its maximum width 6; distal end of gubernaculum somewhat narrower (Fig. 3E,H). Tail slender, 462-816 long.

Female (based on 5 gravid specimens): Length of body 3.08-4.10 mm, maximum width 140-223. Lip-like formations 12-15 long. Size of buccal capsule 12-15 x 12-15. Length of sclerotised horns 12. Length of entire oesophagus 426-455; length of pharynx 15, width 24, of corpus 165-170, width 24-27, length of isthmus 173- 186, width 24-30; bulb 73-84 long and 60-80 wide. Deirids, nerve ring and excretory pore 158-198, 178- 243 and 250-287, respectively, from anterior extremity. Monodelphic. Vulva at short distance, 89-132, anterior to anus; vagina short, directed anteriorly from vulva (Figs. 31, 4E). Uterus containing few developing eggs and/or fully formed larvae (Fig. 31). Ovary short, reflexed, situated some distance posterior to oesophagus. Length of tail 550-590.

Host: Chuco intermedium (Günther) (Cichlidae), Northen check-mark cichlid, TBL 12 cm.

Site of infection: Intestine.

Locality: Lacantún River (Usumacinta River basin), Chiapas State, southern Mexico (collected 17 November 2007).

Prevalence and intensity: 1 fish infected/6 fish examined; 198 nematodes.

Deposition of voucher specimens: IPCAS N-807.

Comments: Gonzalez-Solis and Moravec (2004) described two morphologically similar species of Orientatractis, both intestinal parasites of cichlids (Cichlidae) in Mexico and Nicaragua: Orientatractis campechensis Gonzalez-Solis et Moravec, 2004 from Vieja bifasciata (Steindachner) (type host) in Campeche and C. pearsei in Chiapas (both Mexico); and Orientatractis chiapasensis Gonzalez-Solis et Moravec, 2004 from Ch. intermedium (type host) in Mexico (Chiapas) and Tomocichla tuba (Meek) in Nicaragua. The present specimens were identified as O. chiapasensis mainly based on the rodlike gubernaculum, cup-shaped proximal ends of spicules and the fish host, which is the type host of this nematode species.

González-Solis and Moravec (2004) also mentioned the lengths of spicules as a differential feature between O. campe- chensis (left spicule 400-510 um) and O. chiapasensis (left spicule 200-250 um). However, the left spicule in the present specimens was up to 300 um long, which may be associated with the fact that the present males were somewhat larger than those used for the original species description.

Orientatractis comprises nematodes parasitic in fishes, amphibians and reptiles (chelonians). Currently, this genus includes 11 valid species, of which five are intestinal parasites of freshwater fishes (see Jesus et al. 2023): O. campechensis and O. chiapasensis from cichlids in southern Mexico and Nicaragua; Orientatractis moraveci Cavalcante, Silva, Santos, Chagas-Moutinho et Santos, 2017 from pimelodid catfishes in South America (Bra711); Orientatractis mekongensis Moravec, Kamchoo et Pachanawan, 2015 from pangasiid catfishes in South Asia (Thailand); and Orientatractis brycini Gonzalez-Solis et Mariaux, 2017 from upside-down catfishes 1n Africa (Gabon) (González-Solís and Moravec 2004, Moravec et al. 2015, Cavalcante et al. 2017, Gonzalez-Solis and Mar1aux 2017).

In addition to O. campechensis and O. chiapasensis from Mexico, the only other congeneric species parasitising fishes in the Neotropical Region 1s O. moraveci reported from Pimelodus blochii Valenciennes (Pimelodidae) of the Xapuri and Acre Rivers (Amazon River basin), Acre State, western Brazil (Cavalcante et al. 2017). This species can be differentiated from O. chiapasensis mainly by the length of the larger spicule (161-198 um vs 204-300 pm) and the length of the male tail (225-270 um vs 460- 816 um) (Cavalcante et al. 2017).

In having the mouth surrounded by sclerotised formations (pieces), species of Orientatractis are similar to those of Klossinemella, including the type species Klossinemella iheringi (Travassos, Artigas et Pereira, 1928) parasitising mainly characiform and siluriform fishes in Brazil and Argentina and three other species from turtles (Moravec and Thatcher 1997, Moravec 1998). Whereas species of Orientatractis possess four bicornuate sclerotised pieces and four single horns around the mouth, those of Klossinemella are characterised by the mouth armed with eight Y-shaped sclerotised structures and four single horns (Moravec and Thatcher 1997, Gonzalez-Solis and Moravec 2004, Moravec et al. 2015, Gonzalez-Solis and Мапаих 2017, Cavalcante et al. 2017, Jesus et al. 2023).

The present finding of O. chiapasensis represents only the second record of this parasite since its original description.

Family Cosmocercidae Railliet, 1916

Raillietnema kritscheri Moravec, Salgado-Maldonado et Pineda-López, 1993 Fig. 5

Description (based on specimens from С. pearsei): Small, whitish nematodes. Cuticle finely transversely striated (Fig. SA,B,D). Narrow lateral alae present, initiating at level of about mid-length of oesophagus and extending posteriorly to some distance anterior to cloacal opening in males and to level of anus in females (Fig. 5C,E-G). Somatic papillae indistinct. Cephalic end rounded. Oral aperture triangular; 3 small flat lips present, each provided with broad cuticular membrane (lamella) on inner margin. Four submedian cephalic papillae, 2 on dorsal lip and 1 on each of subventral lip; 2 large oval lateral amphids present (Fig. 5A). Oesophagus with anterior, well-developed and relatively long pharynx; posterior oesophageal bulb with sclerotised apparatus. Nerve ring at mid-length of oesophagus, excretory pore somewhat anterior to level of bulb. Tail of both sexes conical and pointed.

Male (based on 5 specimens): Length of body 2.39- 3.36 mm, maximum width 136-190. Length of entire oesophagus 620-722; length of pharynx 42-54, width 21-30, of corpus 449-530, width 33-48, length of isthmus 36-48, width 33-39; bulb 81-105 long and 90-105 wide. Nerve ring and excretory pore 299-340 and 435-571, respectively from anterior extremity. Preanal region with 2 subventral rows of 5 pairs of large caudal papillae; anterior lip of cloaca with 3 pairs of small papillae and 1 larger unpaired median papilla. One pair of large lateral caudal papillae present approximately at level of cloacal aperture. Mid-region of tail with 2 pairs of large subventral papillae. One pair of small papillae located two-thirds of distance from cloaca to tail tip. Terminal portion of tail with 2 pairs of papillae located close together (Fig. SB,C,H). Spicules short, equally long, somewhat ventrally bent, well sclerotised, with pointed distal tips; spicules 105-126 long and 12- 15 wide. Gubernaculum well sclerotised, 57-66 long and 12-18 wide in lateral view. Length ratio of gubernaculum and spicules 1 : 1.7-2.1. Tail conical, 177-245 long, ending in sharp point (Fig. SC,H).

Female (based on 5 gravid specimens). Length of body 3.60-4.28 mm, maximum width 231-286. Length of entire oesophagus 759-813; length of pharynx 51-60, width 30-39, length of corpus 558-612, width 51-60, length of isthmus 33-45, width 48-60; bulb 105-108 long and 120-129 wide. Nerve ring and excretory pore 340- 381 and 517-694, respectively, from anterior extremity. Uterus amphidelphic; ovaries short, posterior ovary anterior to level of vulva. Vulva distinctly postequatorial, 2.37-2.87 mm, at 66-68% from anterior body end. Vulval lips may be somewhat elevated. Vagina directed anteriorly from vulva. Eggs thin-walled, oval, with uncleaved content, size 163-190 x 136-150; maximum number of eggs in uterus 4-5. Tail conical, 231-299 long, with sharply pointed tip; pair of small lateral phasmids present some distance anterior to tail tip (Fig. 5D,F,G).

Hosts: Cincelichthys pearsei (Hubbs), Pantano cichlid, TBL 12- 29 ст; and Chuco intermedium (Günther), Northern checkmark cichlid, ТВТ, 12 ст (both Cichlidae).

Site of infection: Intestine.

Locality: Lacantún River (Usumacinta River basin), Chiapas State, southern Mexico (collected 18-20 November 2007).

Prevalence and intensity: С. pearsei: 4 fish infected/6 fish examined; 56 to ca. 2,000 nematodes per fish. Ch. intermedium: 1 fish infected/6 fish examined; 10 nematodes in one infected fish specimen.

Deposition of voucher specimens: IPCAS N-596.

Comments: The morphology and measurements of the present specimens are very similar to those of R. kritscheri described by Moravec et al. (1993a) and, therefore, they are considered to belong to this species. Railletnema kritscheri has been known as a parasite of cichlids in southern Mexico (Campeche, Chiapas, Tabasco), being previously reported from C. pearsei (type host) and Vieja melanurus (see Moravec et al. 1993a, Moravec 1998), later also from endemic cichlids Chiapaheros grammodes (Taylor et Miller), Vieja breidohri (Werner et Stawikowsk1) and Vieja hartwegi (Taylor et Miller) from the upper Grijalva River in southeastern Chiapas, the river which joins the branch of the Usumacinta River in Tabasco (all three last-named cichlids are endemic to the Grijalva River basin) (ParedesTrujillo et al. 2020).

The present SEM examination of R. kritscheri enabled a more detailed study of the cephalic structures and male caudal papillae and, for the first time, showed the presence of lateral alae in this species.

Representatives of the cosmocercid genus Raillietnema Travassos, 1927 are mostly parasites of amphibians and reptiles (Baker 1987), only two congeneric species were reported from freshwater fishes: R. kritscheri from cichlids in Mexico (Moravec et al. 1993a) and Raillietnema synodontisi Vassiliades, 1973 from upside-down catfishes (Synodontis spp.) in Africa and from aquarium breedings in former Czechoslovakia (present Czech Republic) (Vassiliades 1973, Moravec and Rehulka 1987). However, based on a re-examination of its type specimens, R. synodontisi Was subsequently transferred to the atractid genus Zabeonema (see Koubková et al. 2008). This was later followed by subsequent authors (e.g., Moravec and Jirka 2017, Moravec 2019).

Although species of Raillietnema and Labeonema are presently listed in two different families of the Cosmocercoidea, their general morphologies are very similar, and the only intergeneric difference should be the presence of two ovaries in Raillietnema and the absence of the posterior ovary in Labeonema (see Moravec and Van As 2004, Koubková et al. 2008, Anderson et al. 2009). Nevertheless, in the phylogenetic analysis of the Cosmocercoidea by Pereira et al. (2015b), Labeonema was considered to be very similar to Raillietnema.

Subsequent phylogenetic analyses of 18S rDNA sequences of cosmocercoid nematodes by Cavalcante et al. (2017) also nested Labeonema synodontisi (Vassiliades, 1973) with Raillietnema sp. According to these authors, the molecular data and previous morphological studies of L. synodontisi indicate that this species should be returned to Raillietnema. Moreover, Koubkova et al. (2008) considered L. synodontisi to be probably identical to Labeonema intermedium Puylaert, 1970, which is the type species of Labeonema. If this 1s the case, then Zabeonema would become a junior synonym of Raillietnema.

As mentioned above, the posterior ovary in К. kritscheri is located approximately at the level of vagina, as it is illustrated for Raillietnema in fig. 6.6 of Anderson et al. (2009), i.e., anterior to the posterior branch of uterus, which looks like a sac without an ovary. Similar location of the posterior ovary as in R. kritscheri was described by Moravec and Rehulka (1987) for R. synodontisi. However, since the posterior ovary is somewhat reduced compared with the anterior ovary, and is partly overlapped by the uterus, it is not easy to observe it in these tiny nematodes.

The presence of the posterior ovary in Raillietnema and Labeonema is also indicated by the location of the vulva near the middle of body and the anteriorly directed vagina, in contrast to the vaginal opening into the rectum forming a cloaca or into the vulva located near the anus in typically monodelphic atractids such as species of Atractis, Orientatractis or Rondonia (see above). In our opinion, future histological studies will confirm synonymy of Labeonema with Raillietnema and, consequently, the allocation of all these nematodes in the Cosmocercidae.

Family Cucullanidae Cobbold, 1864

Cucullanus angeli Cabañas-Carranza et CaspetaMandujano, 2007 Figs. 6-8

New syn. Cucullanus oaxaquensis Caspeta-Mandujano, Salgado-Maldonado et Martinez-Ramirez, 2010

Description: Medium-sized nematodes with whitish, elongate body. Lateral alae absent. Cephalic end slightly asymmetrical in lateral view. Oral aperture dorsoventrally elongate, surrounded by raised narrow membraneous ala (collarette) supported by row of с. 60 minute rod-like basal teeth approximately 10 long (Figs. 6C, 7A,B). Four submedian cephalic double papillae and pair of lateral amphids present (Figs. 6C, 7A,B). Oesophagus muscular, expanded at anterior end to form bulbous pseudobuccal capsule (oesophastome), slightly asymmetrical in lateral view; posterior part of oesophagus also expanded, approximately as wide as oesophastome in lateral view (Fig. GA,B). Oesophagus opens into intestine through large valve. Nerve ring encircles oesophagus at 42-49% of its length. Deirids small, conical, at short distance anterior to oesophago-intestinal junction (Figs. GA,B, 7C). Postdeirids not found. Excretory pore located just posterior to posterior end of oesophagus (Fig. 6A.B). Tail of both sexes conical and sharply pointed at tip.

Male (based on 4 specimens from С. pearsei): Length of body 7.44-8.72 mm, maximum width 258-408; width at level of oesophastome 163-204, at middle of oesophagus 231-245. Length of entire oesophagus 734-830, representing 8-10% of whole body length; length of oesophastome 258-286, its width 136-150; minimum width of oesophagus 82-95; maximum width of posterior part of oesophagus 136-163. Distance of nerve ring from anterior extremity 354-394, representing 46-48% of oesophageal length. Deirids and excretory pore 625-694 and 854-938, respectively, from anterior end of body. Posterior end of body curved ventrally. Ventral sucker present (Fig. OLD). Cloacal region not elevated. Median, transversely oval cuticular elevation present anterior to cloacal opening (Figs. 6G-J, 70-Е, 8C). Spicules equal, 612-734 long, representing 7-9% of body length (Fig. 6J). Gubernaculum well sclerotised, 108-114 long, rod-like in lateral view (Fig. 6G,J) and Y-shaped in ventral view. Caudal papillae 10 pairs: 5 pairs of subventral preanal papillae and 5 pairs of postanal papillae (3 subventral and 2 dorsolateral); first sub- ventral pair of postanals slightly posterior to level of cloacal aperture, second subventral pair of postanals slightly posterior to mid-length of tail and posterior-most pair of subventrals somewhat anterior to tail tip; postanals of dorsolateral pairs approximately at levels of first and second pairs of subventrals (Figs. 6G-J, 7Е Е, 8A-C). Pair of small lateral phasmids somewhat anterior to level of second pair of subventral postanal papillae (Fig. 6H,I). Length of tail 177-190 (Figs. 6G-J, 7E-F, 8A-C).

Female (based on 10 ovigerous specimens from C. pearsei, measurements of 2 specimens with immature eggs from Theraps irregularis in parentheses). Length of body 9.03-13.60 (9.11-9.15) mm, maximum width 299-503 (313-381); width at level of oesophastome 190-245 (177- 218), at middle of oesophagus 190-313 (218-272). Length of entire oesophagus 898-1,074 (802-938), representing 8-10% (9-10%) of whole body length; length of oesophastome 286-367 (299-326), its width 163-190 (150- 177); minimum width of oesophagus 109-136 (82-95); maximum width of posterior part of oesophagus 163-218 (150-177). Distance of nerve ring from anterior extremity 381-462 (394-408), representing 42-49% (43-49%) of oesophageal length. Deirids and excretory pore 775-925 (721-775) and 952-1,156 (911-1,047), respectively, from anterior end of body. Vulva postequatorial, 5.54-8.16 (5.18-5.64) mm from anterior extremity, at 59-63% (57- 62%) of body length; vulval lips slightly elevated (Fig. 6E). Vagina directed anteriorly from vulva. Uterus didelphic, amphidelphic. Fully developed eggs elongate-oval, thin-walled, size 90-105 x 42-57 (-), with uncleaved contents (Fig. 6F). Length of tail 204-258 (190); phasmids situated approximately at its middle (Fig. 6D).

Hosts: Cincelichthys pearsei (Hubbs), Pantano cichlid, ТВТ, 25 ст; and Theraps irregularis Gunther, Arroyo cichlid, TBL not available (both Cichlidae).

Site of infection: Intestine.

Locality: Lacantún River (Usumacinta River basin), Chiapas State, southern Mexico (collected 20 November 2007).

Prevalence and intensity: C. pearsei: 2 fish infected/6 fish examined; 4 and 11 nematodes. 7. irregularis: 2 nematodes found in the only fish examined.

Deposition of voucher specimens: IPCAS N-1289.

Comments: At present the genus Cucullanus Muller, 1777 includes more than a hundred of species (Pereira et al. 2015a) that are intestinal parasites mostly of freshwater, brackish-water and marine fishes throughout the wor1d. Because of their rather uniform morphology and often inadequate descriptions, these parasites are usually dealt with according to their host groups or their zoogeographical region (Petter 1974a, Moravec et al. 1997, Moravec and Justine 2017, 2020).

To date, only five species of Cucullanus have been described from freshwater fishes of the family Cichlidae: Cucullanus angeli Cabañas-Carranza et Caspeta-Mandujano, 2007 and C. oaxaquensis Caspeta-Mandujano, Salgado-Maldonado et Martinez-Ramirez, 2010 from Ch. intermedium (Gunther) and Thorichthys helleri (Steindachner), respectively, in southern Mexico; Cucullanus tucunarensis Lacerda, Takemoto, Marchiori, Martins et PavaneШ, 2013 and Cucullanus opisthoporus Pereira et Luque, 2016 from Cichla piquiti Kullander et Ferreira, and from Cichla melaniae Kullander et Ferreira and Cichla pinima Kullander et Ferreira, respectively, in Brazil; and Cucullanus tilapiae Moravec, 2019 from Tilapia sparrmanii Smith in Africa (Botswana) (Cabañas-Carranza and Caspeta-Mandujano 2007, Caspeta-Mandujano et al. 2010, Lacerda et al. 2013, Pereira and Luque 2016, Moravec 2019).

The general morphology of the present specimens 1s very similar to that of C. angeli, a species reported by Cabañas-Carranza and Caspeta-Mandujano (2007) from the same Mexican locality (Lacantún River). Nevertheless, a major difference 1s in the lengths of spicules; the spicules of C. angeli were originally described as 107-175 um long, whereas those of the present specimens measure 612-734 um. However, the length of spicules given in the description of С. angeli 15 evidently erroneous. It can be determined from the accompanying line drawing and its scale bar (fig. 5) in the paper by Cabañas-Carranza and Caspeta-Mandujano (2007) that in fact the spicules of C. angeli were about 860 um long, i.e., similar to those in the present specimens. Therefore, the present nematodes are identified as C. angeli. The finding of C. angeli in C. pearsei and 7. irregularis represent new host records for this parasite.

In the paper describing C. oaxaquensis, Caspeta-Mandujano et al. (2010) write that this species can be easily distinguished from similar С. angeli based on the length of spicules; however, the spicule length of C. oaxaquensis (575-825 um) is in fact very similar to that in С. angeli (see above). Of other reported interspecific differences allegedly found in С. oaxaquensis, the "two small papillae" near the female tail tip and the "small terminal sclerotised structure" on the tail tip were evidently based on artifacts, as visible in SEM micrographs (figs. 12, 14 and 17) in the paper of Caspeta-Mandujano et al. (2010). A roughly granular (reported as "net-like") superficial sculpture on eggs in C. oaxaquensis was observed using SEM (fig. 13 in Caspeta-Mandujano et al. 2010), but the eggs of C. angeli have not yet been studied using this technique; it may well be that such egg sculpture may occur in both species. Moreover, a markedly granular surface on eggs was also observed in С. tilapiae (see Moravec 2019) and possibly occurs in some other congeners. Consequently, we consider С. oaxaquensis a junior synonym of С. angeli.

Accordingly, C. angeli appears to be a specific parasite of Cichlidae distributed in the Usumacinta and Coatzacoalcos Rivers drainage systems, southern Mexico, from where it was recorded in four host species, Ch. intermedium, С. pearsei, T. helleri and T. irregularis. In addition, Paredes-Trujillo et al. (2020) reported this nematode in three cichlid species, С. grammodes, V. breidohri and Г. hartwegi, from the upper Grijalva River in southeastern Chiapas; all these three fish hosts are endemic to the Grijalva River basin.

Cucullanus tucunarensis and С. opisthoporus, two species reported from Brazil, differ from other species of Cucullanus from cichlids, as well as from all other congeners, in possessing a terminal caudal appendage covered with small spines on its ventral surface. From С. angeli they also differ by the absence of a ventral sucker in the male. The African species C. tilapiae, whose male is not known, can be distinguished from С. angeli by deirids located slightly posterior to the level of oesophago-intestinal junction (vs at a short distance anterior to this junction).

Cucullanus mexicanus Caspeta-Mandujano, Moravec et Aguilar-A guilar, 2000 Figs. 9, 10

Description: Medium-sized nematodes with whitish, elongate body; anterior end of body not narrowed in region of nerve ring. Lateral alae absent. Cephalic end slightly asymmetrical in lateral view. Oral aperture dorsoventrally elongate, surrounded by raised narrow membraneous ala (collarette) supported by row of с. 70 minute basal teeth approximately 4 long (Figs. 9C, 10A,B). Four submedian cephalic double papillae and pair of lateral amphids present (Figs. 9C, 10A,B). Oesophagus muscular, expanded at anterior end to form short bulbous pseudobuccal capsule (oesophastome) slightly asymmetrical in lateral view; posterior part of oesophagus also expanded, approximately as wide as oesophastome in lateral view (Fig. 9A). Oesophagus opens into intestine through large valve. Nerve ring encircles oesophagus at 35-37% of its length. Deirids small, pointed, with broad basal part, situated approximately in mid-way between nerve ring and posterior end of oesophagus (Figs. 9A,D, 10G). Postdeirids not found. Excretory pore located short distance anterior to posterior end of oesophagus (Fig. 9A). Tail of both sexes conical, pointed at tip.

Male (based on 3 specimens): Length of body 10.02- 13.40 mm, maximum width 231-286; width at level of oesophastome 163-190, at middle of oesophagus 190. Length of entire oesophagus 857-884, representing 6-9% of whole body length; length of oesophastome 136-177, its width 109- 136; minimum width of oesophagus 54-68; maximum width of posterior part of oesophagus 136-150. Distance of nerve ring from anterior extremity 313-340, representing 37-39% of oesophageal length. Deirids and excretory pore 558-571 and 721-775, respectively, from anterior end of body. terior end body curved ventrally. Ventral sucker present, weakly developed (Figs. 9B, 10E). Cloacal region not elevated. Spicules equal, 348-360 long, representing 3-4% of body length (Fig. 9B). Gubernaculum weakly sclerotised, 60-81 long, rod-like in lateral view (Fig. 9B,F) and Y-shaped in ventral view. Caudal papillae 10 pairs: 5 pairs of subventral preanal papillae and 5 pairs of postanal papillae (3 subventral and 2 dorsolateral); first subventral pair of postanals slightly posterior to level of cloacal aperture, second subventral pair of postanals slightly posterior to mid-length of tail and posterior-most pair of subventrals somewhat anterior to tail tip; postanals of dorsolateral pairs approximately at levels of first and third pairs of subventrals (Figs. 9B,F, 10C-E). Pair of small lateral phasmids somewhat anterior to level of second pair of subventral postanal papillae (Figs. 9B,F). Length of tail 245-258 (Figs. 9B,E, 10C-E).

Female (based on 1 nongravid specimen): Length of body 17.68 mm, maximum width 340; width at level of oesophastome 163, at middle of oesophagus 218. Length of entire oesophagus 1.05 mm, representing 6% of whole body length; length of oesophastome 163, its width 150; maximum width of posterior part of oesophagus 163. Distance of nerve ring from anterior extremity 367, representing 35% of oesophageal length. Deirids and excretory pore not located. Vulva postequatorial, 10.04 mm from anterior extremity, at 57% of body length. Vagina directed anteriorly from vulva. Uterus didelphic, amphidelphic; empty. Tail elongate, slender, 354 long; small lateral phasmids situated approximately at mid-length of tail (Figs. 9F, 10F).

Host: Rhamdia laticauda (Kner) (Heptapteridae), Filespine chulin, TBL 15 ст.

Site of infection: Intestine.

Locality: Lacantún River (Usumacinta River basin), Chiapas State, southern Mexico (collected 16 and 18 November 2007).

Prevalence and intensity: 2 fish infected/4 fish examined; 3 and 1 nematodes.

Deposition of voucher specimens: Not deposited (paratypes IPCAS N-767).

Comments: To date, the only species of Cucullanus reported from catfishes of the family Heptapteridae is Cucullanus mexicanus, a parasite of the freshwater catfish Rhamdia guatemalensis, described from the brooks in Oaxaca, southern Mexico (Caspeta-Mandujano et al. 2000, Caspeta-Mandujano 2005). In contrast to other congeners, this species was characterised by the presence of a conspicuously elongate ventral sucker in the posterior region of the male (Caspeta-Mandujano et al. 2000).

The present specimens from К. laticauda are somewhat larger than those of C. mexicanus desribed by Caspeta-Mandujano et al. (2000), but their morphology is more or less in agreement with the original description of the species. It should be highlighted that the female nematodes studied by Caspeta-Mandujano et al. (2000) had only immature eggs. In addition to morphological and biometrical similarity of both these nematode forms, 1t is necessary to consider that these were collected from congeneric hosts from nearby regions (Oaxaca and Chiapas) in southern Mexico. Re-examination of the paratypes (male and female) of C. mexicanus deposited 1n the Helminthological Collection of the Institute of Parasitology, BC CAS indicated their conspecificity with the present nematodes, which are considered to belong to this species. The original description of C. mexicanus was based solely on LM examination, so that the present SEM study 15 the first one for this species.

The present finding of C. mexicanus infecting R. laticauda in Chiapas represents new host and geographical records.

Cucullanus potamarii sp. n. Figs. 11-13

ZooBank number for species:

urn:lsid:zoobank.org:act: BD2D8410-8786-4F2C-A948-FC1490A551CD

Description: Medium-sized nematodes with whitish, elongate body; anterior end of body somewhat narrowed in region of nerve ring. Lateral alae absent. Cephalic end slightly asymmetrical in lateral view. Oral aperture dorsoventrally elongate, surrounded by raised narrow membraneous ala (collarette) supported by row of с. 140 minute basal teeth (Figs. 11C, 12A,B). Four submedian cephalic double papillae and pair of lateral amphids present (Figs. 11C, 12A,B). Oesophagus muscular, expanded at anterior end to form short bulbous pseudobuccal capsule (oesophastome) slightly asymmetrical in lateral view; posterior part of oesophagus also expanded, approximately as wide as oesophastome in lateral view (Fig. 11 A,B). Oesophagus opens into intestine through large valve. Nerve ring encircles oesophagus at 30-35% of its length. Deirids small, pointed with broad basal part, situated somewhat posterior to level of oesophago-intestinal junction (Figs. 11B,F, 12D). Postdeirids not found. Excretory pore located far posterior to level of deirids (Fig. 11B). Tail of both sexes conical, pointed at tip.

Male (based on 2 specimens; holotype; measurements of paratype in parentheses): Length of body 17.76 (19.22) mm, maximum width 408 (435); width at level of oesophastome 367 (340), at middle of oesophagus 286 (286). Length of entire oesophagus 1.77 (1.77) mm, representing 10% (11%) of whole body length; length of oesophastome 380 (313), its width 286 (299); minimum width of oesophagus 122 (122); maximum width of posterior part of oesophagus 272 (272). Distance of nerve ring from anterior extremity 571 (530), representing 32% (30%) of oesophageal length. Deirids and excretory pore 1.80 (1.93) mm and 2.58 (2.60) mm, respectively, from anterior end of body. Posterior end of body curved ventrally. Ventral sucker present (Figs. 11G, 12F,G, 13C-F). Cloacal region conspicuously protruded, with broad distal end forming curved rim (Figs. 11G,H,J, 12F,G, 13A,B,D). Small median, papilla-like elevation present anterior to cloacal opening (Fig. 11). Spicules equal, 300 (342) long, repre- senting 1.7% (1.8%) of body length (Fig. 11G). Gubernaculum well sclerotised, 245 (231) long, rod-like in lateral view (Fig. 11G,H) and Y-shaped in ventral view. Caudal papillae 10 pairs: 5 pairs of subventral preanal papillae, 1 pair of subventral adanal papillae and 4 pairs of postanal papillae (2 subventral and 2 dorsolateral); first dorsolateral pair of postanals just posterior to level of cloacal aperture, second dorsolateral pair of postanals near mid-length of tail; papillae of both subventral postanal pairs situated in posterior half of tail (Figs. 11 G,H.J). Pair of lateral phasmids just anterior to level of second dorsolateral pair of postanal papillae (Figs. 11 G,H,J, 13A,B). Length of tail 408 (422) (Figs. 11G,H,J, 12F,G, 13A,B,E,F).

Female (based on 1 ovigerous specimen, allotype): Length of body 15.23 mm, maximum width 313; width at level of oesophastome 286, at middle of oesophagus 245. Length of entire oesophagus 1.51 mm, representing 10% of whole body length; length of oesophastome 326, its width 326; minimum width of oesophagus 109; maximum width of posterior part of oesophagus 204. Distance of nerve ring from anterior extremity 530, representing 35% of oesophageal length. Deirids and excretory pore 1.65 mm and 2.12 mm, respectively, from anterior end of body. Vulva postequatorial, 9.52 mm from anterior extremity, at 63% of body length; vulval lips somewhat elevated (Fig. 111). Vagina directed anteriorly from vulva. Uteri opposed. Fully developed eggs elongate-oval, thin-walled, size 84-105 x 51-54, with uncleaved contents (Fig. 11D). Tail elongate, slender, 435 long; phasmids situated approximately at 2/3 of its length (Figs. 11E, 12E).

Type host: Potamarius usumacintae Betancur-R et Willink (Ariidae), Usumacinta sea catfish, ТВТ, 51 ст.

Site of infection: Intestine (its anterior portion).

Type locality: Lacantun River (Usumacinta River basin), Chiapas State, southern Mexico (collected 20 November 2007).

Prevalence and intensity: 1 fish infected/2 fish examined; 3 nematodes.

Deposition of type specimens (mounted on SEM stub): IPCAS N-1290.

Etymology: The species name of this nematode relates to the generic name of the type host.

Comments: Only seven species of Cucullanus have been described from catfishes of the family Ariidae: Cucullanus arii Yamaguti, 1954 and Cucullanus armatus Yamaguti, 1954, each from Arius sp. off Borneo, Indonesia; Cucullanus bagre Petter, 1974 from Bagre bagre (Linnaeus) off French Guiana (reported also from this host species off Brazil and from Ariopsis assimilis (Gunther) off Mexico); С. baylisi Bharatha-Lakshmi, 2000 from Netuma thalassina (Rúppel) о India; Cucullanus costaricensis Lôpez-Caballero, Osorio-Sarabia et Garcia-Prieto, 2009 from Bagre pinnimaculatus (Steindachner) in Costa Rica; Cucullanus laurotravassosi Petter et Le Bel, 1992 from Arius sp. in Australia; and Cucullanus quadrii Bilgees et Fatima, 1980 from N. thalassina off Pakistan (Yamaguti 1954, Petter 1974b, Bilgees and Fatima 1980, Petter and Le Bel 1992, Bharatha-Lakshmi 2000, A guirre-Macedo et al. 2007, López-Caballero et al. 2009, Pereira et al. 2015a).

Of these, only C. costaricensis and C. laurotravassosi occur in fresh waters of Costa Rica and Australia, respectively, whereas all the remaining five species were reported from marine environments, largely in Asia. It is necessary to note that the fish host of С. potamarii sp. n., i.e., P. usumacintae, is an exclusively freshwater fish (as all its congeners), which is endemic to the Usamacinta River basin (Froese and Pauly 2024).

Based on the general morphology, С. potamarii sp. n. is similar to species of Cucullanus described from ariid hosts in America (C. bagre and C. costaricensis) and that from Australia (C. laurotravassosi) (see also Pereira et al. 2015a), differing from them mainly in the location of deirids, that are distinctly posterior to the level of the oesophago-intestinal junction (vs some distance anterior to oesophago-intestinal junction). The excretory pore of С. bagre is far posterior to the posterior end of oesophagus, resembling thus the new species, whereas that of C. costaricensis is allegedly placed at a short distance posterior to the nerve ring, i.e., anterior to the level of deirids. However, considering that the excretory pore is often difficult to observe in cucullanids and its location in relation to deirids as described for C. costaricensis 1s unusual in these nematodes, the information about the position of the excretory pore in C. costaricensis needs to be verified. In contrast to the new species, the excretory pore of C. laurotravassosi was reported to be "close to the deirids" (Petter and Le Bel 1992), i.e., slightly anterior to the oesophago-intestinal junction.

The location of deirids and the excretory pore far posterior to the end of the oesophagus as in C. potamarii sp. п. is very rare among all Cucullanus spp. and, in fact, this feature was previously reported only for Cucullanus rhamphichthydis Moravec, Kohn et Fernandes, 1997 (described solely from females), a parasite of Rhamphichthys rostratus (Linnaeus) (Rhamphichthyidae, Gymnotiformes) in the Parana River, Brazil (Moravec et al. 1997) and for C. opisthoporus Pereira et Luque, 2016 parasitising Cichla spp. (Cichlidae, Cichliformes) in the Amazon River basin Brazil (Pereira and Luque 2016).

However, in contrast to the new species, specimens of С. rhamphichthydis are smaller (body length 9-12 vs 15- 19 mm), their oesophastome is funnel-shaped (vs approximately spherical) and smaller (258 x 177-190 vs 313- 380 x 286-326 um), the entire oesophagus is shorter (1.17-1. 35 vs 1.51-1.77 mm long) and also the female tail is shorter (313-326 vs 435 um long). Moreover, hosts of both species belong to different fish orders (Gymnotiformes vs Siluriformes). Cucullanus opisthoporus differs clearly from C. potamarii sp. n. in having no ventral sucker in the male, much longer spicules (793-1,180 um vs 300- 342 um) and the presence (vs absence) of numerous minute spines on the tail tip of both sexes. These species also differ in the orders of their fish hosts (Cichliformes vs Siluriformes) and in the geographical region (Brazil vs Mexico).

Cucullanus potamarii sp. n. is the fourth known valid species of Cucullanus parasitising freshwater fishes in Mexico, in addition to C. angeli (syn. C. oaxaquensis), Cucullanus caballeroi Petter, 1977 and C. mexicanus.

Neocucullanus neocucullanus Travassos, Artigas et Pereira, 1928 Figs. 14-16

Syns.: Cucullanus interrogativus Travassos, Artigas et Pereira, 1928; Neocucullanus multipapillatus Petter, 1989

Description: Larger-sized nematodes. Body whitish, elongate, slightly narrower in region between posterior end of pseudobuccal capsule and posterior end of oesophagus (Fig. 14A,B). Lateral cervical alae absent. Cephalic end rounded in lateral view (Figs. 14A, 15A). Oral aperture dorsoventrally elongate, surrounded by raised narrow membraneous ala (collarette) supported by row of c. 100 minute basal teeth (Figs. 14C, 15A-D). Four submedian cephalic double papillae and pair of lateral amphids present (Figs. 14C, 15A-C). Oesophagus muscular, expanded at its anterior portion to form bulbous pseudobuccal capsule (oesophastome); posterior part of oesophagus also expanded, slightly broader than oesophastome in lateral view (Fig. 14A). Six large rounded oesophageal lobes, 3 on each side, visible in mouth in apical view (Figs. 14C, 15C). Oesophagus opens into intestine through large valve. Nerve ring encircles oesophagus at distance representing 37-44% of oesophageal length. Deirids small, rounded, situated at posterior half of distance between nerve ring and posterior end of oesophagus (Figs. 14A, B). Postdeirids absent. Excretory pore slightly posterior to level of deirids (Fig. 14A). Tail of both sexes short, conical to broadly rounded, bearing small terminal cuticular spike (Figs. 14D-F,1, 16A-D,F).

Male (based on 6 specimens): Length of body 9.86- 19.53 mm, maximum width 245-639; width at level of cloaca 245-666. Length of entire oesophagus 1.05-1.32 mm, representing 9-11 (9)% of body length; length of oesophastome 326-394, its width 150-231; minimum width of oesophagus 68-136; maximum width of posterior part of oesophagus 190-245. Distance of nerve ring from anterior extremity 449-517, representing 37-43% of oesophageal length. Deirids and excretory pore 680-870 and 707-966, respectively, from anterior end of body. Posterior end of body curved ventrally. Ventral sucker present, well developed (Fig. 14E,F). Spicules broad, non-alate, equal, 435- 517 long and 24-60 wide (Fig. 14D,F). Gubernaculum absent. Thirteen pairs of caudal papillae: 8 pairs of subventral preanal papillae, 1 pair of adanal papillae and 4 pairs of postanal papillae (3 subventral and 1 lateral); 1 median papilla-like elevation present anterior to cloacal aperture (Figs. 14 D-F, 16A-E). Pair of minute phasmids located laterally close to base of caudal spike (Figs. 14D-F, 16D). Length of tail including terminal spike 150-272; spike 24- 41 long (Figs. 14 D-F, 16 A-D).

Female (based on 3 ovigerous specimens): Length of body 27.57-28.76 mm, maximum width 449-571; width at level of anus 190-394. Length of entire oesophagus 1.33- 1.41 mm, representing 7-9% (7%) of body length; length of oesophastome 408-503, its width 218-231; minimum width of oesophagus 95-122; maximum width of posterior part of oesophagus 231-299. Distance of nerve ring from anterior extremity 517-584, representing 37-44% of oesophageal length. Deirids and excretory роге 897-993 and 911-1,020, respectively, from anterior end of body. Vulva postequatorial, 15.05-15.98 mm from anterior extremity, at 53-56% of body length; vulval lips slightly elevated (Fig. 14H). Vagina directed anteriorly from vulva. Uterus didelphic, amphidelphic. Fully developed eggs elongate-oval, thin-walled, size 51-60 x 30-42, with uncleaved contents (Fig. 14G). Length of tail including terminal spike 204-272; spike 15- 18 long; pair of small phasmids situated laterally at short distance anterior to caudal terminal spike (Figs. 141, 16F).

Host: Brycon guatemalensis Regan (Bryconidae), Machaca; TBL 21-36 cm.

Site of infection: Intestine.

Locality: Lacantún River (Usumacinta River basin), Chiapas State, southern Mexico (collected 15 and 16 November 2007).

Prevalence and intensity: 5 fish infected/8 fish examined; 1-6 (mean 3) nematodes per fish.

Deposition of voucher specimens: IPCAS N-1288.

Comments: Neocucullanus neocucullanus, the type species of Neocucullanus Travassos, Artigas et Pereira, 1928, was described by Travassos et al. (1928) from an unidentified characiform fish (reported as Characidae gen. sp.) in the Paraná River basin of Brazil. Subsequently, the same species was reported from Brazil (States of Minas Gerais, Mato Grosso and Sáo Paulo) by several authors from Brycon sp., Brycon hilarii (Valenciennes), Salminus brasiliensis (Cuvier) and S. hilarii Valenciennes (all Bryconidae, Characiformes) (see Luque et al. 2011). Caspeta-Mandujano et al. (2005) recorded it from Brycon guate- malensis in the Usumacinta River, southerm Mexico. From the same host species in Chiapas it is also reported from the Lacantún River by Salgado-Maldonado et al. (2011b).

Petter (1989) described another congeneric species, N. multipapillatus, from Brycon microlepis (Valenciennes) (= syn. of В. hilarii) of the Paraná River basin in Paraguay and considered Cucullanus interrogativus Travassos, Artigas et Pereira, 1928 from Brazil to represent juvenile forms of N. neocucullanus. Nevertheless, based on the morphological similarity between N. multipapillatus and N. neocucullanus, Moravec (1998) questioned the validity of the former species. Subsequently, N. multipapillatus was formally synonymised with N. neocucullanus by Saraiva et al. (2006), which 15 accepted here.

Small differences were found in the present material when compared with other taxonomic studies of N. neocucullanus. These differences are mainly in the number and distribution of caudal papillae, which may be difficult to observe in LM. Even though Caspeta-Mandujano et al. (2005) and Saraiva et al. (2006) studied the species using SEM, some inconsistencies persist. According to the original description of N. neocucullanus by Travassos et al. (1928), there are 13 pairs of caudal papillae in the males, whereas Petter (1989) and Saraiva et al. (2006) reported 15 and 14 pairs, respectively. The present data are in agreement with the original description of the species and with the observations of Caspeta-Mandujano et al. (2005), reporting 13 pairs of papillae (8 preanal, 1 adanal and 4 postanal) in N. neocucullanus. Petter (1989) reported the presence of a small gubernaculum in N. multipapillatus (= N. neocamallanus), but no sclerotised gubernaculum was observed in Neocucullanus neocucullanus Travassos, Artigas et Pereira, 1928 by Travassos et al. (1928), Caspeta-Mandujano et al. (2005), Saraiva et al. (2006) and in the present study. The shape of spicules in the present specimens was similar to that illustrated by Petter (1989).

The present data show that N. neocucullanus is a parasite of fishes belonging to the characiform family Bryconidae, being distributed from Paraguay and southern Brazil to southerm Mexico (Chiapas).

Ramallo (2012) described a new species of Neocucullanus, Neocucullanus marcelae Ramallo, 2012, from Salminus brasiliensis in the Paraná River basin, Argentina (Province of Salta). She distinguished it from N. neocamallanus mainly by the excretory pore allegedly located near the anterior end of body (far anterior to the level of nerve ring and deirids), long spicules (over 1.60 mm) and a different number (15 subventral pairs and 2 single median papillae) and distribution of male caudal papillae. However, the low-quality SEM micrographs and the respective line drawings indicate that some of these taxonomic features are apparently based on artifacts (e.g., the excretory pore is never anterior to the nerve ring in cucullanids) or inaccurate observations (presence of a ventral median postanal papilla?). Therefore, we consider N. marcelae to be a species inquirenda. Since the host species (S. brasiliensis) and the river drainage system are identical for both N. marcelae and N. neocucullanus, the former species may well prove to be conspecific with the latter one. Nevertheless, the length and shape of the spicules of N. marcelae as reported by Ramallo (2012) are very different from those in N. neocamallanus, resembling rather the spicules in other cucullanid genera, so that the generic misidentifcation of these nematodes cannot be excluded.

Family Camallanidae Railliet et Henry, 1915

Procamallanus (Spirocamallanus) rebecae (AndradeSalas, Pineda-López et Garcia-Magaña, 1994) Moravec, Mendoza-Franco, Vargas-Vazquez et Vivas-Rodriguez, 1995

Syn.: Spirocamallanus rebecae Andrade-Salas, Pineda-López et Garcia-Magafia, 1994

The present specimens of P. (S.) rebecae were morphometrically identical with those previously described by Andrade-Salas et al. (1994) and redescribed by Moravec et al. (1995a) from cichlids in the Mexican States of Campeche and Tabasco, and Yucatan, respectively.

Hosts: Mayaheros urophthalmus (Gunther), Mexican mojarra, TBL 8-15 cm; Thorichthys helleri (Steindachner), Yellow cichlid, TBL 7-15 cm; 7. meeki Brind, Firemouth cichlid, TBL 8-9 cm; Thorichthys sp., TBL 10-13 cm; Vieja melanurus (Gunther), Redhead cichlid, TBL 10-12 cm; and Wajpamheros nourissati (Allgayer), Bluemouth cichlid, TBL 8-11 ст (all Cichlidae).

Site of infection: Intestine, less often stomach or rectum.

Locality: Lacantún River including Laguna El Oaxaco (Usumacinta River basin), Chiapas State, southern Mexico (collected 16-20 November 2007).

Prevalence and intensity: M. urophthalmus: 2 fish infected/5 fish examined; 1 nematode per fish. 7. helleri: 2/4; 1 and 4. Thorichthys sp.: 3/5; 1 and 2. И. melanurus: 2/3; 3 and 6. И. nourissati: 4/6; 1-2.

Deposition of voucher specimens: Not deposited.

Comments: Procamallanus (S.) rebecae seems to be a common and widespread intestinal parasite of many species of cichlids in southern Mexico (States of Campeche, Chiapas, Oaxaca, Quintana Roo, Tabasco, Veracruz and Yucatan) (Caspeta-Mandujano 2005). The species was already reported from the Lacantún River and its tributary Lacanjá River in Chiapas by Caspeta-Mandujano (2005) and Salgado-Maldonado et al. (2011b).

The life cycle of this parasite was experimentally studied by Moravec et al. (1995b) (see also Moravec 1998) in Yucatán, Mexico. As suitable experimental hosts were found copepods Mesocyclops chaci Fiers, Reid, Iliffe et Suárez-Morales (intermediate hosts), guppies Poecilia reticulata Peters (paratenic hosts) and cichlids M. urophthalmus (definitive hosts).

Family Daniconematidae Moravec et Коте, 1987

Mexiconema cichlasomae Moravec, Vidal et SalgadoMaldonado, 1992

The present material comprised only one immature female of this nematode.

Host: Thorichthys helleri (Steindachner) (Cichlidae), Yellow cichlid; TBL 15 ст.

Site of infection: Intestine.

Locality: Lacantún River (Usumacinta River basin), Chiapas State, southern Mexico (collected 19 November 2007).

Prevalence and intensity: 1 fish infected/4 fish examined; 1 nematode.

Deposition of voucher specimen: Not deposited.

Comments: This nematode species 1s a frequent parasite of the abdominal cavity and viscera (rarely of skin) of many species of cichlids in southern Mexico (States of Campeche, Quintana Roo, Tabasco, Veracruz and Yucatán), occasionally also recorded from Yiphophorus hellerii Heckel (Poeciliidae) (Moravec et al. 1992b, 1998, Caspeta-Mandujano 2005).

Infective larvae of M. cichlasomae were recorded from naturally infected Argulus yucatanus Poly (Branchiura) in Yucatán, where these blood-sucking ectoparasites of cichlids apparently serve as the nematode's intermediate hosts (Moravec et al. 1999, Moravec 2006).

Family Rhabdochonidae Travassos, Artigas et Pereira, 1928

Rhabdochona kidderi kidderi Pearse, 1936

Syn.: Rhabdochona ictaluri Aguilar-Aguilar, Rosas-Valdez et Pérez-Ponce de Leon, 2010

Results of detailed studies of this nematode species from fishes of the Lacantún River (part of the present material) have already been published 1n the paper by Moravec et al. (2012).

Comments: Moravec et al. (2012) reported four-stage larvae of Rhabdochona kidderi Pearse, 1936 from the 1ntestine of Eugerres mexicanus in the Lacantún River. In the same host and locality, adults of a new congeneric species, Rhabdochona juliacarabiasae Caspeta-Mandujano, Salinas-Ocampo, Suárez-Rodríguez, Ramirez-Martinez et Matamoros, 2021, were later described by Caspeta-Mandujano et al. (2021). The authors speculated that the larvae reported by Moravec et al. (2012) probably belonged to К. juliacarabiasae. However, the shape of deirids of these larvae was characteristic of R. kidderi, as visible in a SEM micrograph (fig. Зе) of Moravec et al. (2012). In this case, E. mexicanus served as a paratenic or transport host for R. kidderi larvae.

Moravec et al. (2012) synonymised Rhabdochona ictaluri Aguilar-Aguilar, Rosas-Valdéz et Pérez-Ponce de León, 2010 from Ictalurus spp. in Mexico with К. kidderi. However, based on new comparative morphological and molecular studies of R. ictaluri and R. kidderi, the validity of the former species was resurrected by Lagunas-Calvo et al. (2019), reporting the allegedly presence/absence of six basal teeth in the prostom (but no such structures are apparent in their SEM micrographs figs. 1a and 1b) and the number of eggs in the uterus to be the main interspecific morphological differences. However, such features are doubtful and not used in rhabdochonid taxonomy. Therefore, since the molecular differences pointed out by Lagunas-Calvo et al. (2019) are not supported by morphological or ecological differences, we still consider both forms as synonymous for now.

Family Cystidicolidae Skryabin, 1946

Spinitectus osorioi Choudhury et Pérez-Ponce de León, 2001

Results of detailed studies of this nematode species from fishes of the Lacantún River (part of the present material) have already been dealt with in the paper by Moravec et al. (2009).

Spinitectus tabascoensis Moravec, Garcia-Magaña et Salgado-Maldonado, 2002

Syn.: Spinitectus macrospinosus Choudhury et Perryman, 2003

Results of detailed studies of S. tabascoensis from fishes of the Lacantun River (part of the present material) have already been published by Moravec et al. (2009).

Larval nematodes

Family Kathlaniidae Lane, 1914

Falcaustra sp. third-stage larvae

These larvae were already described by Moravec et al. (1995с) from fishes in Texas, USA, and by Moravec et al. (1995d) and Caspeta-Mandujano (2005) from fish hosts in the States of Yucatan, Quintana Roo, Campeche, Tabasco and Chiapas, Mexico.

Host: Rhamdia guatemalensis (Gunther) (Heptapteridae), Pale catfish, ТВТ, 25 ст.

Site of infection: Swimbladder.

Locality: Lacantún River (Usumacinta River basin), Chiapas State, southern Mexico (collected 11 November 2007).

Prevalence and intensity: 1 fish infected/2 fish examined; 15 nematodes.

Deposition of specimens: Not deposited.

Comments: According to Moravec et al. (1995c), these larvae probably belong to one or more species of Falcaustra parasitic in freshwater turtles, where fishes play a role of paratenic hosts. Moravec and Vargas-Vázquez (1998) recorded adults of Falcaustra affinis (Leidy, 1856) from the turtle 7rachemys scripta (Schoepf) in a small lake in central Yucatán.

In Yucatán, larvae of Falcaustra Lane, 1915 were recorded from many fish species (including R. guatemalensis) of different families, being found free or encapsulated in the host's abdominal cavity (mainly on the surface of 1ntestine), liver, gall-bladder, mesentery or brain (Moravec et al. 1995d, Caspeta-Mandujano 2005).

Family Anisakidae Railliet et Henry, 1912

Contracaecum sp. larvae Type 1 of Moravec et al. (1993b)

Descriptions of these larvae were already provided by Moravec et al. (1993b, 1995d) and Caspeta-Mandujano (2005).

Host: Brycon guatemalensis Regan (Bryconidae), Machaca, TBL 29 cm; and Chuco intermedium (Gunther) (Cichlidae), Northern checkmark cichlid, TBL 13 cm.

Site of infection: Surface of intestine.

Locality: Lacantún River (Usumacinta River basin), Chiapas State, southern Mexico (collected 16 and 17 November 2007).

Prevalence and intensity: B. guatemalensis: 1 fish infected/4 fish examined; 9 nematodes. C. intermedium: 1/6, 1.

Deposition of specimens: Not deposited.

Comments: These larvae are characterised by the presence of a markedly long ventricular appendix the length of which usually slightly exceeds that of the intestinal caecum. They were reported from many fish species in BraZil, Venezuela and Mexico (Moravec et al. 1993b, 1995d, Moravec 1998, Caspeta-Mandujano 2005). The adult forms of these larvae are apparently parasitic in fish-eating birds; fishes serve either as the second intermediate hosts or as paratenic hosts.

Contracaecum sp. larvae Type 2 of Moravec et al. (1993b)

Descriptions of these larvae were already provided by Moravec et al. (1993b, 1995d), Moravec (1998) and Caspeta-Mandujano (2005).

Hosts: Atractosteus tropicus Gill (Lepisosteidae), Tropical gar; TBL 52-53 ст; Dorosoma petenense (Gunther) (Dorosomatidae), Threadfin shad, TBL 11-17 ст; Brycon guatemalensis Regan (Bryconidae), Machaca, TBL 18 cm; Ictalurus furcatus (Valenciennes) (Ictaluridae), Blue catfish, TBL 39 cm; Lacantunia enigmatica Rodiles-Hernández, Hendrickson et Lundberg (Lacanthuniidae), Chiapas catfish, TBL 46 cm; Rhamdia guatemalensis (Gunther) (Heptapteridae), Pale catfish, TBL 25 ст; Chuco intermedium (Günther) (Cichlidae), Northern checkmark cichlid, TBL 12 cm; Mayaheros urophthalmus (Gunther) (Cichlidae), Mexican mojarra, TBL 19 cm; Parachromis friedrichsthalii (Heckel) (Cichlidae), Yellowjacket cichlid, TBL 14 cm; Parachromis managuensis (Gunther) (Cichlidae), Jaguar guapote, TBL 24 cm; Petenia splendida Gunther (Cichlidae), Bay snook, TBL 12-23 cm; Thorichthys helleri (Steindachner) (Cichlidae), Yellow cichlid, TBL 10 cm; Thorichthys sp. (Cichlidae), TBL 13 cm; and Gobiomorus dormitor Lacepede (Eleotridae), Bigmouth sleeper, TBL 38 cm.

Sites of infection: Abdominal cavity, surface of stomach and intestine, mesentery and liver.

Locality: Lacantún River (Usumacinta River basin), Chiapas State, southern Mexico (collected 12-20 November 2007).

Prevalence and intensity: A. tropicus: 2 fish infected/4 fish examined; 1-3 nematodes. D. petenense: 2/9; 12. B. guatemalensis: 1/9; 1. 1. furcatus: 1/12; 3. L. enigmatica: 1/2; 4. R. guatemalensis: 1/2; 6. Ch. intermedium: 1/6; 1. M. urophthalmus: 1/6; 1. P. friedrichsthalii: 1/1; 2. P. managuensis: 2/2; 1. P. splendida: 3/4; 1-6. Т. helleri: 1/4; 1. Thorichthys sp.: 1/5; 1. G. dormitor: 1/1; 1.

Deposition of specimens: Not deposited.

Comments: This type of larvae is mainly characterised by having the anterior intestinal caecum conspicuously longer than the ventricular appendix. Larvae of this type were reported from many fish species, e.g., 1n Brazil, Argentina, Venezuela, Cuba and southern Mexico (Moravec et al. 1993b, 1995d, Moravec 1998, Caspeta-Mandujano 2005). The adult forms of these larvae are mainly parasites of fish-eating birds; fishes serve either as the second intermediate hosts or as paratenic hosts.

Family Gnathostomatidae Railliet, 1895

Gnathostoma binucleatum Almeyda-Artigas, 1991 third-stage larva

The morphology and measurements of the only available larva was in agreement with the description of G. binucleatum third-stage larvae provided by Moravec (1998).

Host: Rhamdia guatemalensis (Gunther) (Heptapteridae), Pale catfish; TBL 25 cm.

Site of infection: Intestine.

Locality: Lacantún River (Usumacinta River basin), Chiapas State, southern Mexico (collected 11 November 2007).

Prevalence and intensity: 1 fish infected/2 fish examined; 1 nematode.

Deposition of specimen: Not deposited.

Comments: The definitive hosts of G. binucleatum are ocelots (Leopardus pardalis Linnaeus) and feral cats (Felis catus Linnaeus), in which adult nematodes occur in nodules in the stomach. This parasite develops with the participance of copepod intermediate hosts, whereas fishes serve as paratenic hosts. In Mexico, many freshwater and oligohaline fish species of different families were recorded to harbour G. binucleatum larvae in the musculature (Pérez-Ponce de León et al. 1996, Moravec 1998, Caspeta-Mandujano 2005). Most of the records of congeneric larvae unidentified to species from fishes in Mexico also relate to G. binucleatum larvae (Almeyda-Artigas 1991). The finding of this larva (as Gnathostoma sp.) in К. guatemalensis of the Lacantún River has already been reported by Salgado-Maldonado et al. (2011a,b). This nematode species is considered as the causative agent of human gnathostomosis, a common disease among inhabitants 1n the region of the lower Papaloapan River basin in Mexico (Moravec 1998).

Spiroxys sp. third-stage larvae

Descriptions of Spiroxys larvae from fishes in Mexico were already provided by Moravec et al. (1995d) and Caspeta-Mandujano (2005).

Hosts: "Cichlasoma" sp., TBL 16 ст; Mascaheros argenteus (Allgayer), White cichlid, TBL20 cm; Mayaheros urophthalmus (Gunther), Mexican mojarra, TBL 19-20 cm; Thorichthys sp., TBL 10 em (all Cichlidae); and Rhamdia guatemalensis (Günther), Pale catfish (Heptapteridae), TBL 25 cm.

Sites of infection: Abdominal cavity (surface of stomach and intestine) and mesentery.

Locality: Lacantún River (Usumacinta River basin), Chiapas State, southern Mexico (collected 17-19 November 2007).

Prevalence and intensity: "Cichlasoma" sp.: 1 fish infected/1 fish examined; 1 nematodes. Mas. argenteus: 1/2; 26. May. urophthalmus: 2/6; 3 and 68. Thorichthys sp.: 1/5; 3. R. guatemalensis: 1/2; 6.

Comments: These larvae belong to one or more species of Spiroxys Schneider, 1866, parasites mainly of freshwater turtles. In Mexico, Moravec and Vargas-Vázquez (1998) recorded adults of Spiroxys contortus (Rudolphi, 1819) infecting the turtle 7rachemys scripta from a small lake in central Yucatan and this widely distributed parasite is also reported from congeneric turtles in Brazil (Mascarenhas and Muller 2015). Therefore, it is probable that most records of Spiroxys larvae from Mexican fishes relate to this species. In Mexico (States of Chiapas, Morelos, Nayarit, Veracruz and Yucatan), larvae of Spiroxys sp. were reported from many fish species belonging to different families (Caspeta-Mandujano 2005). Spiroxys larvae in fishes of the Lacantun River were previously reported by Salgado-Maldonado et al. (2011a,b).

The intermediate hosts of Spiroxys spp. larvae are various copepods (Cyclops, Macrocyclops, Mesocyclops), whereas fishes, amphibians and some invertebrates (freshwater snails, dragonfly nymphs) serve as paratenic hosts (Moravec et al. 19954).

DISCUSSION

The Lacantun River is a river in the State of Chiapas, southern Mexico, the lower portion of which runs through the Lacandon Forest. It is an important tributary of the Usumacinta River (river in southeastern Mexico and northwestern Guatemala), which flows northwestward, receiving the Lacantun River, forming the border between Mexico and Guatemala at the Chiapas State border, and emptying into the Gulf of Mexico in Tabasco. The large and isolated Usumacinta River basin is well known to house a vast biodiversity (Britannica 2024).

As far as freshwater fish nematodes in the Lacantun River are concerned, to date 12 nominal species of adult nematodes, viz. Cucullanus angeli, Cucullanus caballeroi, Mexiconema cichlasomae, Neocucullanus neocucullanus, Orientatractis chiapasensis, Procamallanus rebecae, Raillietnema kritscheri, Rhabdochona acuminata (Molin, 1860), Rhabdochona juliacarabiasae, Rhabdochona kidderi, Spinitectus osorioi and Spinitectus tabascoensis were reported from this stream (Cabañas-Carranza and Caspeta-Mandujano 2007, Moravec et al. 2009, 2012, Salgado-Maldonado et al. 2011a,b, Caspeta-Mandujano et al. 2021), ten of which as a part of the material collected by us in November 2007. Three more nominal species of adult fish nematodes, Atractis vidali, Cucullanus mexicanus and Cucullanus potamarii sp. n., are reported from the Lacantún River in this paper for the first time. Most of the 15 above-mentioned species of adult fish nematodes recorded in the Lacantún River are probably widely distributed within the Usumacinta River basin in Chiapas and Tabasco (see Caspeta-Mandujano 2005, Caspeta-Mandujano et al. 2005, Salgado-Maldonado 2006, Salgado-Maldonado et al. 2011a,b), C. potamarii sp. n. and R. juliacarabiasae being probably endemic to this drainage system.

The great majority of nematode larvae (Contracaecum spp. types 1 and 2, Falcaustra sp., Gnathostoma binucleatum, Spiroxys sp.) reported now from fishes of the Lacantún River were previously recorded from other localities in the Usumacinta River basin (Salgado-Maldonado 2006, Salgado-Maldonado et al. 2011a,b).

The present study confirms a diverse, rich-in-species fauna of nematodes parasitising fishes of the Usumacinta River basin, which is evidently of a Neotropical character. Nevertheless, further studies are needed to recognise this fauna in detail, including studies on the life cycles and biology of individual species of these interesting parasites.

Acknowledgements. We thank Guillermina Cabafias-Carranza, Jessica Pérez-Sánchez, Jesús Montoya-Mendoza and Aliberth Mora-Bonilla, then students of the Institute of Biology, Universidad Autónoma de México (UNAM), Mexico City, and Ana Santana-Piñeros from Universidad de Mar (UMAR), for their help with collecting and examining fishes in Chiapas State. Thanks are also due to the staff of the Laboratory of Electron Microscopy, Institute of Parasitology, Biology Centre CAS, an institution supported by the MEYS CR (LM2015062 Czech-Biolmaging) and ERDF (No. С7..02.1.01/0.0/0.0/16_013/0001775), for support with obtaining the scientific data presented in this paper, and to Blanka Skorikova of the same Institute for help with the illustrations. This study was supported by the Mexican research projects PAPIIT IN217723 of the Universidad Nacional Autónoma de México (UNAM), Dirección General del Asuntos del Personal Académico, DGAPA, and CONAHCyT A-1S-19959; partly also by the grant 524/06/0170 from the Grant Agency of the Czech Republic and the Institute of Parasitology, BC CAS (RVO: 60077344).

Author contribution. Guillermo Salgado-Maldonado initiated the study, organised the research expedition to Chiapas, carried out the identification of fishes and actively participated in fish examinations. Frantisek Moravec participated in examining fishes, carried out the identification and morphological analysis of nematodes and drafted the manuscript. David Gonzalez-Solis participated in fish examinations and fixed and sorted the helminth material. All authors contributed substantially to the manuscript text, participated in its revision and appoved its final draft.

Sidebar

Received 31 January 2025

Accepted 10 April 2024

Published online 4 July 2025

References

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