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Introduction
Tapeworms (Cestoda) are parasitic flatworms (Platyhelminthes: Neodermata) that occur as adults almost exclusively in the intestinal tract of all major vertebrate groups, including elasmobranchs (Chondrichthyes: Holocephali and Elasmobranchii; here called elasmobranchs for simplicity) and ray-finned fishes (Actinopterygii; here called teleosts for simplicity). They are characterized by (1) the absence of a digestive tract (intestine), (2) the presence of hair-like structures called microtriches on their surface, (3) the anterior end of the body called the scolex and (4) a usually long, dorsoventrally flattened body (strobila) that contains multiple sets of genital organs (proglottids) in most groups (except for the earliest diverged orders called monozoic, i.e. with a single set of genital organs) (Khalil et al., 1994; Caira and Jensen, 2017). Adult fish tapeworms vary in overall length from less than a millimetre up to 2 m (Eubothrium crassum), but their larvae (metacestodes) are usually much smaller (Williams and Jones, 1994; Chervy, 2002). ‘Fish tapeworms’ here refer to all tapeworms that mature in elasmobranchs and teleosts.
Caira and Jensen (2014) and Scholz and Kuchta (2017a) provided an overview of the current knowledge on elasmobranch and teleost tapeworms, respectively. Caira et al. (2014) provided evidence for the unexpected radiation of elasmobranch tapeworms and its importance in deciphering the evolutionary history of all tapeworms, and finally Caira and Jensen (2017) summarized updated information on all tapeworms in a comprehensive monograph. Data on nominal species of all tapeworms, including hosts, localities and authorities not listed herein, are available (and continually updated) in the freely available on-line Global Cestode Database (Caira et al., 2022).
Here we briefly review the history of fish tapeworm research, focusing on the recent application of molecular tools, diversity of fish tapeworms, their systematics, host–parasite associations and distribution.
Pre-molecular era
Research on tapeworms has a long tradition, mainly because some species infecting humans (such as the pork tapeworm Taenia solium and beef tapeworm Taenia saginata or the broad fish tapeworm Dibothriocephalus latus) were known since the Middle Ages and were already known to Linnaeus (1758). However, intensive research on tapeworms began much later, especially in the 20th century, with relatively few fish tapeworms described in the 18th and 19th centuries (Fig. 1).
Fig. 1.
Species diversity of fish...