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The branched colonies of several hydroids are a suitable substrate for sessile (Hughes, 1975, 1978a,b) and vagile epibiontic organisms (Lagardère & Tardy, 1980; Bavestrello et al., 1996). Generally the hydroid does not represent for these organisms a mere low-competition substratum but several examples of relationships occurring between the host and its epibionts are here described. Lagardère & Tardy (1980) recorded a complex behaviour of a syllid polychaete that fixes its tubes immediately under the polyps of Tubularia larynx. In this situation, the polychaete eats part of the food captured by the polyps, and is able to defend the host from nudibranch predation. Some species of caprellid amphipods living on Eudendrium are able to use the polyps as traps to collect planktonic prey. When a polyp has caught and engulfed prey, the caprellid opens the polyp mouth by its gnatopods and extracts and eats the prey (Bavestrello et al., 1996). It was observed that the polyps of Eudendrium never eat the harpacticoids living on their colonies (Bavestrello et al., 1996), while it was demonstrated that polyps of this genus are generalistic predators engulfing all the prey with a diameter compatible with their hypostome (Barangè, 1988).
Recently it was demonstrated that several microorganisms, such as protists and bacteria, are able to live on hydroid colonies. Stabili et al. (2006) have shown that chitinolitic, luminous bacteria of the genus Vibrio live on the perisarc of several species and Romagnoli et al. (2007) have recorded bacterial and cyanobacterial mats on the branches of Eudendrium.
Benthic diatoms are very common on the hydroid branches where they produce dense assemblages or exploit more specific microenvironments such as the inner side of hydrothecae or the narrow grooves between the annuli of branches (Di Camillo et al., 2005; Romagnoli et al., 2007). Sessile ciliates such as Ephelota have commonly been recorded on several hydroid species (Di Camillo et al., 2005), while Vorticella is a typical epibiont of some species of Aglaophenia where it settles on the edge of the theca (unpublished).
The aim of this paper is to summarize recorded data about the protists associated with marine hydroids, pointing out some ecological aspects of these epibiontic communities.
Diversity and abundance