Co-Occurrence of Gastropods of the Genus Melanella (Mollusca: Eulimidae) Parasitizing the Black Sea Cucumber Holothuria leucospilota in Central Japan: Implications for Their Geographic Distribution and Parasitic Ecology

Snails of the family Eulimidae are parasites of echinoderms in all five extant classes. Despite long years of taxonomic research on Eulimidae in Japan, their local species richness remains to be investigated, and few studies have focused on a eulimid fauna of a certain echinoderm taxon, even if it is a common species. Here, we conducted a comprehensive sampling of <i>Melanella</i> species parasitizing the black sea cucumber <i>Holothuria leucospilota</i> in Shirahama, Wakayama, central Japan. The data used in this study were partly obtained during a summer education program of the Seto Marine Biological Laboratory, Kyoto University. <i>Melanella kuronamako</i> and <i>M. spina</i> parasitized the external surface, and two unidentified species were found inside the host's body cavity. This study represents the first record of <i>M. spina</i> in Japanese waters and eulimids exploiting the internal habitat of <i>H. leucospilota</i>. Morphological and molecular comparisons between the present four species and other Japanese <i>Melanella</i> specimens suggested that they generally exhibit wide geographic distributions and low host specificity. Additionally, the two species from the body cavity were probably seldom observed in central Japan, as inferred from the accumulated results of the dissection of <i>H. leucospilota</i> through a long-running, annual education program at Shirahama.