Habitat selection in two species of Eulimidae (Mollusca: Gastropoda) parasitic on the black sea cucumber Holothuria atra.

Parasites exhibit high diversity in various ecosystems. Habitat selection has played an important role in the diversification of parasites by facilitating intra-host speciation and repeated shifts to a single host species by multiple parasite taxa. In this study, we investigated habitat selection in two gastropod species, Melanella cf. acicula and Peasistilifer nitidula (Caenogastropoda: Vanikoroidea: Eulimidae), that are parasitic on the black sea cucumber Holothuria atra in southwestern Japan. The number and position of parasites were observed for 335 hosts; in total, 89 individuals of M. cf. acicula and 194 of P. nitidula were found. Melanella cf. acicula exhibited strong habitat selection, with 70 individuals (78.7%) attached around the host's mouth, whereas P. nitidula had relatively low infection site specificity. This suggests that the two parasite species have different feeding habits. Molecular phylogenetic analysis verified their non-monophyletic relationship, indicating that they likely made independent host shifts to H. atra. The observed difference in habitat preference, as well as the abundance of suitable habitats, may have contributed to the repeated host shifts and sympatric distribution of M. cf. acicula and P. nitidula. Our phylogenetic analysis also revealed the non-monophyly of eulimids parasitising holothuroids, as in asteroid and ophiuroid parasitic taxa. [ABSTRACT FROM AUTHOR]