Revisiting Accumulated "Discoveries": Museum Deep-sea Fish Specimens Can Provide New Insights into Their Parasites

A growing body of knowledge on biodiversity suggests that more than a half of all extant species on the earth may be parasites; this is also easily supposed from the fact that there are no free-living organisms without parasites in the wild. Nevertheless, our understanding of biodiversity and ecology on parasites has lagged far behind that of free-living groups, particularly in deep-sea ecosystems.

Natural-history specimens in museums are recognized as important resources for taxonomy and natural-history study of the organisms, providing fundamental knowledge on biodiversity. In this study, I focused on museum specimens of deep sea fishes from the perspective of parasitology. The specific aims were to examine whether 1) fish parasitic crustaceans (Cymothoidae: Isopoda) are found from museum fish specimens collected mainly from the Pacific Ocean and 2) such parasite specimens provide any novel scientific knowledge on the host-parasite relationships.

Through the surveys at Japanese museums across various taxa of deep sea fish (12 orders, 18 families, 29 genera), I found more than 5 species of a deep-sea cymothoid genus, which includes poorly-known species and recently described species. These parasite specimens provided various new insights into the parasite taxonomy and ecology. For example, the prevalence of Elthusa moritakii Saito & Yamauchi, 2016, a recently described species from Japanese waters, of the museum fish specimens I examined was over 50% (out of 22 fish, 12 were infected), suggesting high prevalence even in the wild. These results suggest that comprehensive re-surveys on deep-sea fish specimens may promote an understanding of deep-sea parasite biodiversity and the relationships with their hosts.