A study on the biodiversity of opisthobranchiate mollusks : diet specialization and speciation

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This research project has made marked contributions to the biodiversity study on the order Sacoglossa, whose members are mostly known as specialist herbivores of macroalgae. Sagami Bay and Okinawajima, both are well-known for a high species diversity of opisthobranchs, are the main study areas of this project. The diversification of feeding habit as well as the species diversity of this opisthobranchiate order was especially high in Okinawajima where the diversity of sacoglossan's primary food, macro-green algae, was also high. Major accomplishments of this project are 1) ca. 20 unrecorded species in Japan were found, many of which are likely to be undescribed ; 2) two cryptic species of a common Japanese sacoglossan, Placida sp. (sensu Baba, 1986), were found ; these two cryptic species were found to have narrower diet width than Placida sp. (sensu Baba, 1986) ; 3) another possible cryptic species complex was found in the genus Ercolania ; 4) some sacoglossans previously recognized as distinct species based on radular morphology were shown to be intraspecific variation; it was shown radular morphology might be highly variable in some sacoglossan species ; 5) food algae for many sacoglossan species were demonstrated both field associations and laboratory experiments; for some sacoglossans food algae were recorded for the first time ; 6) detailed ecological surveys on several common sacoglossan species revealed that some species have much wider diet width than previously thought, while more specialized food preference was suggested in some sacoglossan species ; 7) with a review of past researches on Japanese sacoglossans it was shown that there are a few challenges for the biodiversity study of Sacoglossa. Finally, diverse achievements of this project have shown several important future research directions in the biodiversity study not only on sacoglossans but also on other opisthobranchs.