Subtropical Sacoglossans in Okinawa—At “Special Risk” or “Predictably Rare”?

On low intertidal and shallow subtidal shores on the west coast of Okinawa, Japan, we investigated the trophic associations of sacoglossan opisthobranchs associated with Bryopsidalean green algae. During 11 short research visits (55 days total) from 2002 to 2008, we recorded almost 500 specimens of 11 species. These sacoglossans include a new record for Japan (Caliphylla A. Costa, 1867), a recent record for Japan (Placida daguilarensis Jensen, 1990), two undescribed species (Placida Trinchese, 1876 and Elysia Risso, 1818), one unnamed (but well-described) species (Placida sp. sensuBaba 1986), and six other Indo-Pacific species. Not only did we record more sacoglossan species but also we found higher slug abundances than other colleagues in Okinawa or the Indo-Pacific region. Quantitative population attributes and feeding preferences are described for these sacoglossans. In contrast to temperate geographic regions, several of these Japanese sacoglossans specialized on a single algal genus rather than two or more genera in different families. This specificity is consistent with narrower host-plant associations in high-diversity communities; yet monophagy has not yet been demonstrated in this guild of Okinawan sacoglossans. Given the broad geographic ranges, restricted host ranges, often predictable populations, and high frequency of life cycles with planktotrophic larvae, western Pacific subtropical sacoglossans should be considered “predictably rare” (sensuRabinowitz 1981) rather than at “special risk” (sensuClark 1994).