Meeting Abstract

S11-1.4  Monday, Jan. 7  Diverse and plastic sexual systems in barnacles YUSA, Yoichi*; SAWADA, Kota; YAMAGUCHI, Sachi; Nara Women’s University; The Graduate University for Advanced Studies; Kyushu University yusa@cc.nara-wu.ac.jp

Barnacles (Crustacea: Thoracica) show diverse sexual systems, including simultaneous hermaphroditism, androdioecy (hermaphrodites + males), and dioecy (females + males). When males occur, they are always smaller than conspecific hermaphrodites or females (called “dwarf males”). Since Darwin found this, many scientists have been fascinated by the diversity. While most barnacles are hermaphroditic, females and dwarf males tend to occur in symbiotic or deep-sea species. We hypothesized that dwarf males had evolved in response to low sperm competition among hermaphrodites in small mating groups. Females might have evolved in very small groups, where large individuals have little chance to fertilize conspecifics. Using a phylogenetic comparative method, the data from 48 species of barnacles supported the hypothesis that dwarf males and females evolved when group size was small. In some hermaphroditic species, we observed that small individuals were attached to a specific site of large conspecifics. To test if the small individuals act as dwarf males, we investigated their reproductive state in Octolasmis warwickii. The small individuals on large conspecifics had a well-developed testis and a longer penis as compared with others of the same body size. Thus, these conspecific-attached individuals act as “dwarf males”. A transplanting experiment using small individuals of O. lowei suggested that those transplanted on conspecifics emphasized male function than those on plastic plates. Overall, our study shows that the distinction between hermaphrodites and dwarf males is sometimes obscure. We suggest that sexual expression of barnacles is more continuous and plastic than previously considered.