Diverse, Continuous, and Plastic Sexual Systems in Barnacles

Abstract

Barnacles (Crustacea: Thoracica) show diverse sexual systems, including simultaneous hermaphroditism, androdioecy (hermaphrodites + males), and dioecy (females + males). When males occur, they are always much smaller (called dwarf males) than conspecific hermaphrodites or females. Ever since Darwin made this discovery, many scientists have been fascinated by such diversity. In this study, we provide an overview of (1) the diversity of sexual systems in barnacles, (2) the continuity between different sexual systems in some genera or species, and (3) the plasticity in sexual expression in several species. First, although most barnacles are hermaphroditic, both theoretical and empirical studies suggest that females and dwarf males tend to occur in species with small mating groups. Low sperm competition among hermaphrodites and little chance to act as a male are both associated with small group sizes and identified as the forces promoting the evolution of dwarf males and pure females, respectively. Second, in some groups of barnacles, the distinction between hermaphrodites and dwarf males is unclear because of the potential of dwarf males to become hermaphrodites. As many barnacle species tend toward protandric simultaneous hermaphroditism (develop male function first and then add female function without discarding male function), the dwarf males in such cases are best described as potential hermaphrodites that arrest growth and emphasize male function much earlier because of attachment to conspecifics. This is presumably advantageous in fertilizing the eggs of the host individuals. The distinction between hermaphrodites and females may also be obscured in some species. Third, sex allocation and penial morphology are plastic in some species. We also report the results of a transplanting experiment on small individuals of the pedunculate barnacle Octolasmis angulata, which suggests that individuals transplanted onto conspecifics developed longer and broader penises than did control individuals. Overall, the diversity, continuity, and plasticity in the sexual systems of barnacles are sources of important insights into the evolution and maintenance of the diversity of sexual systems.

Sex determination in the androdioecious barnacleScalpellum scalpellum(Crustacea: Cirripedia)

“…On the other hand, in several pedunculated and balanomorphan barnacles, the dwarf males are virtually small hermaphrodites that attach to the conspecifics and mature much earlier as males and retard (or in most cases cease) the development of the female sexual organs (Crisp, ; Yusa et al ., , ; Sawada et al ., ). Here, environmental effects appear to operate in sex determination although a GSD component has not been tested (Yusa et al ., ). In between, we suggest that both ESD and GSD are active in S. scalpellum .…”

Different effects of mating group size as male and as female on sex allocation in a simultaneous hermaphrodite

“…The cost of building and keeping the penis can be large for barnacles living in wave‐exposed shores or in dense populations (Hoch, ; López, Catalán, Barriga, & López, ; Neufeld & Palmer, ). Moreover, as some barnacles mature at very small sizes (dwarf males; Yusa, ; Yusa et al, ) and the penis is renewed in each reproductive season (Crisp & Patel, , ; Klepal & Barnes, ), investing in a penis is comparatively larger cost for them (Crisp, ; Dreyer et al, ). Such fixed cost may affect the optimal sex allocation, and ultimately, the evolution of sexual systems (Charnov, ).…”

“…Moreover, barnacles are the original model organisms considered in Charnov (, ) and have been used for testing sex allocation theory. They show various degrees of sex allocation, to the extent that even pure males (i.e., sex allocation = 1) and females (0) are known in several species (Darwin, ; Yusa, ; Yusa, Takemura, Sawada, & Yamaguchi, ). In barnacles, MGS (not distinguished into MGSm and MGSf) has different effects on sex allocation depending on the species.…”

The rediscovery of a rare pedunculate cirripede Paralepas maculata (Cirripedia: Heteralepadidae) on a sea urchin Prionocidaris sp. in Okinawa, southern Japan

“…See Høeg (1995) and Yusa et al (2010)). Dwarf males with diverse morphological characteristics are also found in thoracican barnacles in general (Klepal, 1987;Høeg, 1995;Yusa et al, 2013) and may be important for the adaptive significance and life history strategy of dwarf males (Yamaguchi et al, 2007(Yamaguchi et al, , 2012. Dwarf males on hermaphrodites are also reported in Paralepas klepalae Kolbasov & Zevina, 1999and Heteralepas vetula Pilsbry, 1911(see Newman, 1996 in the Heteralepadidae.…”