Labile sex allocation and sex ratio in the androdioecious barnacle <i>Octolasmis unguisiformis</i>

Abstract Theoretical models predict that the sex allocation of hermaphrodites is affected by the number of individuals who can donate sperm to each other [mating group size (MGS)]. However, theory does not make clear predictions for solitary hermaphrodites (i.e. MGS = 1). Moreover, few empirical studies exist on the sex allocation of hermaphrodites coexisting with males (androdioecy). We studied male function (testis and seminal vesicle volumes), female function (ovary volume), sex allocation [male/(male + female functions)], and the male-to-hermaphrodite ratio in the androdioecious barnacle Octolasmis unguisiformis, with hermaphrodites carrying dwarf males. We analysed the effects of MGS on these traits separately for MGS 1 vs. 2 or larger (i.e., whether hermaphrodites are solitary or gregarious) and for MGS 2–4. The male function of mature hermaphrodites presented a positive relationship with MGS in both MGS 1 vs. 2 or larger and MGS 2–4, and female function presented a negative relationship with MGS when comparing MGS 1 vs. 2 or larger. Moreover, the sex allocation of hermaphrodites was positively related to MGS in the analyses of both MGS 1 vs. 2 or larger and MGS 2–4. Remarkably, we found many ‘hermaphrodites’ lacking sperm in MGS 1, suggesting that they are functional females. In addition, more dwarf males per hermaphrodite were present in small MGS. These results generally agreed with the theoretical predictions and revealed a highly plastic sexual system of this barnacle.