Sex allocation and maintenance of androdioecy in the pedunculated barnacle Scalpellum scalpellum (Crustacea: Cirripedia: Thoracica).

Recent studies have shown that the number of individuals that can mate with each other (the mating group size, MGS) is a major driving force in the evolution of sexual systems in animals. Androdioecy is an important yet little-studied sexual system linking dioecy and hermaphroditism. Here, the individual sex allocation of hermaphrodites carrying dwarf males in the androdioecious barnacle Scalpellum scalpellum was studied. In accord with theoretical models, it was predicted that solitarily settled hermaphrodites have (1) a lower allocation of own male functions (estimated from testis size and penis length) but (2) instead a higher allocation of own female functions (estimated from embryo numbers) with (3) more dwarf males attached as a result of their lower MGS. As predicted, solitary hermaphrodites had relatively smaller testes and shorter penises than gregarious hermaphrodites of a similar size. This indicates that the solitary hermaphrodites can adaptively adjust sex allocation. Contrary to our prediction, these did not have larger brood sizes or carried significantly more males than gregarious hermaphrodites, although the probability of having any dwarf males present was significantly higher. We relate our results to sex allocation theory and the evolutionary maintenance of androdioecy in the species. [ABSTRACT FROM AUTHOR]