Reversed sex roles and parental energy investment in zygotes of two pipefish (Syngnathidae) species

In pipefishes, males brood their offspring on their body or in a brood pouch. In 2 species of pipeflsh, Siphonostorna typhle and Nerophis ophidion, sexual dimorphism, dichromatism, and sex role reversal differed in degree between the species. N. ophidion females were larger than males, whereas in S. typhle sexes were equally large. Furthermore, N. ophidion females have pronounced sexual colourings and possess dorso-ventral skin folds, in contrast to males, whereas sexes in S. typhle look alike. The hypothesis that the males in the more reversed sex dimorphic species, N. ophidion, should invest relatively more energy in their young than the males of the species with smaller differences in sex roles and sex dimorphism was investigated by measuring energy contents in adults and different zygote stages as well as in vitro respiration of zygotes, also of different stages. Our results showed that in both species males provide nourishment to their broods. Contrary to expectation, however, males of N. ophidion invest less energy per zygote than do the females, while in S. typhle the 2 sexes make about equal investments. The strong dimorphism in N. ophidion therefore is likely to have evolved in response to some constraint other than energy investment imposed by males on the female's reproductive success.