Variations of hydra reproductive strategies arising from its modular structure. Two aspects of the modular reproductive effect

Clonal populations in which, after switching to sexual reproduction, simultaneous hermaphrodites may coexist with variable proportions of hermaphrodites functioning only as male or female provide an opportunity to test sex ratio bias and sex investment within a simultaneous hermaphrodite, which are collectively referred to as the modular reproductive effect in this study. An individual-based model (IBM) was used to determine how sex investment and sex ratio bias affect the cnidarian Hydra circumcincta’s fitness. Its clone-mates represent a genet, and after switching to sexual reproduction, they can be simultaneous hermaphrodites, hermaphrodites functioning as females, or hermaphrodites functioning as males, depending on their body size. The results of the model indicated that under low competition among polyps to fertilize eggs, hermaphrodites functioning as females and those functioning as males coexist, with the numerical superiority of the latter being more successful than simultaneous hermaphroditism independently of male-biased allocation of sex functions. When mate competition increases, the coexistence of hermaphrodites functioning as females and those functioning as males with the fundamental sex ratio 1:1 and females able to change their investment in eggs is the most successful. When there is strong mate competition, simultaneous hermaphroditism with strongly female-biased allocation of sex functions is the optimal strategy, except in scenarios with constrained investment in eggs. In such scenarios, various strategies, observed in natural populations of hydras, become successful.