A comparative study of mobility in three Hydra species with different reproductive strategies

Sessility or isolated habitat can significantly reduce the number of encounters with potential mating partners and thus favour evolution and maintenance of self-fertilisation. Mobility analysis of phylogenetically related species that differs in reproductive mode and self-fertilisation ability could support the widely accepted but not clearly validated theory of selfing being favoured by sessility. Here, we compare mobility in three species of Hydra (Cnidaria, Hydridae) that differ in reproductive strategy: a simultaneous hermaphrodite able to self-fertilise, a sequential hermaphrodite, and a gonochoric species. The results indicate that mobility expressed as distance traversed by individuals is significantly reduced in Hydra circumcincta, a species that is able to self-fertilise, compared to sequential hermaphroditic Hydra vulgaris and gonochoric Hydra oligactis. Differences in mobility were not correlated to individual body size. It is likely that habitat isolation and limited mobility is associated with the evolution of self-fertilisation in such animals like hydras.