The circatidal rhythm of the estuarine gastropod Hydrobia ulvae (Gastropoda: Hydrobiidae)

摘要

Intertidal animals display a suite of cyclic behaviours that evolved as adaptations to the predictable cycle ofinundation and exposure. In estuarine habitats, mud snails from the genusHydrobiaare among the most abundantgrazers, and have received considerable attention with respect to the behavioural mechanisms mediating locomo-tion, dispersal, and feeding, although the nature of the control of these processes has remained elusive. Inparticular, it is not clear whether endogenous activity patterns are related to periodic changes of microphyto-benthos biomass at the sediment surface, or whether they are timed to the tidal cycle at all. In the present study,we address the crawling activity ofHydrobia ulvaeunder constant conditions, as well as the effects of individualsize and previous short-term exposure to tides of different range, by recording immersed individual snails underconstant dark conditions. We show that the species displays an overt circatidal pattern of crawling, with activitypeaks around high water, and that the start of inundation may act as an entrainment agent of the rhythm.Moreover, the results obtained indicate that smaller snails display higher levels of activity, although neither thesize nor previousin situinfluence of tidal range has an effect on the period and on the amplitude of the rhythm.These findings suggest that fluctuations of microphytobenthos biomass are not a sufficiently strong selectivepressure to have shaped locomotor activity inH. ulvae. Moreover, feeding ofH. ulvaeshould take place mostlyduring high water and be independent of periodic fluctuations of microphytobenthos biomass at the surface of thesediment.