Heritability of circatidal vertical migration rhythms in zoea larvae of the crab Carcinus maenas (L.)

Abstract

Previous studies showed that in the crab Carcinus maenas, endogenous circatidal vertical migration rhythms are exhibited by zoea-1 collected from the field and by larvae newly hatched in the laboratory from field collected ovigerous females. In the present studies, non-ovigerous females were brought into laboratory and kept in non-tidal conditions until eggs were produced. About 100 such females spawned in the laboratory during 1992–1994. Observations were then made on swimming behaviour patterns of groups of larvae hatched, either naturally or from detached eggs, from more than 20 of these laboratory-spawned females, some of which had been kept in the laboratory for nearly a year and one of which spawned twice with an interval of 8 months. Most groups of larvae hatched under such circumstances exhibited vertical migration rhythms of circatidal periodicity despite never having experienced tidal conditions in their life time, even as embryos. Such behaviour is particularly surprising in larvae hatched from detached egg masses. The results indicate that the periodicity of larval circatidal rhythms is inherited and raise the question as to how such rhythms are synchronised. In exceptional cases larvae hatched from detached eggs of two laboratory-spawned females, and which had been incubated under light-dark cycles, showed weak circadian swimming rhythms. In contrast, exposure of detached eggs from one of those females to successive 2 h episodes of water agitation every 12 h, resulted in expression of circatidal rhythmicity in the newly hatched larvae. This simulated tidal agitation appears to mimic the mechanical stimulation produced by females during the hatching process which has been shown previously to set phase of the larval circatidal swimming rhythms. Synchronisation of circatidal migration rhythms in larvae hatched from detached eggs remains speculative but could occur by mutual entrainment. The study appears to be the first to demonstrate artificial entrainment of a circatidal rhythm in a planktonic organism.