ABSTRACT: Employing a highly sensitive infra-red light actography system, circatidal rhythms of vertical migration were recorded in first stage zoea larvae of the common shore crab Carcinus maenas (L.), freshly collected from coastal waters. The rhythms persisted in constant conditions in the laboratory for several days, ascent occurring at the times of expected ebb tides at collection sites, and descent during expected flood tides. Timing was identical in larvae sampled at different stage of spring/neap cycle and from sites where different hydrodynamic conditions prevail. These experiments, supported by repeated surface plankton samples which confirmed that zoea-1 larvae of C. maenas were most abundant at the surface during ebb tides, suggest that the vertical migration behaviour might be linked with avoidance of stranding and offshore dispersal of newly released larvae. The fact that tidal migration rhythm was exhibited by larvae collected from different sites implies that the behaviour is probably widespread for the species. So far, endogenous tidal rhythms of vertical migration in plankton have been demonstrated solely in a few estuarine forms, and have been interpreted only as estuarine retention or reinvasion mechanisms. Present results appear to be the first to demonstrate that endogenous tidal rhythms also exist in the planktonic larvae of a coastal invertebrate and suggest that these larvae may have an adaptive mechanism to avoid stranding and enhance offshore dispersal in the open sea.

KEY WORDS: Carcinus maenas . Zoea-1 . Tidal vertical migration . Endogenous . Timing . Surface abundance . Dispersal . Avoidance of stranding

Published in MEPS Vol. 132 (1996) on February 29
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1996