Vertical migration behaviour in the larvae of the shore crab Carcinus maenas from a microtidal system (Gullmarsfjord, Sweden)

Tide-related vertical migration behaviours which assist in horizontal transport of marine larvae have been documented in many species from meso-tidal systems; however, little is known about larval behaviour in areas where the tides do not produce predictable changes in water cur- rents. We assessed the vertical migration behaviour in larvae of the shore crab Carcinus maenas (L.) from the Swedish west coast, to explore possible transport mechanisms in a micro-tidal system. Repli- cated plankton samples were collected at a fixed station from 2 depths during flood and ebb tides, and analysed for effects of tides and light on temporal and vertical patterns in abundance. In a com- plementary experiment, first-stage zoeae were hatched in the laboratory from field-collected females and studied in constant darkness for endogenous circa-tidal or circa-diel rhythms. The abundance of different larval stages decreased from the first to the fourth zoeal stage by a factor 10, whereas mega- lopal densities were similar to those of the first zoea. This pattern in abundance indicates an off- shore/onshore transportation of larvae during ontogeny, suggesting a behavioural component in larval dispersal. However, contrary to what has been demonstrated in larvae from meso-tidal areas, none of the larval stages displayed a vertical or temporal pattern in abundance related to the tide, although a clear semi-diurnal tidal change of approximately 0.3 m was registered throughout the sampling periods. Instead, the highest densities of all larval stages were consistently sampled during low light conditions, indicating a diel pattern in swimming behaviour. This behaviour appears to be controlled by exogenous cues, since the laboratory experiment with first zoeae in constant darkness did not indicate any consistent tidal or diel rhythm in vertical migration. The lack of an endogenous circa-tidal vertical migration rhythm suggests that Swedish shore crabs are reproductively isolated from the British Isles populations, where this behaviour has been demonstrated to be inherited. The possibility that C. maenas larvae in micro-tidal systems may use predictable sea and land breezes as a mechanism for cross-shelf transport is discussed.