Vertical migration and selective tidal stream transport in the megalopa of the crab Carcinus maenas

Abstract

Megalopae of Carcinus maenas (L.) were intensively sampled in the Canal de Mira (Ria de Aveiro, Portugal) during the years of 1990 and 1991, with the use of a pump and nets, respectively. Sampling was carried out along 25 h plankton sampling cycles conducted at fixed stations, at pre-determined depths along the water column. In 1990, the average depth of distribution of megalopae throughout the water column was affected by tidal phase: the larvae were closer to the surface during flood than ebb. Stepwise linear regression showed that a shallower distribution of the larvae was associated with higher salinity. Other hydrological variables did not account well for the vertical distribution of megalopae. Analysis of megalopal instantaneous transport velocity indicated that their horizontal velocity depended on phase-of-tide: during ebb megalopae were transported downstream at a lower velocity than the vertically integrated water column velocity; during flood, the vertically integrated velocity of the larvae and water column were similar. Thus, a shift of vertical position during the tidal cycle controlled the transport velocity of megalopae, which was due to vertical water velocity shear current differences. During the 1991 study, significantly higher densities of megalopae were collected during flood. Main effects of phase-of-day and depth of sampling on density were not significant. However, highest densities during flood were found at mid-water, in both day phases, whereas during ebb megalopae were evenly distributed throughout the water column during the day, with low densities, or showed increasing abundances towards the bottom during the night. These observations are consistent with the hypothesis that megalopae of C. maenas undergo active vertical migration synchronised with the tidal cycle, which can be classified as a selective tidal stream transport mechanism to travel up estuaries. A conceptual model for the reinvasion of estuaries by C. maenas megalopae is proposed. According to the model, vertical movements between the bottom and the water column are controlled by exogenous factors associated with the tidal cycle.