Vertical migration cycles of crab larvae and their role in larval dispersal
Abstract:Larvae of the estuarine crab, Rhithropanopeus harrisii, undergo regular vertical migrations both in their natural habitat and in the laboratory. These migrations have amplitudes equivalent to a large fraction of the total height of the water column. When they are observed in the field, they form complex patterns made up of cycles of various periods. This complexity evidently results from the interaction of several exogenous stimuli such as salinity change or submarine irradiance level, which elicit stereotyped larval responses, and possibly also from passive redistribution via turbulence. In the laboratory, larval vertical migrations are much simpler, and under constant conditions they reduce either to aperiodic changes in depth or to repeated cycles of circatidal or circadian rhythmicity. Larvae of R. harrisii seem strongly predisposed to express tidal rhythms, and even newly hatched larvae from crabs of a strongly tidal estuary have circatidal vertical migrations. The adaptational significance of larval vertical migrations is twofold: they keep the larvae in dimly lit regions of the water column where they may feed but where there is little risk of visually directed predation, and they allow the larvae to ride the residual non-tidal currents of the estuary so that they are retained in the upper estuarine reaches. Larvae of other genera of estuarine brachyurans, such as Uca or Callinectes, are apparently not retained within the parent estuary; it will be of considerable interest to learn their vertical migratory behavior.
Document Type: Research Article
Publication date: September 1, 1986
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