A model for behavioral regulation of depth of negatively buoyant zooplankters is used as the basis for discussion of research on depth regulation by brachyuran larvae. Behavioral responses to conservative environmental stimuli modify the effects of negatlve buoyancy. Basic distribution patterns which result may be further modified by responses to increasingly more variable stimuli. Techniques and experimental results for orientation responses to gravity and light are reviewed, as are kinesis responses to light, temperature, salinity, and hydrostatic pressure. Other modifying regulators such as endogenous rhythms in locomotor activity, starvation, and pollutants are discussed. A summary of results by stimulus is provided. Negative geotaxis and high barokinesis promote upward migration in first stage larvae of most species. Ontogenetic changes in sign of geotaxis and presence of barokinesis result in less precise depth regulation and thus vertical scattering in later zoeal stages. Megalopae regain behavioral traits which yield precision in depth regulation. The tactic contributes to dispersal by early stages and recruitment to favorable adult habitats by later instars. Implementation of the model in experimental systems is discussed and its utility in the study of dispersal and recruitment tactics in brachyurans is described, using examples from the literature.

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Estuarine invertebrates that have planktonic larvae must maintain their populations in the face of net seaward flow of water. One mechanism for this is active retention within the estuary of sufficient off spring to at least replace the population ( Sandif er, 1 975 ; Scheltema, 1 975 ) .Retention mechanisms that combine behavioral adaptations with characteristic estuarine circulation have been described for larvae of several estuarine invertebrates, including barnacles and mud crabs (Bousfield, 1955), oysters (Carriker, 1951Wood and Hargis, 1971) and decapods (Sandifer, 1975). However, it is unlikely that all planktonic larvae will be retained in the face of net downstream flow of water. Indeed, larvae of many estuarine crabs are present in the waters of the continental shelf off the east coast of North America (Nichols and Keney, 1963 ;Sandifer, 1973;Dudley and Judy, 1971). Among the most common are species of the genus Callincctcs, including the blue crab CaUincctes sapidus Rathbun (Sandifer, 1975;Goy, 1976).C. sapidus inhabits entire estuaries as an adult, but spawns only in high salinity regions near estuary mouths (Hopkins, 1943;Van Engel, 1958). These circumstances increase the probability of loss of larvae from the estuary. It is critical to an understanding of the recruitment process, and hence population dynamics, to determine whether larvae exported from estuarine spawning grounds to ocean waters represent a significant loss from the adult habitat and, if so, whether they may be recruited back to it in significant numbers.

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