Behavioural significance of pressure responses in megalopa larvae ofCallinectes sapidusandMacropipussp.

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Abstract
The primarily benthic megalopa larvae of Callinectes sapidus and Macropipus holsatus swim upwards for a short distance from the bottom when subjected to pressure increased above ambient values, particularly when illuminated from above. Larvae exposed to stepwise increments of pressure exhibit a threshold in the barokinetic response which is lower in Callinectes (0.4 atm) than in Macropipus (0.8–1.0 atm). Such behaviour would tend to inhibit swimming in surface waters of stratified estuaries where net flow is seawards, but would promote slight upward swimming when on the bottom in deeper water which has a net flow inwards, thus substantiating other evidence that the megalopa is a stage at which recruitment to estuaries takes place. The lower threshold of the barokinetic response in Callinectes may be related to its occurrence in more uniformly shallow estuaries on the south and east coasts of N. America. In both species swimming is markedly inhibited after a slight reduction in pressure. Such responses would be of value to larvae running the risk of being washed ashore in surf Sudden reductions in pressure by surf action would induce sinking into the undertow which would carry larvae offshore into deeper water.