Behavioral Responses to Changes in Hydrostatic Pressure and Light During Larval Development of the Lobster Homarus gammarus

Abstract

Larvae of the lobster Homarus gammarus hatched and reared under laboratory conditions in shallow water responded to changes in hydrostatic pressure in a depth-regulatory manner. Responsiveness to instantaneous pressure increments ranging from 1 to 20 psi increased linearly and there was no threshold of sensitivity within the range tested. Pressure sensitivity was retained throughout larval life up to and including stage IV (megalopa) but responsiveness weakened with age at least up to stage III. Stage IV larvae were highly responsive to pressure changes but stage V (1st juvenile) lobsters were nonresponsive. Older larvae accommodated to sustained pressure more rapidly than younger larvae and larvae quickly became nonresponsive to repeated pressure changes. After being held at high pressure, larvae responded to slight pressure decreases by downward movement even though still held at pressures to which they would initially respond by vigorous upward movement. Larvae were more responsive to pressure in overhead light than in darkness but light intensity had no apparent effect on the initial response nor on the rate of accommodation to sustained pressure, but in darkness the initial response was slower. Responsiveness to sudden changes in light intensity was much less marked than to pressure changes and it also weakened with age. In the sea older larvae showed a stronger tendency for downward movement when released at the surface and a decreasing tendency for upward movement when released at depths of 25 ft (7.61 m) or more. The average rate of upward movement decreased over a 5-min period and there was no consistent difference in this rate or in the rate at which it decreased when larvae were released at different depths. The average rate of upward movement tended to decrease with age.