Behavioral responses of the larvae ofcancer magisterDana (1852) to light pressure, and gravity

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Abstract
Under laboratory conditions planktonic larvae of C. magister exhibit both ontogenetic and diurnal changes in behavioral responses to cues which may regulate their vertical position in the water column. During the day C. magister larvae orient consistently with respect to gravity, and show both increased activity and rapid movement toward directional light following increases in pressure up to 2.5 atm. All zoeal instars display a positive phototaxis to light of 0.06 or 0.3 Wm-2. First and 2nd zoeal instars do not move toward light of 72 Wm-2 as do 3rd, 4th and 5th instar zoeae. Megalopae swim toward light of 0.06 Wm-2, or toward light of 1.4 Wm-2 when subjected to increased pressure. At night, larvae orient to gravity and increase their activity following a rise in pressure. Each of the larval instars responds differently to greater pressure. First instar zoeae maintain their vertical position under elevated pressures of up to 2.5 atm. Second instar zoeae move upward when pressure is raised 1.5 or 2.5 atm and hold their position for pressure increases of 0.5 and 1 atm. Fourth instar zoeae do not respond consistently to increased pressure, but tend to drift downward under all but 1.5 atm. Fifth instar zoeae move up when pressure is raised to 1, 1.5 or 2.5 atm. Predictions based on laboratory data suggest that during the day and night early zoeal instars should be at approximately 20 m, later zoeal instars should be in the upper 2 m, and megalopa should occur near 20 m.