Swimming behaviour of larvae of the ocean quahog Arctica islandica in response to pressure and temperature

Abstract

Trochophore larvae of the ocean quahog Arctica islandica swim continuously. At a constant temperature of 12 "C they are negatively geotactic, have no phototactic response and exhibit no change in swimming behaviour in the pressure range 1 to 3 bar. In a vertical thermal gradient from 9 to 25 "C the trochophores swim throughout the experimental chamber and show no temperature preference. Veliger larvae of A. islandica alternate between periods of active upward swimming in vertically oriented helices, that is a negative geotaxis, and periods of passive sinking with the velum either trailing or retracted between closed valves. They do not swim in the horizontal plane and, in the length range 160 to 202 pm, show no phototactic response. When exposed to sequential increases and decreases in hydrostatic pressure at 12 "C, larvae in the length range 160 to 196 pm consistently exhibited a net upward movement following an increase in pressure and a net downward movement following a decrease in pressure. The threshold pressure change to elicit response is < 0.5 bar. Larvae of 170 p n l length respond to increased pressure by decreasing the diameter of the helix and increasing the height gain per rotation. Larvae of 196 p m length respond to increased pressure by increasing height per rotation and vertical velocity. Larvae of 202 pm length exhibit no significant change in swimming behaviour with increased pressure. In a vertical thermal gradient early veligers swim in the range 7 to 23 "C with preferential aggregation, depending upon size, in the range 12 to 18 "C. Larvae of 204 p m length show no temperature preference in the range 6 to 20 'C. The implication of the observed behaviour on seasonal depth distribution of A. islandica larvae in the Middle Atlantic Bight is discussed.