Respiratory Pumping Behavior in the Marine Snail Aplysia californica as a Function of Ambient Hypoxia

Abstract

Respiratory pumping (RP) behavior was monitored in intact, unrestrained Aplysia californica. Respiratory pumping rates in ambient normoxic seawater (4.3-4.8 mL O₂/L) were compared with those during (1) long-term (24 h) exposure to rapidly developed extreme hypoxic seawater (≤ 1.8 mL O₂/L), (2) short-term (3-4 h) exposure to three different rapidly induced hypoxic levels (i.e., "mild" hypoxia of 3.0-3.8 mL O₂/L, "medium" hypoxia of 1. 8-2. 7 mL O₂/L, and "extreme" hypoxia of 0.5-1.0 mL O₂/L), and (3) exposure to slowly developed (approximately 5 h) "extreme" hypoxia. Long-term exposure to hypoxic seawater depressed all forms of RP activity. However, during the time of developing hypoxia, RP activity increased and was inversely related to the level of hypoxia developed; the most effective hypoxic level was ≤ 1.8 mL O₂/L. Attenuation of this developing-hypoxia-induced rise in RP occurred by slowing down the rate of developing hypoxia. Aquarium reoxygenation to normoxia reestablished normoxic RP rates although the occurrence of a patterned series of RP responses, termed an RP seizure, was more frequent during reoxygenation than during exposure to normoxic or hypoxic seawater. These results suggest that A. californica at first attempts to maintain its O₂ uptake through increased RP when ambient dissolved O₂ declines, but when such hypoxia persists, RP and associated aerobic metabolism may be limited by O₂ availability.