BIMODAL GAS EXCHANGE AND THE REGULATION OF OXYGEN UPTAKE IN HOLOTHURIANS

¹ Department of Zoology, University of Maine, Orono, Maine 04469

In air-saturated sea water, 60% of the total O₂ uptake by Cucumaria frondosa (epifaunal) and Sclerodactyla briareus (infaunal) occurs through the respiratory trees. When the cloacal opening of C. frondosa is occluded, weight-specific O₂ uptake decreases with weight more than in intact specimens, indicating an increasing dependence of large individuals on their respiratory trees. Likewise, cloaca occlusion results in a proportionately greater loss of oxyregulatory ability in large C. frondosa. Neither of these results occurs in S. briareus, which compensates for loss of the trees by increasing O₂ uptake across the general body surface.

In intact animals, the greater oxyregulatory ability of S. briareus relative to C. frondosa is also due to the former's having less of a decline in frequency combined with a more pronounced increase in volume per pumping cycle of the respiratory tree irrigation rhythm, resulting in an increase in total volume of water pumped, as P_o2 declines; in C. frondosa, this volume declines continuously with P_o2. Also, a greater increase in extraction efficiency occurs in S. briareus as P_o2 declines. Such differences between these species represent adaptation to habitats differing in availability and variability of O₂.