OXYGEN CONSUMPTION OF THE PURPLE SEA URCHIN WITH SPECIAL REFERENCE TO THE REPRODUCTIVE CYCLE

1. The rate of respiration (Qo2) of the sea urchin Strongylocentrotus purpuratus measured with an oxygen electrode parallels but is somewhat lower than that determined manometrically on the same individuals under the same conditions. The higher values of the Qo2 obtained manometrically are attributable to the higher partial pressure of oxygen (ppO2) in the fluid of the manometric vessels continuously equilibrated with air as compared to the closed chamber used with the oxygen electrode, in which the ppO2 is continuously falling.2. The effects of a number of factors on Qo2 were determined: ppO2 (including oxygen enrichment), relation between ambient oxygen partial pressure (AO2), perivisceral oxygen partial pressure (PvfO2) and effect of body size on Qo2, oxygen consumption of isolated and intact body wall; and the main thrust of this investigation—the change in oxygen consumption during the reproductive cycle.3. The sea urchin is an oxygen conformer, its oxygen consumption being dependent upon the oxygen partial pressure from above-air saturated partial pressure following enrichment with oxygen down to 10 mm Hg. The oxygen partial pressure of the perivisceral fluid is also dependent upon the ambient oxygen partial pressure. The strict conformity of body wall tissue is the determining factor in the response of the whole body Qo2 to ambient oxygen partial pressure. Internal oxygen partial pressures are the result of low internal oxygen availability after the body wall oxygen demand has been met, and do not determine the whole body Qo2 in S. purpuratus.4. S. purpuratus exhibits no systematic change in the Qo2 as the reproductive cycle(as measured by the gonad index) reaches its peak, although the data are quite variable. Relatively low perivisceral fluid oxygen partial pressures obtain throughout the year, and the gonadal tissue is under a nearly anaerobic condition at all times. The low oxygen availability to the gonadal tissues results in their having little input into the whole body Qo2 of the sea urchin.5. S. purpuratus cannot withstand ambient oxygen partial pressures below the 80 to 100 mm Hg level for more than one or two hours due, apparently, to the sensitivity of the body wall to attenuated ambient oxygen. This is an important factor in restricting this species to habitats near the air saturation level of ppO2.