RESPIRATION DURING THE REPRODUCTIVE CYCLE IN THE SEA URCHIN, STRONGYLOCENTROTUS PURPURATUS

¹ Hopkins Marine Station and Department of Biological Sciences, Stanford University, Stanford, California

1. The oxygen consumption of small sea urchins differing in gonad index, studied by Warburg-Barcroft manometry, is 26 to 31 µl. O₂ per hour per g. wet weight, or between 3.0 to 5.6 µl. O₂ per hour per mg. nitrogen.

2. The rate of oxygen consumption per unit wet weight is approximately the same regardless of gonad index of the animals.

3. The gonad index measures the size of gonad relative to the entire body weight. In a sea urchin the wet weight does not change appreciably within a reproductive cycle as the gonad index increases because the perivisceral fluid, of much the same specific gravity as the tissues, is displaced by the gonads.

4. With increase in gonad index, however, the bulk of organic material as measured by nitrogen content in a sea urchin increases, ultimately doubling. Consequently, when the rate of respiration is measured per unit nitrogen content, it is found to fall with rise in gonad index.

5. When gonads and other body components are removed from the body, and oxygen consumption measured in sea water in equilibrium with air, their rate of respiration is approximately the same regardless of size or gonad index of the animal from which they came. When the gonad increases in size, the expected total respiration of the sea urchin, as determined by summing the respiration of its body components, is therefore greater than the respiration measured on the intact animal in water in equilibrium with air.

6. The rate of respiration of sea urchins is probably limited by the respiratory surface and convection transport of oxygen to the internal organs. Increase in the bulk of internal tissue due to gonad growth does not result in increased total respiration by the animal. Presumably, instead, coelomic oxygen tension becomes attenuated, resulting in the relative oxygen unsaturation of the internal tissues.