OXYGEN-SUPPLY AND LIMITS ON AGGREGATION OF EMBRYOS

Effects of oxygen supply on development rates of embryos in gelatinous masses were tested with natural masses of three species of opisthobra nch gastropods and artificial masses made with embryos of a sea urchin and agarose gel. When diffusive exchange was restricted for embryos i n such masses, increased oxygen alone was sufficient to maintain devel opment rates of embryos. Diffusive supply of oxygen is therefore a lim iting factor for embryos clustered in gelatinous masses. Development r ates were constant over a wide range of experimentally altered oxygen concentrations but retarded with oxygen below about 10% of air saturat ion. Some opisthobranch embryos exposed to low oxygen concentrations h atched with shorter shells. Delayed hatching and shorter shells at hat ching were both associated with a central position in the globose gela tinous mass of one of the opisthobranch species, even in air-saturated and vigorously stirred water. The pH near the centres of the masses w as lower when embryos were at more advanced stages; thus the intracaps ular stirring caused by cilia of embryos at later stages does not comp ensate for their greater metabolic rates. The pH in some egg masses wa s about seven or lower, and this pH affected development in separate e xperiments. The masses approach limits for removal of wastes, although oxygen becomes limiting for rates of development before accumulation of wastes becomes Limiting. Embryos of opisthobranchs that are normall y aggregated and embryos of sea urchins that are normally dispersed we re similarly tolerant of low oxygen concentrations and low pH. In resp ect to tolerance of hypoxia and accumulating wastes, embryos in many c lades are preadapted for aggregation into masses, but there are Limits on the size of the aggregation. Egg masses are commonly at those limi ts.