The Ontogeny of Osmoregulation and Its Neurosecretory Control In the Decapod Crustacean, Rhithropanopeus harrisii (Gould)

Laboratory-hatched larvae of this estuarine crab were reared at 25°C in seawater of 25‰ salinity for 18 days covering zoeal Stages I to IV and a megalops. Three-day periods between zoeal stages represent intermolt stages of circadean metecdysis, diecdysis, and proecdysis. Larvae were exposed to either a series of seawater salinities from 5-40‰ in 5‰ increments or of 10-40‰ in 10‰ increments for one hour during each day of their development. The osmoconcentrations of 20-80 nanoliter hemolymph samples from each of four larvae were measured separately by determinations of freezing point depression. Eyestalkless larvae in metecdysis of zoeal Stage II were exposed to the same osmoconcentrations as unoperated controls to test for osmoregulation by eyestalk nerve tissue. Larvae tend to be isosmotic with seawater of 30-40‰ salinity (S) and to hyperregulate in more dilute media except for larvae in their first diecdysis which remain isosmotic. Larvae in the last few hours of proecdysis hyperregulate against 40‰ S as well, presumably to insure inflow of water to establish a greater body volume during hardening of the exoskeleton. They are consequently isosmotic in the very early metecdysis. The presence of eyestalks at the first metecdysis (Stage II) keeps zoeas hyperosmotic to 5-30‰ S, but prevents them from hyperregulation against 40‰ S. Eyestalkless zoeas become isosmotic with 5-30‰ S and hyperregulate against 40‰ S like late proecdysal larvae. Lack of eyestalks makes diecdysal animals hyperregulate against a medium with which normal animals are isosmotic. The eyestalk influence affects second metecdysal (Stage III) larvae in a way similar to those in first metecdysis except that it apparently also prevents a curious tendency to hyporegulate in 5-30‰ S. Similarly, in this stage, the eyestalks prevent hyperosmosity in 40‰ S seawater as they do during the first day of zoeal Stage II. Eyestalk nerve tissue reduces the degree to which diecdysal larvae of this stage remain hyperosmotic to media of 10‰ S and 20‰ S and apparently causes larvae to be hypoosmotic at 40‰ S. Preliminary data indicate that removal of eyestalks has little effect on proecdysal larval osmoregulation or on regulation of Stage IV zoeas. In other experiments ablation of eyestalks caused Stage II larvae to lose the ability to osmoregulate against 10-30‰ S seawater within two hours after the operation. The same zoeas did not hyperregulate against 40‰ seawater until four hours after removal of eyesta