SHAFER, T.H.*; ROER, R.D.; GAY, D.M.; ALEXANDER, M.S.; University of North Carolina at Wilmington: Changes in hyposaline tolerance during the development of blue crab embryos (Callinectes sapidus)

Ovigerous blue crabs (Callinectes sapidus) are found throughout the estuaries of southeastern NC. Therefore, their externally developing embryos are exposed to a range of ambient salinities. By the time of hatching, however, the “sponge”-bearing female crabs migrate to inlets or river mouths. Successful development of the planktonic larvae occurs in full-salinity sea water (> 30 ppt). We show that embryos become less tolerant of hyposaline conditions as they develop. By the “heart beat” stage, they cannot survive longer than 24 hr in sea water diluted to 5 ppt. All embryos increase in volume in hyposaline treatments, but swelling is more rapid later in development. Embryos at the “eye placode” stage increase in volume by more than 100% in less than 24 hr. The developmental difference in blue crab embryo swelling leads to the hypothesis that the loss of hyposaline tolerance is result of some change in the vitelline envelope. Transmission electron micrographs indeed show that some layers of this “embryo capsule” are lost at the heart beat stage and that the entire structure appears more granular, suggesting a possible lessening of resistance to osmotic swelling, as the embryo matures. On the other hand, a more physiological hypothesis cannot be ruled out, namely that embryos become less able to regulate intracellular osmotically-active substances as they develop. Loss of intracellular osmoticum in younger embryos after transfer to low salinity would help avoid detrimental water uptake. We are testing this hypothesis by measuring the osmolality of embryo masses of different ages during experimental salinity changes. Methods have been developed to account for the contribution of interstitial water to these measurements.