Responses of embryonic Cyathura polita (stimpson) (Isopoda: Anthuridea) to varying salinities

The salinity responses of the embryonic stages of the isopodCyathura polita have been studied. The development of cleaving eggs and embryos from the two cell stage through juveniles leaving the marsupium was observed in salinities of 0, 0.5, 10, 20, 30 and 40‰ The role of the embryonic membranes in osmoregulation was examined by various combinations of membrane removals and determinations of freezing point depressions of embryonic fluids in the various salinities. The marsupial stages are the most salinity sensitive stages in the life cycle. Normal development can proceed in water with salinities in the interval between 0 and 30‰ The vitelline and embryonic membranes osmoregulate, maintaining concentration differences in dilute and concentrated media. Membrane shedding is an osmotic process and is the developmental event most vulnerable to osmotic disruption. Fluids within the vitelline and embryonic membranes are favorable to water uptake in 0 to 20‰, but not in 30 and 40‰ After hatching, membrane-less juveniles are limited hyperosmotic regulators. The distribution ofC. polita conforms to the salinity limitations of the marsupial stages.