A COMPARISON OF OSMOLALITY AND SPECIFIC ION CONCENTRATIONS IN THE FLUID COMPARTMENTS OF THE REGULAR SEA-URCHIN LYTECHINUS-VARIEGATUS LAMARCK (ECHINODERMATA, ECHINOIDEA) IN VARYING SALINITIES

Lytechinus variegatus were exposed to four salinity treatments (20, 25 , 30 and 35 ppt S) for 32 days. All compartmental fluids showed osmoco nformity at all salinities. Sodium levels were slightly, but significa ntly, higher in the lumenal fluids of the stomach and intestine as com pared to the external medium at all salinities. Chloride levels were s ignificantly lower in the coelomic fluid in comparison to the seawater and the stomach and intestinal lumenal fluids. We hypothesize that ch loride moves from the lumen of the stomach and intestine into the coel om, then into the external medium, generating an ion flux aiding in th e co-transport of nutrients or other molecules. Potassium levels in th e lumen of the stomach and intestine remained significantly higher and similar in all salinity treatments, while potassium levels in other c ompartment fluids varied directly with salinity. At low salinity, rela tively high stomach and intestine lumenal fluid levels of potassium ma y represent a stress response and ultimately reflect physiological mec hanisms leading to the death of the individual.