Comparative biochemical studies of osmo-regulation in Sipunclul--I; Steady-state characteristics of two sipunculids in full-strength sea water.

l. Chemical analyses are reported for centrifuged coelomic fluid, body wall, retractor muscles and coelomocytes of the Sipunculids Themiste dyscritum and Phascolopsis gouldi acclimated to full-strength sea water. 2. Both species are isosmotic to sea water. The major coelomic fluid solutes are sodium and chloride; free amino acids (FAA) and glucose do not significantly contribute to the coelomic fluid osmotic pressure. 3. Relative to ambient conditions, P. gouldi is slightly hyperionic in Na and iso-ionic in Ca; T. dyscritum is iso-ionic in Na and hypo-ionic in Ca. Both species show elevated levels of K and lower levels of Cl, Mg, SO, and orthophosphate. 4. All tissues studied appear closely isosmotic to coelomic fluid (T. dyscritum). Half to three-fourths of the total measured solute concentration of the tissues is contributed by FAA, phosphate and K. 5. Glycine is the major constituent (cu. 80%) of the FAA pool of T. dyscritum retractor muscle. Also present in greater than 10 mM concentrations are glutamate, isoleucine, threonine, serine and aspartate. 6. Orthophosphate appears more concentrated in Sipunculid muscle tissue (body wall, retractor muscle) than in coelomocytes. Coelomocytes are possibly distinct in having a higher intracellular glucose level.