SOLUTE ADJUSTMENTS IN THE COELOMIC FLUID AND MUSCLE FIBERS OF A EURYHALINE POLYCHAETE,NEANTHES SUCCINEA, ADAPTED TO VARIOUS SALINITIES

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Abstract
1. The solute composition of the coelomic fluid and ventral musculature of a euryhaline polychaete, Neanthes succinea, acclimated to various salinities was examined. 2. The coelomic fluid of these worms conforms osmotically and ionically (with the exception of potassium) with salinities as low as 35% SW. Below this point hyperosmotic regulation of the coelomic fluid occurs as a result of the retention of NaCl. Calcium and magnesium are also slightly concentrated in the regulating range but they do not contribute significantly to the hyperosmotic plateau. The means of solute retention in the coelomic fluid are considered. 3. The extracellular space of the muscle tissue remains unchanged over the salinity range, although the tissue does become more hydrated. Thus, the water gain upon adaptation to dilute conditions must be proportionally distributed between the intra- and extracellular spaces. 4. The sum of the apparent concentrations of the intracellular solutes measured was not a constant proportion of the coelomic fluid osmotic activity, possibly indicating that either the solvent volume or the extent of solute binding varies with the external osmotic activity. 5. Intracellular solute concentrations fall with decreasing salinity. The reduction in concentration of K, PO4 fractions, Ca, and Mg may be explained merely on the basis of increases in cell hydration, while the lower concentrations of Na, Cl, and Nin + N are not wholly accounted for by the influx of water. The removal of a large fraction of the free amino acid pool is the major osmotic adjustment observed in the muscle fibers of this worm.

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