Osmoconformer

Abstract: Specimens of Chlamys opercularis, Modiolus modiolus, Mytilus edulis, Crassostrea gigas, Scrobicularia plana and Mya arenaria were exposed to both gradual (sinusoidal) and abrupt (square-wave) salinity fluctuations and measurements made of osmotic, Na+, Mg2+ and Ca2+ concentrations in the hemolymph and where applicable in the mantle fluid. In both sinusoidal and square-wave regimes fluctuating between 100 and 50% seawater (100%=ca. 32‰ S), the hemolymph Na+, Mg2+, Ca2+ and osmotic concentrations followed the concentrations of the external medium in Chlamys opercularis. The hemolymph and mantle fluid osmotic Na+, Mg2+ and Ca2+ concentrations of Modiolus modiolus, Mytilus edulis, Crassostrea gigas and S. plana followed those of the external medium as long as the molluscs' shell valves remained open. There were no changes in the ionic or osmotic concentrations of the hemolymph or mantle fluid of any of these species during periods of shell-valve closure. The hemolymph osmotic, Na+ and Mg2+ concentrations of wedged-open Modiolus modiolus, Mytilus edulis, C. gigas and S. plana followed those of the external medium. Hemolymph Ca2+ concentrations showed a damped response in C. gigas and Mytilus edulis. The hemolymph osmotic, Na+, Ca2+ and Mg2+ concentrations of Mya arenaria fluctuated in a similar manner to the external medium, but were damped. Wedged-open Mytilus edulis exposed to fluctuating salinity and supplied with a constant supply of 10 mM Ca2+ showed greater changes in hemolymph ionic and osmotic concentrations than M. edulis exposed to the same salinity fluctuation without a constant Ca2+ supply. Chlamys opercularis and Modiolus modiolus survived in a 50% seawater minimum sinusoidal salinity fluctuation for 10 days; wedged-open M. modiolus survived only 3 days. Burrowing had no effect on the osmotic, Na+, Mg2+ or Ca2+ concentrations of the hemolymph of Mya arenaria or S. plana exposed to fluctuating salinities. All of the species studied were shown to be osmoconformers.

Abstract: The relationship between the osmotic pressures of the blood and the ambient medium was determined for 4 species of bivalve molluscs whose habitats represent distinct salinity regimes within the range from fresh to full seawater. These organisms included 3 corbiculids: Corbicula manilensis (freshwater); Polymesoda caroliniana (brackish-water); Pseudocyrena floridana (marine) and 1 unionid: Elliptio lanceolata. On the basis of the data and similar measurements from the literature, we have placed the molluscs into 5 categories: marine stenohaline, marine euryhaline, oligohaline, fresh-water euryhaline and freshwater stenohaline. Marine stenohaline and euryhaline species are osmoconformers. They differ only in the size of the free amino acid pool available for intracellular volume regulation, and thus in the range of salinities that they tolerate. Oligohaline species tolerate salinities from seawater down to freshwater; they not only possess a large capacity for volume regulation, but can also osmoregulate below 3‰ S. Freshwater species also osmoregulate below 3‰ S, but they are usually limited to salinities below 2‰. Presumably, in evolving from the marine to the freshwater habit, they have lost the ability to volume-regulate in response to hyperosmotic stress. We propose that the varying physiological characteristics underlie the well-known relationship that species abundance declines from both freshwater and full seawater to a minimum between 3 and 5‰ S. We have related this species minimum to physical-chemical discontinuities in the ionic composition of seawater which are, again, reflected in the physiological mechanisms of the molluscs.

Abstract: 1. A detailed study of intracellular osmoregulation in skeletal muscles has been carried out in two species of toads, Bufo viridis and Bufo boreas, adapted to various external salinities between fresh water and 50% sea water (salinity 16%‰).2. Both species are osmoconformers, changes in plasma osmotic concentrations being due almost entirely to changes in NaCl concentrations. Muscle dry weight, however, is more stable, increasing by less than 35% in both forms in the face of osmotic concentration increases of 80-135% in the plasma. Muscle extracellular volume (inulin space) is constant, independent of changes in plasma concentration.3. Relative stability of muscle hydration is due to accumulation of intracellular solutes. Changes in intracellular osmotic concentration are broadly partitioned: 47% inorganic ions (Cl, Na, K), 33% free amino acids and related compounds, 20% urea in B. viridis; 43% inorganic ions, 40% free amino acids and related compounds, 17% urea in B. boreas. Free carbohydrates appear to ...