Paracellular Solute Uptake in the Freshwater Bivalves Corbicula fluminea and Toxolasma texasensis.

Two species of freshwater bivalve were exposed to hyperosmotic solutions of various nonelectrolytes to compare the paracellular permeability of their gill epithelia. In Corbicula fluminea, exposure resulted in an elevation of blood solutes that was primarily due to dehydration. After 36 h of exposure, the concentration of Na in the blood decreased precipitously, and the nonelectrolyte accumulated. When lanthanum was added to the solution as a diffusion tracer, its electron-dense precipitate was rarely observed to penetrate the paracellular spaces of the gill epithelial cells in the absence of hyperosmotic stress. In contrast, precipitated lanthanum was commonly observed in the paracellular junctional complexes of the gill in animals that were subjected to hyperosmotic conditions. When the second species, Toxolasma texasensis, was exposed to hyperosmotic solutions of nonelectrolyte, dehydration appeared to be minimal and a seemingly normal concentration of ions was maintained in the blood. This, however, was because of the simultaneous loss of ions and water and a small gain in nonelectrolytes. Longer exposure (12 h or more) produced a precipitous decrease in most blood solutes and an extensive accumulation of nonelectrolyte. More lanthanum precipitate was seen in the paracellular spaces of both control and hyperosmotically stressed T. texasensis than in identically treated C. fluminea. We conclude that the epithelial junctions found in C. fluminea are relatively tight, which probably contributes to the ability of this species to maintain the solute in its body fluid at concentrations higher than are possible in T. texasensis.