The water balance of Modiolus (Mollusca: Bivalvia: Mytilidae): Osmotic concentrations in changing salinities

• 1.

  1. Comparative measurements of the osmotic concentrations in three fluid compartments of species of Modiolus from different regions of the marine environment show that the animals are osmotic conformers. However, the body fluids remain hyperosmotic to the environment by a constant amount over the non-lethal salinity range.

• 2.

  2. The hyperosmotic condition appears to be the result of a passive equilibrium rather than active regulation and is probably characteristic of all osmoconformers.

• 3.

  3. The osmotic difference between the blood and the pericardial fluid precludes the possibility of ultrafiltration from the blood into the pericardial cavity using the systolic pressure of the heart as the driving force.