osmoconformers

Osmoconformers are organisms whose body fluids have an osmolarity that changes in concert with the osmolarity of their surrounding environment, rather than actively regulating to maintain a constant internal level. In contrast to osmoregulators, osmoconformers let their internal osmolality track external seawater or freshwater conditions.

Most osmoconformers are marine invertebrates, including many mollusks, echinoderms, and tunicates. Seawater has an osmolarity of about 1000 mOsm/L (roughly 1 osmol/L). In stable saline environments, these animals' body fluids approximate that value; when environmental salinity changes, their internal osmolarity shifts accordingly, with some adjustments in cellular osmolytes to maintain volume and function.

Physiologically, osmoconformers rely on passive diffusion and regulated ion exchange across epithelia rather than active, energy-intensive regulation of solute concentrations. They typically maintain an ionic composition compatible with their environment, and any osmoregulatory work is limited to maintaining cell volume and acid-base balance within a narrow range.

Ecologically, osmoconformers are well suited to stable marine habitats but are often less tolerant of rapid or large salinity fluctuations. This strategy minimizes energy expenditure but can constrain geographic and tidal range compared with osmoregulators.