an aquatic animal that is incapable of maintaining more or less constant osmotic pressure of body fluids when there is a change in water salinity. The bodily osmotic pressure in poikilosmotic animals is equal to or slightly greater than that of the surrounding medium.
Poikilosmotic animals include lower invertebrates, bivalve mollusks, many annelids, and echinoderms. In contrast to homoiosmotic animals, they are incapable of maintaining an osmotic pressure lower than that of the surrounding medium. There are stenohaline or euryhaline poikilosmotic animals. In the latter, the internal osmotic pressure changes within broad limits corresponding to changes in the salinity of the surrounding medium. Some animals, such as beach fleas (Amphipoda), are homoiosmotic until the salinity of their environment reaches a level that is too high, at which time the organisms become poikilosmotic.