The Water Balance of a Serpulid Polychaete, Mercierella Enigmatica (Fauvel)

The serpulid polychaete Mercierella enigmática is found naturally in a wide range of salinities – from fresh water to 150% sea water (< 1–55‰, < 25·8–1421 mOsm). Changes in body weight, blood volume and blood osmolality have been measured both during and after equilibration of animals with media of altered salinity. The blood remains similar in osmolality to the external medium over a very wide range of salinity (43–1620 mOsm); osmoregulation occurs only at the lowest limit of the natural range. Mercierella enigmática shows volume regulation; after 4 days of equilibration with a medium of altered salinity the blood volume shows much less change than the blood concentration. During equilibration there appear to be passive movements of both water and salts between the animals and their environment.