Seawater salinity and blood acid-base balance in the lugworm, Arenicola marina (L.).

The kinetics of variations in the blood acid base balance (ABB) were investigated in a moderately euryhaline osmoconformer, the lugworm Arenicola marina (L.), exposed to natural and experimental hypo- or hyperosmotic shocks. In natural as well as in experimental conditions, a hyposmotic shock induced a transient and essentially metabolic acidosis, probably linked to the ionic readjustments following the shock, which was rapidly overridden by a metabolic alkalosis. In field conditions, a new ABB equilibrium was then attained, the metabolic alkalosis being neutralized by the respiratory and metabolic acidosis occurring normally in the lugworm during low tide. Conversely, in the normoxic conditions of our laboratory experiments, a new ABB equilibrium was never reached. Under experimental conditions, a hyperosmotic shock always induced a respiratory and metabolic acidosis. In the field, this phenomenon must occur at the beginning of high tide and must help to restore normal blood ABB in lugworms submitted to a moderate hyposmotic shock during low tide. All the observed blood ABB variations reveal the complex intracellular processes through which the lugworm submitted to moderate osmotic shocks tentatively regulates, sometimes without any real success, its osmoticity and volume. Obviously, complementary physical, physiological and behavioral mechanisms allow the lugworm to live in sediments washed by almost fresh water during a 7-8 h 'low tide'.