Abstract
Measurements were carried out on the total osmotic concentration, the electrolyte concentration and the refractive index of the coelomic fluid of Arenicola marina acclimated to various salinities, as well as during acclimation to sea water of low and high salinity. In addition changes in body weight in air and underwater were measured. Under steady-state conditions the coelomic fluid is almost isoosmotic to the medium, and (except at low salinities) a Donnan-equilibrium exists between the ionic and non-electrolytic solutes in the coelomic fluid and electrolytes in the medium. The average molecular weight of the organic solutes derived from osmosity and refractive index, was found to be independent of salinity, lacking evidence for involvement of association-disassociation phenomena in the regulation of the concentration of dissolved substances in the coelomic fluid. No evidence for selective permeability for anions or cations across the body wall was found. Arenicola marina displays distinct capacity for volume regulation both after increase and decrease in ambient salinity. The ionic and non-electrolyte solutes in the coelomic fluid show rapid initial responses to salinity changes; then maintain levels consistent with a Donnan equilibrium across the body wall. In the volume regulatory response to decreased salinity, water loss is associated with a decrease in organic solutes in the coelomic fluid; in high salinities, the regulative water gain is associated with an increase in electrolytes. The rapid changes in the internal concentrations following salinity change reflect a high permeability of the body wall. This tends to annihilate the damping effect of the voluminous coelomic fluid on ambient salinity variations.
Published Version
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