Osmotic and Ionic Regulation in the Isopod Crustacean <i>Ligia Oceanica</i>

Abstract

ABSTRACT Osmotic pressure of the blood of Ligia oceanica, measured by the freezingpoint depression, has a mean value of Δ 2·15 ±0·04° C. (≡ 3·58 % NaCl on weight/ volume basis). Osmotic pressure of Ligia blood is much higher than that of other terrestrial isopods: Oniscussp. Δ1·04° C.; Armadillidiumsp. Δ1·18° C. -, Porcellio sp. Δ1·30° C. or of the fresh-water Asellus sp. Δ0·500 C. The osmotic pressure of the blood increases during the process of moulting, but no subsequent decrease is observed in the 4 days following. Animals kept at low humidities lose water. They may be desiccated without permanent adverse effects until Δblood is 3·48° C. (≡ 5·8% NaCl). Recovery to a normal level takes about 24 hr. in moist conditions. In well-aerated sea water between 50 and 100% concentration, animals survive without much alteration in Δblood. Above and below this range Δblood rises and falls. In animals kept on filter-paper moistened with distilled water Δblood may fall to 1·44° C. (=2’4% NaCl) without permanent adverse effects. Analyses of inorganic ions in the blood show that sodium, potassium and chloride are all higher in concentration than in sea water; calcium is much more concentrated ; and magnesium and sulphate much reduced.