Coelomic Water Volume Control in the Antarctic Sea-star Odontaster validus

Abstract

ASTEROIDS are usually considered to have little or no control over the volume of water in the perivisceral coelom, the body wall being freely permeable to water and most ions1. The Antarctic asteroid Odontaster validus Koehler, however, apparently regulates coelomic water volume to the extent of maintaining seasonal fluctuations2. This species is a “cushion” type asteroid with small, loosely scattered ossicles in a thin body wall permitting considerable flexibility in body shape. Individuals often markedly swollen with coelomic water were collected, while others were “normal” or even shrivelled. Many animals lost weight during successive weighings, and in some cases the decrease was more than 40 per cent of the original wet weight. Other animals maintained their original weight, or gained up to 30 per cent of their original wet weight, during successive weighings. Animals in the field had significantly less coelomic water in March and April, and significantly more in June, than at other times during the year. This seasonal fluctuation did not correlate with fluctuations in gonadal, pyloric caecal or intestinal caecal size. Several experiments, carried out at McMurdo Station, Antarctica, and showing other possible aspects of coelomic water volume regulation in O. validus, are described here.