THE REGULATION OF ASEXUAL REPRODUCTION AND INDETERMINATE BODY SIZE IN THE SEA ANEMONEANTHOPLEURA ELEGANTISSIMA(BRANDT)

Abstract

The sea anemone, Anthopleura elegantissima, forms distinct clonal aggregations in the field by longitudinal fission (Hand, 1955, Ford, 1964, Francis, 1973a, b, 1976, Sebens, 1977). Field observations indicate that division takes place most often in the fall and winter when individual anemones are decreasing in size (Sebens, 1977). Mean size of clonal individuals decreases with increasing intertidal height (Sebens, 1977) and with increasing tidepool temperature at a given height. In the laboratory, asexual division is inhibited when A. elegantissima is fed continuously. Such division occurs more rapidly and in a larger fraction of the population as ambient water temperature increases. Starvation, defined as weight loss, is not in itself sufficient stimulus to initiate the division process. Shrinking individuals, given food, do not divide even though that food may not be efficiently assimilated (as at 20° and 25° C). Strong light or darkness has no significant effect on the division process although it has a marked effect on weight change. Finally, it appears that only the larger members of the population undergo fission, as in the field (Sebens, 1977). Weight change in experimental groups is significantly affected by temperature, feeding regime, and illumination. Evidence that zooxanthellae are a significant positive factor in the anemone's energetic regime is supported. All anemones had zooxanthellae at the beginning although many anemones at 25° C egested them during the course of the experiment. Weight gain in fed anemones was greater, and weight loss during starvation less, when given illumination than in darkness. The effect of illumination and feeding regime changed at 20° C and at 25° C, at which point all treatment groups lost weight rapidly. Analysis of variance indicates significant interaction between temperature and feeding regime, and between temperature and light (in the fed group) as factors affecting weight loss or gain. Light and feeding regime do not show significant interaction. The results of laboratory experiments support the hypothesis that seasonal control of longitudinal fission is affected through the energetic regime, commencing during periods of starvation. More rapid weight loss and lesser feeding time in high intertidal pools may result in division at a smaller size and thus clones where mean individual size is smaller than in the low intertidal.