NEW FISSION PROCESSES IN THE ZOANTHID PALYTHOA CARIBAEORUM: DESCRIPTION AND QUANTITATIVE ASPECTS

摘要

Populations of the zoanthid Palythoa caribaeorum (Duchassaing and Michelotti, 1861) were monitored in a 2-yr study at several sites on the Sao Paulo coast, Brazil, to describe, quantify, and compare fission processes occurring in moderately vs. highly stressed environments. The effects of depth and season were also considered. Palythoa caribaeorum was found to reproduce by two new general processes of fission. The first process requires: A) the formation of crevices throughout the colony and the maintenance of basal coenenchyme throughout; B) the production of polyp-clusters which are connected to each other by thin basal coenenchyme, not readily visible; and C) the subsequent severance of the basal coenenchyme between polyp-clusters, creating true ramets. The first process included two variants: edge fission and polyp-cluster release. The second fission process was characterized by directional growth of tissue and a subsequent breaking away and dispersing of the ramets. It also included two variants: tissue outgrowth and polyp-ball production. Of the monitored colonies, 55% (n_t = 579) exhibited at least one variant of fission, yielding 1304 ramets over 1 yr. Edge fission was the dominant form of asexual reproduction in this species. We found no evidence to support the hypothesis that higher environmental stress results in increased ramet production among sites, depths, or seasons. Fission occurred year-round and in colonies of all sizes. The minimum size of a colony reproducing by fission was 5 cm~2. A significant positive linear relationship was found between colony area and number of ramets produced per colony. Ramet release also occurred year-round, although the frequency of release was significantly lower during the winter, when environmental conditions were harsher. This is the first report of tissue outgrowth, polyp-ball production, and polyp-cluster release for the genus Palythoa, and the first report of polyp-ball production within the Zoanthidea. There appear to be no analogues of polyp-cluster release within the Cnidaria. Fission in P. caribaeorum appears to be a conservative trait over a wide geographical range. It also seems to be endogenously controlled (genetically programmed) responding to colony growth constraints. Exogenous factors, however, may help to define the variant of fission used and the quantity of ramets produced at a given time. The adaptive value of fission for P. caribaeorum lies in its contribution to the number of clone-mates (increasing population size). It represents a critical and important form of asexual reproduction, helping to explain the ecological and evolutionary success of this species in the western Atlantic.