Fragmentation is an important characteristic of the life history, population dynamics, and genetic structure of at least five metazoan phyla. This study represents a first report of modes of fragmentation and their relative importance to population growth in Palythoa caribaeorum
(Cnidaria, Zoanthidea). Several populations at two coastal sites in Brazil (Praia Portinho, on São Sebastião island, and Ponta Recife, on the São Paulo coast) were followed for 1 yr. Two depths in the sub-tidal were examined at each site: 0.5–1.5 m and 2.5–4.0
m. These sites represented different habitats characterized by different levels of environmental stress. Fragments were produced by two methods: (1) biotic fragmentation (tissue isolated due to biologically caused lesions); in this case, primarily due to disease causing partial colony mortality
in the colony; and (2) physical fragmentation (colonies broken into several pieces by storms, currents, or tides). Both modes involved incidental fragmentation, not actively directed by the parent colony. Of the 579 colonies monitored, 6.9% exhibited at least one mode of fragmentation over
the period of 1 yr, producing 64 new ramets. The frequency of fragmentation, or total number of fragments produced from both modes of fragmentation, was independent of site and season, but not of depth. Significantly higher fragmentation was observed in shallower waters. This pattern could
be explained by higher biotic fragmentation due to disease in these colonies. The frequency of colonies fragmenting by physical disturbance (storms) was found to be independent of site, depth, and season. At least 57% of the population exhibited signs of partial colony mortality (lesions),
which on the average affected <5% of the total colony area. Partial colony mortality rarely resulted in ramet formation (9.7%). Of this 9.7%, 5.5% developed fragments as a result of partial mortality due to disease and 4.1% due to storm damage. A significant positive linear relationship
was found between colony area and degree of partial mortality. When compared to other species of zoanthids, fragmentation plays an important role in asexual reproduction and potential contribution to population growth. It is not, however, as important as some other reproductive modes also
utilized by this group.
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