CLONAL PROPAGATION AND POPULATION-DYNAMICS OF A GORGONIAN CORAL

In the San Blas Islands, Panama, populations of Plexaura A are found in dense aggregations that develop clonally by fragmentation. In a 5 m2 transect containing 284 colonies, all successful recruits developed from branches that had broken off large colonies and had reattached. Fragment production was generally restricted to colonies >20 cm in height. Fragment production varied by close to an order of magnitude between months. Fragment survival and reattachment was temporally variable in survivorship experiments; 1-yr survivorship of 6-branch fragments ranged from 0.0-0.23. Fragments with 6 branches tended to have higher 1-yr survival rates than either 1- or 18-branch fragments. Although mature colonies produce c104 larvae each, in 6 yr there were at most 7 observations of larval recruits. Survival of attached colonies on the transect was positively related to colony size. Large (>20 cm tall) colonies had yearly survivorship of 0.95 over a 4-yr period. Growth rates were extremely variable among colonies, and many colonies suffered partial mortality and decreased in size between years. Partial mortality was most likely caused by grazing. A simple model of colony growth indicates that, due to their initial size, fragment-formed colonies have up to a 25-fold greater chance of reaching reproductive size than do larval recruits. The advantage of large fragment-formed colonies can be attributed to the higher mortality experienced by small colonies and the high probability of negative growth, which keeps small colonies in the high-mortality region of the life table. It may be more accurate to characterize vegetative propagation as a strategy of clonal growth than as a reproductive strategy. -from Author