Clonal Propagation and Population Dynamics of a Gorgonian Coral

Abstract

Clonal taxa frequently produce vegetative propagules with attributes that are dramatically different from the products of sexual reproduction. Among taxa with size dependent demographics, large vegetative propagules may avoid a section of the life table with slow growth and high mortality. Through this @`escape in size," vegetative propagation can lead to high population growth rates. To better understand this phenomenon I followed the population dynamics of the clonal Caribbean gorgonian Plexaura A and measured the magnitude of the escape in size by comparing the abundance and potential survivorship of vegetative propagules and larvae. In the San Blas Islands, Panama, populations of Plexaura A are found in dense aggregations that develop clonally by fragmentation. Rates of reproduction, survivorship, and growth were determined by monitoring mapped colonies along transects and by following survivorship of artificially generated propagules. In a 5—m 2 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, over 44 mo of monitoring. Fragment survival and reattachment was temporally variable in survivorship experiments, and 1—yr survivorship of six—branch fragments ranged from 0.0 to 0.23. Fragments with six branches tended to have higher 1—yr survival rates than either 1—or 18—branch fragments. Larval recruitment was monitored in the 5—m 2 transect over 6 yr. Although mature colonies produce °10 4 larvae each year, there were at most seven 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. Temporal variation in these demographic parameters suggests that Plexaura A population structure has a large historical component. 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. This suggests that factors such as partial mortality may be important in establishing the relative advantages of vegetative propagation. The difference in survival between larvae and fragments further enhances the relative success of vegetative propagation in Plexaura A. There is no evidence of a trade—off between larval reproduction and vegetative propagation in Plexaura A, and it may be more accurate to characterize vegetative propagation as a strategy of clonal growth than as reproductive strategy.