Population Dynamic Modeling of the Corkscrew Sea Anemone Bartholomea Annulata on Caribbean Coral Reefs

Abstract

The corkscrew anemone Bartholomea annulata is a large, common sea anemone on Caribbean coral reefs. The holobiont of B. annulata, endosymbiotic microalgae (zooxanthellae), and ectosymbiotic cleaner shrimps may have positive cascade effects in coral reef communities by serving as fish cleaning stations. Thus, collection of B. annulata for the ornamental aquarium trade may indirectly impact reef fish diversity, yet this fishery is largely uncontrolled and has drastically reduced the abundance of these sea anemones in some parts of the Caribbean. The dynamics of 2 unfished populations of B. annulata in the US Virgin Islands were modeled here, to determine how they vary spatially (between habitats) and temporally (among 4 seasons). Seven types of demographic models were used to project future population size and structure at each of the 2 study sites. Population dynamics differed significantly between the 2 examined study locations, with higher mortality at the outer reef site (Flat Cay) and higher rates of transition between size-classes (growth and shrinkage) at the inner reef site (Brewers Bay). There was no significant difference in the rate of total recruitment between these 2 populations, but both the number of recruits per m2 and recruits per resident anemone per m2 were significantly higher at Brewers Bay, while the number of recruits per resident anemone was significantly higher at Flat Cay. Light intensity, sediment grain size, percent stony coral cover, and relative water motion all were significantly greater at the outer site of Flat Cay than at the inner protected site in Brewers Bay. These differences in site characteristics may influence the demography of B. annulata at each site, in particular rates of anemone growth and mortality. Matrix models based on current population trends projected the survival of these populations at both sites. Closed population models for both sites projected exponential growth and population sizes much larger than the potential carrying capacities of the sites. In contrast, the open and semi-open models projected stabilization of the anemone populations, though the equilibrium sizes of the populations varied among models. Elasticity analyses of the matrix models indicated that the smallest individuals contributed the most to population growth, and suggest a need for establishing a minimum body size for the collection for these organisms. When outside recruitment was removed from the open and semi-open models, anemone populations were projected to plummet to zero within about 3 years, revealing the importance of outside recruitment for population growth and stability. Thus, unfished marine preserves near fished areas of these anemones may be important as sources of outside recruitment. It is concluded that the establishment of unfished marine preserves adjacent to fished populations, and a minimum body size for collection, are recommended for the development of a sustainable fishery on this giant sea anemone on Caribbean coral reefs.