LOCAL AND GEOGRAPHIC-VARIATION IN THE DEFENSIVE CHEMISTRY OF A WEST-INDIAN GORGONIAN CORAL (BRIAREUM-ASBESTINUM)

We sampled the between-colony and between-habitat variation in defensive chemistry of colonies of Briareum asbestinum, one of the most ubiquitous and chemically deterrent of the West Indian gorgonians. The basis of deterrence appears to reside in a diverse complement of 5 to 15 briarane and asbestinane diterpenoids found in individual colonies. Variation in chemistry of colonies is extensive throughout the Bahamas and the Virgin Islands: the complement of compounds is slightly different at each site in the Bahamas and colonies from all the Bahamian sites are distinct from those at the St. Croix (US Virgin Islands) site. St. Croix colonies not only differ in their composition of compounds, but also have fewer major diterpenoids than colonies from shallow water sites in the Bahamas. The shallow St. Croix extracts also differ in being dominated by asbestinanes instead of briaranes as in the Bahamas. The qualitative chemistry of colonies also varies locally between depths near San Salvador and Chub Cay, 2 islands in the Bahamas. At both sites, the ratio of briarane to asbestinane diterpenoids was higher in shallow (1 to 12 m) water colonies than in deeper ones (25 to 40 m). To determine whether the qualitative differences in chemistry between depths could be induced or were fixed, colonies were transplanted from shallow to deep reefs in San Salvador. After 7 mo, newly grown tissue from transplanted colonies still retained the same shallow water diterpenes. This experiment suggests that the qualitative differences in chemistry observed with depth are fixed in adult colonies. We also sampled quantitative variation in colonies from St. Croix and the Bahamas. Although we expected higher levels of compounds in shallow colonies because both productivity and numbers of predators are higher in shallow waters, at both sites levels of compounds were higher in deeper waters. Briareum asbestinum is unusual in being heavily invested with a diversity of extremely deterrent diterpenoids. The outstanding feature of this chemical defense system is the qualitative variation in the chemistry of colonies from different habitats, which appears to be genetically fixed, raising the question of what selective agents maintain the variation in chemistry among different habitats.