Chemical and structural defenses of Caribbean gorgonians ( Pseudopterogorgia spp . ) : Tntracolony localization of defense

We have adopted an integrated approach to assessing the function of gorgonian defenses by analyzing within-colony patterns in the distribution of both structural elements and chemical compounds in two morphologically similar species of Rseudopterogorgia. In both species, crude extract concentrations are higher in polyp-bearing regions of colonies than in more proximal, nonpolyp-beari ng regions of colonies. The concentration of the ichthyodeterrent terpene from Pseudopterogorgia acerosa (pseudopterolide) is also higher in polyps than in axial tissue. In Pseudupterogorgia rigida, an ichthyodeterrent terpene (curcuhydroquinone) occurs almost exclusively in the polyps and is undetectable in most samples from the axial coenenchyme. The distribution of the calcium carbonate structural elements (sclerites) within colonies is negatively correlated with the concentration of extract. Sclerites comprise the largest percent composition in the axial tissue and a very small proportion of branch tissue. Geographic variation in the negative within-colony correlation between chemical and structural components indicates the range of variation possible in these defensive traits. Modular organisms (sensu Harper and Bell 1979) such as plants and colonial invertebrates often exhibit striking within-individual asymmetries in deployment of defenses against consumers (Thompson 1982; Harvell 1986). It is difftcult, however, to interpret the function and adaptive significance of these asymmetries without understanding the mechanisms and constraints underlying their development (Smith et al. 1985). For example, Harvell (1984) documented within-colony variation in palatability of a marine bryozoan. Since the regions of reduced palatability correlated with the location of reproductive tissue, the reAcknowledgments This work was primarily conducted on RV Columhz4.s Zsclin (University of Miami) in the Grenadine Islands (supported by NSF grant CHE 86-20217 to W. Fenical). The research was supported by a Du Pont Faculty Award and Hatch Grant (NYC-l 83408) to C. D.H. WC are particularly grateful to Klaus Ruetzler and the Smithsonian Institution for providing facilities for part of this research at Carrie Bow Cay, Belize. Charles Greene helped with fieldwork and commented on the manuscript. We thank Richard Helling, Jessica Marks, and Dan Turlington for assistance with sclerite analysis. duced palatability could be due to selection for differential defense within the colony. Since both the palatability and reproductive gradients were also correlated with an ontogcnetic gradient, however, the possibility that the entire pattern is due to developmental or physiological constraint cannot be ruled out. Similarly, although higher defensc of reproductive tissue in plants may be a result of selection for defense of valuable tissue (Thompson 1982), an equally valid hypothesis is that of physiological constraint in the mechanisms of deployment of secondary compounds. Thus the distribution of defensive elements within modular organisms is likely a complex compromise between selection for optimal defensive patterns and developmental, architectural, or physiological constraints. Very little is known, however, about predatorimposed selection favoring the evolution of these patterns or the developmental constraints limiting attainment of some optimal defensive profile. Although the potential defensive role of gorgonian secondary compounds has been repeatedly investigated (Fenical 1982; Bakus et al. 1986; Pawlik et al. 1987; Harvell