Chemical defenses of the Caribbean sponges Agelas wiedenmayeri and Agelas conifera

Previous studies have determined that Caribbean reef sponges of the genus Agelas are chemically defended from fish predation by brominated pyrrole alkaloids, and that the compounds responsible for this defense have been elucidated for 1 species, A. clathrodes. In this study, we expand our understanding of chemical defense in this common sponge genus to include the characterization of defensive metabolites in the tissues of A. wiedenmayeri and A. conifera. Bioassay-directed isolation of defensive metabolites was undertaken using fish feeding assays carried out in laboratory aquaria and in the field. A. wiedenmayeri contained the same 2 major metabolites as A. clathrodes, 4,5-dibromopyrrole-2-carboxylic acid (1), and oroidin (2), in addition to a small amount of bromoageliferin (7). The 2 major metabolites were present at higher concentrations in samples of A. wiedenmayeri than in A. clathrodes, and their relative concentrations were reversed, with A. wiedenmayeri on average containing more 4, 5-dibromopyrrole-2-carboxylic acid (1) (2.0 mg ml(-1)) than oroidin (2) (0.8 mg ml(-1)). A. conifera contained a mixture of dimeric bromopyrrole alkaloids dominated by sceptrin (3), with <10% each of dibromosceptrin (5), bromoageliferin (7), dibromoageliferin (8), ageliferin (6), and bromosceptrin (4). Mean concentration of sceptrin (3) in sponge tissue was 5.3 mg ml(-1); this compound deterred feeding of reef fish in aquarium assays at 1.0 mg ml(-1), the lowest concentration assayed. Sceptrin (3) concentrations were higher in sponges collected in the southern Bahama Islands than in those collected in the middle Bahamas, but the reasons for this variation remain unclear. The structure-activity relationship of the pyrrole group was investigated by assaying derivatives of the active metabolites. Feeding deterrent activity of the molecule was enhanced by the addition of bromine to the pyrrole group, but not affected by exchange of the heteroatom from N to O or S. Combining an understanding of the structure-activity relationship of Agelas metabolites with an understanding of the variation in these metabolites across the genus may provide insight into the evolution of defensive chemistry in this highly successful taxa of pan-tropical sponges.