Chemical defenses of the Caribbean sponges Agelas wiedenmayeri and Agelas conifera

Previous studies have determined that Caribbean reef sponges of the genus A gelas are chemically defended from fish predation by brominated pyrrole alk aloids, and that the compounds responsible for this defense have been eluci dated for 1 species, A. clathrodes. In this study, we expand our understand ing of chemical defense in this common sponge genus to include the characte rization of defensive metabolites in the tissues of A. wiedenmayeri and A. conifera. Bioassay-directed isolation of defensive metabolites was undertak en using fish feeding assays carried out in laboratory aquaria and in the f ield. A. wiedenmayeri contained the same 2 major metabolites as A. clathrod es, 4,5-dibromopyrrole-2-carboxylic acid (1), and oroidin (2), in addition to a small amount of bromoageliferin (7). The 2 major metabolites were pres ent at higher concentrations in samples of A. wiedenmayeri than in A. clath rodes, and their relative concentrations were reversed, with A. wiedenmayer i 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 mixtu re of dimeric bromopyrrole alkaloids dominated by sceptrin (3), with <10% e ach of dibromosceptrin (5), bromoageliferin (7), dibromoageliferin (8), age liferin (6), and bromosceptrin (4). Mean concentration of sceptrin (3) in s ponge 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. Sce ptrin (3) concentrations were higher in sponges collected in the southern B ahama Islands than in those collected in the middle Bahamas, but the reason s for this variation remain unclear. The structure-activity relationship of the pyrrole group was investigated by assaying derivatives of the active m etabolites. Feeding deterrent activity of the molecule was enhanced by the addition of bromine to the pyrrole group, but not affected by exchange of t he heteroatom from N to O or S. Combining an understanding of the structure -activity relationship of Agelas metabolites with an understanding of the v ariation in these metabolites across the genus may provide insight into the evolution of defensive chemistry in this highly successful taxa of pan-tro pical sponges.