Chemical warfare among scleractinians: bioactive natural products from Tubastraea faulkneri Wells kill larvae of potential competitors.

Competition for space among scleractinians by overgrowth, overtopping, extracoelenteric digestion and the use of sweeper tentacles is well recognized, but another potential mode of competitive interaction, allelopathy, is largely uninvestigated. In this study, chemical extracts from Tubastraea faulkneri Wells were tested for deleterious effects on competent larvae of 11 other species of coral belonging to seven genera of four scleractinian families. Larvae exposed to extract concentrations from 10 to 500 µg ml(-1) consistently suffered higher mortality than larvae in solvent controls. Larvae of Platygyra daedalea (Ellis and Solander) and Oxypora lacera (Verrill) were the most sensitive, experiencing high mortality even at the lowest extract concentration. The toxic compounds from T. faulkneri did not kill any conspecific larvae. The estimated concentrations of active compounds within T. faulkneri tissues were 100-5000 times higher than the experimental concentrations. Pure compounds isolated from bioactive fractions of the extract were indole alkaloids identified as aplysinopsin, 6-bromoaplysinopsin, 6-bromo-2'-de-N-methylaplysinopsin and its dimer. The first three occur in other non-zooxanthellate corals in the same family as T. faulkneri, whereas the dimer is novel. These compounds could act as allelochemicals that prevent potential competitors from recruiting in the vicinity of T. faulkneri colonies and help to pre-empt interactions with competitively dominant species.