TOXIC PREY DISCRIMINATION IN A HIGHLY SPECIALIZED PREDATOR CHAETODON-MELANNOTUS (BLOCK ET SCHNEIDER) - VISUAL VS CHEMICAL CUES

The specialized predator Chaetodon melannotus (Bloch et Schneider) (Vertebrata, Pisces), which feeds on highly toxic and sometimes allelopathic octocorals, initially locates its prey using visual rather than chemical cues. A novel controlled experiment was carried out to further understand the basis of food preference and selection. Organic extracts were interchanged between colonies of three species of octocoral: Alcyonium molle (Dean), Clavularia inflata (Schenck) and Sinularia flexibilis (Quoy et Gaimard) (Coelenterata, Octocorallia). Extracts were prepared by stepwise solvent extraction of the octocoral; first with ethanol, then with mixtures containing dichloromethane in ethanol, and finally with dichloromethane. All extractable organic material was removed. Concentrated extracts (in ether) were then reintroduced into the extracted colonies in the six possible permutations. In three smorgasbord-type experiments using two octocoral species at a time, C. melannotus were offered the six types of modified colonies, including controls without any extract, yielding a balanced matrix of visual and chemical cue combinations. Responses of the fish to prey choices were nonrandom in all cases. Alcyonium colonies with original extracts reintroduced were strongly preferred over Clavularia. Clavularia colonies and any colony containing their extracts received low preference. When offered with Sinularia, Clavularia colonies devoid of organic components received high preference. Chemical cues were determined to be important in this selection process. The Sinularia colonies in this situation received moderate acceptance. Sinularia colonies with their own extract reintroduced, received high preference when offered with Alcyonium. This indicated that there might be a subtle change in appearance or texture of Alcyonium (the preferred choice in an earlier study) as a result of the extraction. Responses were significantly different from that expected if the fish were cueing solely on the basis of either visual or chemical cues. A combination of visual and chemical cueing, possibly in sequence, appears to have produced the observed results. These results suggest that C. melannotus use visual cues to locate their prey and probably further select on the basis of chemical cues. We believe that the technique used here is valuable for use in the separation of visual and chemical cues for feeding.