Defense by symbiotic crustacea of host corals elicited by chemical cues from predator

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Abstract
Observations and experiments carried out on a coral reef off the Pacific coast of Panamá demonstrated that shrimp (Alpheus lottini) and crab (Trapezia spp.) symbionts that protect their host coral (Pocillopora elegans) can detect an approaching sea star predator (Acanthaster planci) by chemical cues. Simulated feeding attacks by Acanthaster in sealed transparent bags elicited only 0.5 defensive responses (snipping at spines and tube feet, jerking the sea star, and snapping) per 3 min; defensive behavior increased significantly to 4 and 5 responses, respectively, for Acanthaster in perforated bags and for Acanthaster in direct contact with coral. Neutralized (boiled) Acanthaster elicited only 3 defensive interactions per 3 min compared with 12 interactions for live Acanthaster. Simulated feeding attacks by Oreaster, a non-corallivorous sea star, elicited only 0.5 defensive responses per 3 min, whereas Oreaster introduced with “Acanthaster water” increased the level of defensive responses to 7. These results suggest that chemical, and to a lesser extent visual (physical presence and movements of sea star), cues stimulate the defensive behavior of the symbiotic crustaceans. The ability to detect a predator at a distance is probably advantageous because in responding only to an actual threat it minimizes the time the defending symbionts spend in an exposed position on the terminal branches of the host coral and because it alerts the crustaceans to sea stars feeding at night.