Diel Variation in Prey Capture Behavior by the Corallimorpharian Discosoma sanctithomae: Mechanical and Chemical Activation of Feeding.

The feeding biology of corallimorpharians is poorly understood. This paper describes an envelopment method of prey capture by a Caribbean species, Discosoma (= Rhodactis) sanctithomae, and further examines the stimuli that elicit envelopment and subsequent ingestion of prey. The corallimorpharians exhibited a diel pattern of expansion and contraction of the oral disc margin and tentacles. This was correlated with the density of zooxanthellae in these tissues. The tentacles expanded during the day to expose abundant zooxanthellae. The oral disc margin contained relatively few zooxanthellae and was contracted and turned down in a convex posture during the day. At night the margin was expanded and turned up to give the oral disc a bowl-shaped posture. This allowed effective execution of the envelopment response that was successful in the capture of crustaceans, polychaetes, and small fishes that were most abundant in the plankton at night. The stimuli that elicited this predatory behavior were both mechanical and chemical. Mechanical stimulation elicited envelopment at night but was not as effective during the day. Envelopment was also activated by the imino acid proline and the tripeptide reduced glutathione at low concentrations, and four other amino acids at high concentrations. Continued contraction and ingestion responses required chemical activation. Consistent ingestion responses were caused only by reduced glutathione.