Feeding Behavior in the Hawaiian Zoanthids Palythoa and Zoanthus

Abstract

Palythoa psammophilia Walsh & Bowers and Zoanthus pacificus Walsh & Bowers are two subtropical, subtidal zoanthids closely related phylogenetically but widely different in their feeding methods. Palythoa is a predator that feeds on zooplankton, particularly crustaceans. Of a vast array of items offered to Zoanthus in the laboratory, only small pieces of freshly killed or frozen fish elicited positive feeding responses. In natural situations Zoanthus has never been observed to feed. Palythoa polyps capture live prey with their tentacles and use these to hold the prey against the peristome. Very few nematocysts are discharged and they do not paralyze the prey. The behavioral response called "tentacle protrusion" allows the polyps to capture additional prey while ingesting one just caught. The optimum density of zooplankton, which causes the fastest response and largest capture, was found to be 200 Artemia per m^3. This represents approximately 10 times the zooplankton density calculated for the natural environment of Palythoa. After the food has been obtained, either by capture (Palythoa) or seizure of items (Zoanthus), both animals show the same complex and orderly series of steps which is called the feeding reaction and consists of the following: (1) Lip formation. A group of tentacles seizes the food, the edge of the disc carrying these tentacles first contracts, then rises up and turns inward, thereby folding tentacles and food toward the mouth; (2) Mouth opening. Upon contact with the food, the mouth borders separate and the food is swallowed; and (3) Ingestion response. Food disappears in the coelenteron and the borders of the mouth close over it.