Sweeper Tentacles in a Gorgonian Octocoral: Morphological Modifications for Interference Competition

Abstract

Elongate tentacles serve an agonistic function in sea anemones and scleractinian corals. Although certain octocorals (soft corals: Octocorallia: Alcyonacea) produce and exude allelochemicals that damage neighboring scleractinian corals, no specialized structures used in agonistic behavior have previously been identified in this large cnidarian subclass. Here, we describe the first evidence of the occurrence and use of specialized agonistic structures, sweeper tentacles, in an octocoral. The encrusting gorgonian Erythropodium caribaeorum Pallas (Octocorallia: Gorgonacea) is abundant on shallow reefs in the Caribbean, and competes for space with numerous coral species, sea anemones, and other cnidarians. Zones of contact between this gorgonian and several coral species were observed and recent damage to the coral colonies was noted. Furthermore, the gorgonian develops fields of modified polyps along such borders. These polyps have elongate tentacles termed `sweeper tentacles,' as in scleractinian corals. Such tentacles lack the side branches (pinnules) characteristic of octocorals in general, and bear a bulbous tip (acrosphere) densely packed with nematocysts. Transplant experiments showed damage to corals placed in contact with the gorgonian's sweeper tentacles and sweeper tentacles were induced when scleractinian corals contacted Erythropodium colony borders having exclusively normal tentacles. Thus, sweeper tentacles may contribute to the competitive success of this species in habitats where space is limiting and where there are a number of competing species, many with agonistic mechanisms of their own.