Marginal tentacles of the corallimorpharian Rhodactis rhodostoma. 1. Role in competition for space

The corallimorpharian Rhodactis rhodostoma (Ehrenberg, 1934) forms aggregations that dominate patches on some coral reef flats in the Red Sea. The outcomes and mechanisms of competition for space between this corallimorpharian and other sessile organisms are poorly understood. Polyps of R. rhodostoma were observed to overgrow zoanthids, hydrozoan corals, sponges and encrusting macroalgae on a fringing reef at Eilat, northern Red Sea. R. rhodostoma polyps also damaged, and in some cases overgrew, reef-building corals in the families Poritidae, Acroporidae and Pocilloporidae, most of which form branching colonies with small polyps that are subordinate in coral competitive hierarchies. In contrast, most stony corals in the families Faviidae and Mussidae had standoff interactions with R. rhodostoma, in which they prevented the corallimorpharians from damaging them or approaching closer than 1 to 3 cm. The latter corals are ranked at the top of competitive hierarchies for Indo-Pacific corals, and they form massive colonies of large polyps which may develop aggressive organs termed sweeper tentacles. Some soft corals that exude allelopathic chemicals also avoided overgrowth by the corallimorpharians. Tentacles along the oral disk margin of R. rhodostoma polyps were swollen and bulbous during contacts with cnidarians. These bulbous marginal tentacles had significantly thicker ectoderm and a higher proportion of holotrichous nematocysts than did the normally filiform marginal tentacles of R. rhodostoma polyps. It is concluded that, on the reef flat at Eilat, this corallimorpharian damages and overgrows a variety of sessile competitors, including branching stony corals, via the application of specialised marginal tentacles filled with penetrating nematocysts. R. rhodostoma is an intermediate competitor in the aggressive hierarchy among Indo-Pacific Anthozoa, including the reef-building corals. Received: 1 July 1998 / Accepted: 24 March 1999