Sweeper tentacles of the brain coral Platygyra daedalea: induced development and effects on competitors

Abstract

Benthic marine organisms utilize an array of defensive and aggressive mechanisms that affect competition for space on hard marine substrata. The sweeper tentacles of stony corals are inducible aggressive organs used during competition, but they also may serve a pre-emptive defen- sive function. About half of the colonies of the brain coral Platygyra daedalea at Eilat, northern Red Sea, possess sweeper tentacles, many of which are not directed toward neighboring corals. These randomly oriented sweeper tentacles may be produced in order to detect the settlement or advance of corals occurring at >5 cm distance from the colony. Of coral colonies <5 cm distant from P. daedalea, about 43% exhibit tissue damage facing the interaction area. Adjacent corals with the most damage belong to the genera Favites and Leptastrea, while colonies of Millepora and congeneric Platygyra exhibit significantly less damage. Tissue damage of neighboring coral colonies decreases significantly with increasing distance from P. daedalea. The presence of sweeper tentacles on brain corals correlates significantly with colony diameter, but not with the number of neighboring colonies. The aggressive reach of P. daedalea in Eilat is 5.3 ± 3.0 cm, longer than previously reported for mem- bers of this genus. Under laboratory conditions, sweeper tentacles develop on P. daedalea colonies about 30 d following initial contact with colonies of the common massive coral F. complanata, and at approximately 50 d they reach a maximum length of about 6.5 cm, 10 × longer than feeding tentacles. Sweeper tentacles cause increasing tissue damage to F. complanata colonies over 2 mo. During mor- phogenesis, their tip-to-stalk ratio and ectoderm thickness doubles, indicating acrosphere develop- ment, but maximal width of the tentacle stalk does not change. Sweeper tentacles appear to be a common agonistic mechanism among stony corals, and may serve also as a defensive mechanism that allows the persistence of some species in crowded reef habitats.