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 defensive 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 members 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 morphogenesis, their tip-to-stalk ratio and ectoderm thickness doubles, indicating acrosphere development, 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.

Long-term effects of competition on coral growth and sweeper tentacle development

“…The colonies were removed carefully from the substratum using a chisel and hammer, and transported in seawater to the Interuniversity Institute for Marine Science (IUI) at Eilat. They were acclimated for 3 wk in outdoor running seawater tanks at the IUI, then transferred to experimental aquaria at the adjacent Underwater Observatory Marine Park (after Lapid et al 2004).…”

“…Illumination was supplied by Philips 36w/w blue type light bulbs suspended over each aquarium. Light intensity was 100 µE m -2 s -1 , equivalent to the light level at approximately 15 m depth on the reef slope (details in Lapid et al 2004). This light intensity was lower than at the depth where colonies were collected (2 to 6 m, see above) due to lighting constraints of the culture facility.…”

“…Damage was measured to the nearest millimeter as the surface area of exposed skeleton on each dome-shaped coral colony, using a fine flexible ruler made of transparent underwater paper accurate to 1 mm (after Lapid et al 2004). The maximal and minimal diameters through the center of the damaged area on each coral were measured, and the area of the ellipse was calculated.…”

“…During the first 2 mo, observations were made every 3 to 4 d; the development of sweeper tentacles during this initial period is reported elsewhere . Sweeper tentacles were defined as long tentacles possessing an enlarged tip relative to the stalk, in contrast to feeding tentacles, which were defined as short, tapered tentacles (after Lapid et al 2004). Random sweeper tentacles were defined as those that appeared at unpredictable locations on the colony, apparently as chance events and not directed toward competitor contact.…”

On the run: free-living mushroom corals avoiding interaction with sponges

“…Scleractinian corals may come into contact with other organisms, such as algae (River and Edmunds 2001;McCook 2002, 2003;Nugues and Bak 2006;Haas et al 2010;Benzoni et al 2011), ascidians (Bak et al 1981(Bak et al , 1996Sommer et al 2009), barnacles (Benzoni et al 2010), corallimorpharians (Chadwick 1991;Chadwick and Adams 1991;Langmead and Chadwick-Furman 1999;Kuguru et al 2004), polychaetes (Samini Namin et al 2010), soft corals (Sammarco et al 1983;Dai 1990), and particularly by sponges Coles and Bolick 2007;de Voogd 2007;Benzoni et al 2008) and may be overgrown and even killed by them. Scleractinian coral species also show aggressive reactions to one another, which may depend on coral size, secretion of bioactive compounds, and the presence of sweeper tentacles (Sheppard 1981;Bak et al 1982;Koh and Sweatman 2000;Lapid et al 2004;Lapid and Chadwick 2006;Chadwick and Morrow 2011).…”