Abstract
Soft corals of the family Xeniidae are particularly abundant in Red Sea coral reefs. Their success may be partly due to a strong defense mechanism against fish predation. To test this, we conducted field and aquarium experiments in which we assessed the antifeeding effect of secondary metabolites of 2 common xeniid species, Ovabunda crenata and Heteroxenia ghardaqensis. In the field experiment, the metabolites of both investigated species reduced feeding on experimental food pellets in the natural population of Red Sea reef fishes by 86 and 92% for O. crenata and H. ghardaqensis, respectively. In the aquarium experiment, natural concentration of crude extract reduced feeding on experimental food pellets in the common reef fish Thalassoma lunare (moon wrasse) by 83 and 85%, respectively. Moon wrasse feeding was even reduced at extract concentrations as low as 12.5% of the natural concentration in living soft coral tissues. To assess the potential of a structural anti-feeding defence, sclerites of O. crenata were extracted and mixed into food pellets at natural, doubled and reduced concentration without and in combination with crude extract at 25% of natural concentration, and tested in an aquarium experiment. The sclerites did not show any effect on the feeding behavior of the moon wrasse indicating that sclerites provide structural support rather than antifeeding defense. H. ghardaqensis lacks sclerites. We conclude that the conspicuous abundance of xeniid soft coral species in the Red Sea is likely a consequence of a strong chemical defence, rather than physical defences, against potential predators.
Highlights
Soft corals (Cnidaria, Alcyonacea) are a major component of the sessile coral reef benthos and are highly diverse in tropical Indo-Pacific coral reefs (Dinesen 1983, Fabricius & Alderslade 2001), including the Red Sea (Benayahu & Loya 1977, 1981, Benayahu 1985, Reinicke 1997)
A mean (± SE) of 97 ± 2.5 and 92 ± 2.9% of the control pellets were eaten by the reef fishes, while only 14.4% (± 3.9) and 8.7% (± 3.2) of the extract containing pellets of Ovabunda crenata and Heteroxenia ghardaqensis were eaten, Fig. 1
Moon wrasse, the control pellets were always eaten, In the aquarium experiment, the fish quickly while the treatment pellets containing the natural adapted to the new condition and readily accepted concentration of crude extracts were rejected to the control food pellets
Summary
Soft corals (Cnidaria, Alcyonacea) are a major component of the sessile coral reef benthos and are highly diverse in tropical Indo-Pacific coral reefs (Dinesen 1983, Fabricius & Alderslade 2001), including the Red Sea (Benayahu & Loya 1977, 1981, Benayahu 1985, Reinicke 1997). Some soft corals of the families Xeniidae and Alcyoniidae contribute to the diet of coral reef fishes (Gohar 1940). Secondary metabolites of some soft corals have been shown to possess ecological functions including anti-predatory. These chemical defenses may be as effective as biomineralized skeletons in that they protect hermatypic corals from predation by most reef fishes (Sammarco & Coll 1992). Some Alcyoniidae species (e.g. Sinularia polydactyla, Rhytisma fulvum fulvum) were shown to possess secondary metabolites, which protected the soft corals against predation by carnivorous fish (Wylie & Paul 1989, Van Alstyne et al.1994, Kelman et al 1999).
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