Chemical versus structural defense against fish predation in two dominant soft coral species (Xeniidae) in the Red Sea

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.

红海珊瑚礁生境中，异花软珊瑚科（Xeniidae）的软珊瑚种群尤为繁盛。这类珊瑚的高丰度或许部分得益于其针对鱼类捕食的强效防御机制。为验证这一假说，我们开展了野外与水族箱实验，以评估两种常见异花软珊瑚物种——具褶卵软珊瑚（Ovabunda crenata）与哈氏异软珊瑚（Heteroxenia ghardaqensis）——的次生代谢产物的拒食效应。野外实验结果显示，两种受试物种的代谢产物分别使红海礁鱼自然种群对实验饵料球的取食率降低了86%（具褶卵软珊瑚）与92%（哈氏异软珊瑚）。水族箱实验中，两种物种的粗提物在自然浓度下，分别使常见礁鱼新月锦鱼（Thalassoma lunare，俗称月斑盔鱼）对实验饵料球的取食率降低了83%与85%；即便粗提物浓度仅为活体软珊瑚组织自然浓度的12.5%，月斑盔鱼的取食行为仍受到显著抑制。为探究结构型拒食防御的潜力，我们提取了具褶卵软珊瑚的骨针，并将其以自然浓度、加倍浓度与降低浓度三个梯度混入饵料球，同时分别设置仅含对应梯度骨针、以及骨针与25%自然浓度粗提物联合添加的组别，随后在水族箱实验中开展测试。实验结果显示，骨针并未对月斑盔鱼的取食行为产生任何影响，这表明骨针的作用仅为提供结构支撑，而非充当拒食防御手段。哈氏异软珊瑚并不具备骨针结构。综上，红海异花软珊瑚科物种的显著高丰度，大概率是其针对潜在捕食者具备强效化学防御（而非物理防御）的结果。