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.

穗珊瑚科（Xeniidae）的软珊瑚在红海珊瑚礁中尤为繁盛。它们的繁盛或许部分得益于抵御鱼类捕食的强效防御机制。为验证这一推测，我们开展了野外与水族箱实验，对两种常见穗珊瑚——细叶伞形软珊瑚（Ovabunda crenata）与哈尔达格异指软珊瑚（Heteroxenia ghardaqensis）的次生代谢产物的拒食活性进行了评估。野外实验结果显示，两种受试物种的次生代谢产物可使红海礁栖野生鱼类对实验饵料颗粒的取食率分别降低86%（细叶伞形软珊瑚）与92%（哈尔达格异指软珊瑚）。水族箱实验中，两种受试珊瑚的天然浓度粗提物可使常见礁栖鱼类新月锦鱼（Thalassoma lunare，俗称月斑隆头鱼）对实验饵料颗粒的取食率分别降低83%与85%。即便粗提物浓度仅为活体软珊瑚组织天然浓度的12.5%，月斑隆头鱼的取食行为仍会受到抑制。为探究结构型拒食防御的潜力，我们提取了细叶伞形软珊瑚的骨针，将其分别以天然浓度、2倍浓度与稀释浓度混入饵料颗粒，并设置仅添加骨针（无粗提物）与骨针联合25%天然浓度粗提物两种添加方式，随后在水族箱实验中开展验证。实验结果显示，骨针并未对月斑隆头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。哈尔达格异指软珊瑚则不具备骨针结构。综上，红海海域穗珊瑚科软珊瑚物种的显著繁盛，大概率是其具备强效化学防御机制以抵御潜在捕食者的结果，而非依赖物理防御手段。