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.

Effects of ocean acidification caused by rising CO2 on the early development of three mollusks

Increasing atmospheric CO2 can decrease seawater pH and carbonate ions, which may adversely affect the larval survival of calcareous animals. In this study, we simulated future atmospheric CO2 concentrations (800, 1500, 2000 and 3000 ppm) and examined the effects of ocean acidification on the early development of 3 mollusks (the abalones Haliotis diversicolor and H. discus hannai and the oyster Crassostrea angulata). We showed that fertilization rate, hatching rate, larval shell length, trochophore development, veliger survival and metamorphosis all decreased significantly at different pCO2 levels (except oyster hatching). H. discus hannai were more tolerant of high CO2 compared to H. diversicolor. At 2000 ppm CO2, 79.2% of H. discus hannai veliger larvae developed normally, but only 13.3% of H. diversicolor veliger larvae. Tolerance of C. angulata to ocean acidification was greater than the 2 abalone species; 50.5% of its D-larvae developed normally at 3000 ppm CO2. This apparent resistance of C. angulata to ocean acidification may be attributed to their adaptability to estuarine environments. Mechanisms underlying the resistance to ocean acidification of both abalones requires further investigation. Our results suggest that ocean acidification may decrease the yield of these 3 economically important shellfish if increasing CO2 is a future trend.