Grazers improve health of coral juveniles in ex situ mariculture

Scleractinian coral mariculture plays an increasingly important role in reef rehabilitation and the aquarium trade, but its efficiency can be impeded by fouling macroalgal blooms, thus compromising coral health and increasing maintenance costs. In this study, the sea urchin Salmacis sphaeroides and the gastropod Trochus maculatus were introduced into ex situ coral mariculture tanks as biological controls of fouling macroalgae. We examined the effects of both grazers on the overall health of Pocillopora damicornis juveniles by quantifying the changes in growth, colour saturation and extent of tissue lesion of the corals for 24 weeks. The juvenile corals exposed to S. sphaeroides and T. maculatus grew seven to eleven times faster than the controls, and attained final ecological volumes of 14104 ± 2955 mm3 (25.1 ± 0.9 mm diameter), 11913 ± 2644 mm3 (24 ± 0.7 mm diameter) and 1924 ± 620 mm3 (12.2 ± 1 mm diameter) respectively. The juvenile corals in the grazer treatments were also significantly darker and showed fewer lesions than that in the control. In the tanks with (S. sphaeroides, T. maculatus) and without (control) grazers, the mean algal cover was 37.5%, 57.4% and 78.8%, respectively, and the final algal biomass was 13.9 ± 3.9 mg, 27.3 ± 8.1 mg and 71.9 ± 7 mg, respectively. Taken together, the results showed that S. sphaeroides and T. maculatus are effective biological controls of fouling macroalgae in ex situ coral culture.