Microscopic observation of symbiotic and aposymbiotic juvenile corals in nutrient-enriched seawater

Symbiotic and aposymbiotic juvenile corals, which were grown in the laboratory from the gametes of the scleractinian coral Acropora digitifera and had settled down onto plastic culture plates, were observed with a microscope under different nutrient conditions. The symbiotic corals successfully removed the surrounding benthic microalgae (BMA), whereas the aposymbiotic corals were in close physical contact with BMA. The areal growth rate of the symbiotic corals was significantly higher than that of the aposymbiotic corals. The addition of nutrients to the culture seawater increased the chlorophyll a content in the symbiotic coral polyps and enhanced the growth of some of the symbiotic corals, however the average growth rate was not significantly affected, most likely because of the competition with BMA. The comparison between the symbiotic and aposymbiotic juvenile corals showed that the establishment of a symbiotic association could be imperative for post-settlement juvenile corals to survive in high-nutrient seawater.

► The growth of juvenile corals was microscopically observed. ► Symbiotic corals can actively remove the surrounding benthic microalgae. ► Symbiotic corals grow much faster than aposymbiotic corals. ► The establishment of coral symbiosis is imperative in nutrient-enriched seawater.