by Kimberly Coombs
Climate change is known for causing adults corals to become bleached, but it is also affecting the early life stages of corals. The larval stage of a coral reef’s lifecycle is very important to its survivorship. Corals disperse their larvae out into the water, then the larvae are responsible for finding a suitable substrate to settle on. After settlement, corals are able to start growing accumulate symbionts. Several studies have observed how different symbionts influence juvenile coral growth rates and thermotolerance; however, no data currently show if there are any influences from symbionts on the coral larvae before settlement occurs.
Winkler et al. (2015) conducted a study to observe how symbionts may affect coral larvae habitat choice, survival, and settlement success. They looked at Acropora millepora from the Great Barrier Reef, a coral species that usually associates with sensitive or thermally tolerant symbionts, and exposed the larvae of this species to four different temperature treatments ranging from 22.5 to 28.5. The larvae were also exposed to seven different symbiont treatments that involved three different species of Symbiodinium: C3 (usually found in adults of this species), C15 (thermotolerant and not normally associated with this species), and D1 (thermotolerant and found in this species in certain regions). Habitat choice was documented based on where the larvae settled (on the top, back, or vertical surfaces of tile and on crustose coralline algae).
Lower temperatures were found to negatively impact A. millepora habitat settlement orientation and recruitment success, while other studies have found that higher temperatures negatively affect larvae success. Winkler et al. attribute their findings to their highest temperature being set at 28.5, where as other studies have had their highest temperature at 30 or 32. This temperature was used in preference to 30 or 32 in order to represent a 3 increase from the seasonal average temperature at the Great Barrier Reef, as only a 1–2 increase in temperature is expected to occur at the Great Barrier Reef by 2100.
When temperatures were favorable (higher than 24.5), coral larvae settled on vertical surfaces. Vertical surfaces in the natural environment represent areas where low algal growth levels are maintained and there is no destructive grazing on the corals. Coral larvae also settled on crustose coralline algae in favorable temperatures. Crustose coralline algae represents a habitat that is suitable for settlement and later metamorphosis for coral larvae. Lower temperatures caused the coral larvae to choose suboptimal substrate selection (coral larvae chose to not settle on crustose coralline algae nor on vertical surfaces), therefore, this reduces the recruitment success.
The different symbionts that the larvae were exposed to showed different temperature-dependent crustose coralline algae selection. At a temperature greater than 26, larvae offered the symbiont D1 and C3 had a similar crustose coralline algae settlement selection, yet at lower temperatures, larvae offered symbiont D1 chose crustose coralline algae as their settlement more than larvae with symbionts C15 and C3. Larvae given the symbiont C15 chose to settle least on crustose coralline algae across the different temperatures.
The results that Winkler et al. found suggest a link between symbionts and settlement on crustose coralline algae. Crustose coralline algae metabolism is reduced at lower temperatures; as a result, this reduces the metabolites that attract symbionts and decreases the number of symbionts settling on crustose coralline algae. The results indicate that either different symbionts influence the crustose coralline algae settlement cue or that settlement is influenced by different symbionts behaving as a co-inducer.
Winkler, N.S., Pandolfi, J.M., Sampayo, E.M., 2015. Symbiodinium identity alters the temperature-dependent settlement behaviour of Acropora millepora coral larvae before the onet of symbiosis. Proceeding of the Royal Society B, 282: 20142260.