Corals that help build tropical, shallow-water reefs get some metabolic help from the microscopic protists they host within their cells. These dinoflagellates fix carbon, assimilate nitrogen, and transport essential compounds like lipids and glucose in their coral hosts – helping them grow, form skeletons, and reproduce. But before coral even reach this reef-forming stage, they start off as free-swimming larvae. Many coral colonies expand by brooding, then releasing their tiny, planktonic larvae (called planulae), which disperse over large distances before settling to form new colonies. As they’re dispersing, the planulae rely on energy reserves derived from their mother colonies for sustenance, and until now, it’s been unclear how much dinoflagellates contribute.
Well, according to a new Science Advances study, corals in this early life stage rely minimally on their dinoflagellate symbionts, unlike adult corals. A better understanding of coral biology would help researchers prepare for, or perhaps slow, the impacts of climate change on these important reef builders.
Christophe Kopp of École Polytechnique Fédérale de Lausanne and colleagues collected newly released planulae from a large mother colony of the cauliflower coral Pocillopora damicornis. These corals were grown in the open-system Océanopolis Aquarium in France, with natural seawater pumped from the Brest estuary. To study how dispersing coral larvae get and use energy, the team labeled the planulae with isotope tracers.
They found that nutrition and energy contribution from dinoflagellates is minimal in coral larvae, compared to adult colonies. Instead, the planulae obtain most of their energy early on from their own, maternally-derived proteins and lipids. These lipids make up as much as 70 percent of their weight.
The team thinks that this might have to do with the lower density of dinoflagellates on planulae. They likely gain energy from metabolic interactions with their symbiotic partners later on during their development. Recent work with 22- to 27-day-old Pocillopora damicornis planulae found that dinoflagellate symbionts translocate up to 70 percent of photosynthetically fixed carbon to the tissues of the coral host – which is similar to the proportion seen in adult corals.