Hunting corals

Corals feed differently than expected

When it comes to nutrition, corals have a few tricks. Most of their nutrients come from microscopic algae that live in them, but when these algae do not provide enough food, corals can grab and eat tiny prey with their tentacles.

A recent study by researchers from the Woods Hole Oceanographic Institution (WHOI), the University of New Mexico and the Scripps Institution of Oceanography shows that more nutrients from coral come from this type of hunt than previously thought - information that can help fate predict coral reefs as global ocean temperatures rise. The study has now appeared in the journal Functional Ecology.

"In a heatwave, corals start to bleach, symbiotic algae that live in corals and provide them with most of their nutrients are excreted from their bodies, and if corals remain bleached too long, they starve to death," says Michael Fox of the WHOI and Main author of the study. "But if a coral has the ability to eat a lot before being bleached, it can survive on its fat stores long enough to recapture these symbionts as the water cools, so that we can better understand when, where, or why corals are eating. In future bleaching events, we may be able to understand why they survive better in some places than others."

Researchers conducted their study using samples collected at Palmyra Atoll in the Central Pacific. After being returned to the lab, the researchers removed the coral polyps from their skeletons and then separated the coral and their symbiotic algae in a centrifuge. The team then extracted essential amino acids from the corals, their symbionts, and the tiny zooplankton that corals eat often.

"Essential for the survival of an animal are essential amino acids, but most corals cannot make them. They have to get the essential acids either from their symbionts or from something they've just eaten," says Fox. "But each of these sources produces amino acids in different ways, giving the molecules different chemical properties."

These signatures can be used to identify the source of the amino acid. By measuring chemical differences in six individual amino acids, researchers were able to determine how much of the food comes from a coral of symbionts or captured prey. This new method of measuring coral nutrition allows scientists to estimate the contribution of different food sources to coral nutrition and provides a more accurate view of their diet than previous methods.

"As far as I know, this has never been done with corals," says Fox. "Our results suggest that some corals eat much more than we thought before, which has a big impact on the survival of reefs during climate change, and we've also learned that individual corals of the same species can have very different diets - that can be an important source of variation that we need to consider to understand how corals will respond to future changes."