The Healthiest Corals Are Also the Fattest

The survival of many ocean species depends on the survival of coral reefs. But with many species of coral threatened by climate change, some scientists believe we should focus our preservation efforts on those most able to adapt—the fattest ones.

Corals are essentially colorless; the brilliant greens, yellows and browns that we associate with them are actually the colors of symbiotic algae living within the corals' cells.

Every so often, corals endure a period of heat stress called bleaching, when they dump the algae from their cells, which gives them a pale appearance. They can recover by growing more algae or acquiring new algae once water temperatures return to normal.

The Healthiest Corals Are Also the Fattest

Normally, bleaching is a rare event. But, by 2025, Caribbean waters are expected to be hot enough that the coral living there will be stressed to the point of bleaching once a year. The rest of the tropics are expected to experience annual bleaching by 2050.

Scientists at Ohio State University decided to study which types of corals would be able to survive bleaching events that occurred two years in a row. They found that:

Corals can supplement their diet by eating plankton, but they get most of their energy from their symbiotic relationship with algae. The algae get nutrients from the coral, and the corals get to siphon off sugars that the algae produce in photosynthesis. Like humans, corals can store excess energy as fat.

Two key survival strategies emerged in this study: the most resilient corals stored up fat reserves in times of plenty, and were willing to switch to a new dominant algal type in order to gather food in times of stress. Corals that didn't store fat or were stuck with their algal partner didn't fare as well.

The real winner was finger coral, which switched completely from one algal partner type to another over the course of the study, and had the largest fat reserves—47 percent higher than the boulder coral or mustard hill coral. The finger coral was barely even affected by the second bleaching and maintained a healthy growth rate.

The good news: We know that some species will be able to adapt to climate change. The bad news: As other coral species die out, there will be less biodiversity in reefs, where the different sizes and shapes of coral provide specialized habitats for fish and other creatures.

As the Earth's temperature continues to rise, that's the closest we get to a happy ending.