This guest post was written by Ben Maxie. Ben is an undergraduate researcher at Old Dominion University who studies zooxanthellae genetics with Dan Barshis. Aside from marine biology, he is interested in beer brewing, car modification, and hiking.

All over the world, coral are under attack, and the biggest threat seems to be high sea surface temperature. Coral, which are colonies of tiny animals akin to jellyfish or sea anemones, are transparent on their own. These animals get their color from algae harbored inside their cells, which essentially act as power plants for the coral. These algae, called zooxanthellae (zoox for short), give the animal some of the carbs that they make in photosynthesis. High temperatures cause coral to ‘bleach,’ or lose their zoox. These temperatures aren’t likely to drop anytime soon, so coral need another way to adapt.

Coral Bleaching. Image from BBC

One way that coral can adapt is from the zoox themselves. There are several different types of zoox, and the types are usually called ‘clades.’ A recent study showed that some of these clades seem to be more tolerant of high temperature than others.

Mechanics of Coral Bleaching. Image from NOAA

First the researchers put several coral with clade C zoox, the most common in the wild, into different tanks with temperatures generally agreeable to the coral (24 C), and a bit warmer (29 C). Then they raised the temperature higher than the coral could withstand. These coral, which were all dominated by clade C zoox, bleached. The temperatures were then dropped back down to what they had been before the bleaching. The colonies that recovered were dominated by clade D zoox, which were at undetectable levels before bleaching. Coral that weren’t in bleaching tanks kept their clade C zoox, although the coral in the 29 C tank had more clade D zoox than the coral in the 24 C tank.

The researchers then bleached some of the coral again. The coral in the tank that stayed at 24 C throughout these bleaching events still showed no clade D zoox, while the coral that stayed in the 29 C tank all showed some clade D zoox at the end of the six-month experiment. Among the ones that were bleached, the colonies that recovered were all dominated by clade D zoox. Interestingly though, the coral that recovered from bleaching in the cooler 24 C tank lost more clade D symbionts and took on more clade C zoox than coral that recovered in a 29 C tank. A group of coral that were bleached the first time and then sat in a comfortable 24 C tank lost most of their clade D zoox after six months.

So what does any of that mean? It seems like whenever coral were exposed to heat, they took in the apparently more heat tolerant clade D zoox, and kicked out their clade C zoox. The fix seems to be temporary, though. The coral seem to keep clade D zoox around for as long as there is a threat, but when they’re put into more agreeable water, they bring back their pals from clade C. This is along the lines of what we’re seeing in the wild, also. Clade D zoox have been flaring up in the Caribbean lately. Coral may survive ocean warming yet.