While diving in Thailand in 2010, Mark Eakin, a scientist with the NOAA, saw some clownfish behaving in strange way — and for a very concerning reason.

Clown fish live in sea anemones — jellyfish-like polyps with toxic tentacles — mainly for safety reasons. But the clown fish he saw had stopped doing that.

"Instead of darting into their anemone for safety, the clown fish were going to nearby bleached corals, because something was wrong with their anemones," Eakin told The Dodo. "They didn't consider that a safe home anymore."

The clown fish had changed their behavior right after a mass coral bleaching event that happened in the area. Coral bleaching happens when sea temperatures rise, and corals expel the algae living inside their tissues, which turns the corals white.

This kind of thing is happening all over the world, and now it's hit the Great Barrier Reef in Australia, the largest coral reef system in the entire world. And this is bad news for the sea animals who depend on the reef.

Aerial view of the bleached coral on the Great Barrier Reef Aerial view of the bleached coral on the Great Barrier Reef Arc Centre of Excellence

In 2016, 67 percent of the northern region of the Great Barrier Reef died. This year, more bleaching has occurred, although scientists are still figuring out just how bad it is. Unfortunately, some are fearing the worst.

"The severity of the 2016 bleaching was off the chart," Terry Hughes, the director of the ARC Centre of Excellence for Coral Reef Studies, said in a statement. "It was the third major bleaching to affect the Great Barrier Reef, following earlier heatwaves in 1998 and 2002. Now we're gearing up to study a potential number four."

Bleached coral on the Great Barrier Reef in AustraliaBleached and dead coral on the Great Barrier ReefArc Centre of Excellence

When coral reef systems die, animals lose their homes. Not just a few animals — millions of them.

Bleached coral on the Great Barrier Reef in AustraliaA fish swimming next to bleached, dead coralArc Centre of Excellence

"You're losing countless different species — small organisms and vertebrates and reefs," Eakin said. "And of course, the fish communities are being affected. They're just gone."

Bleached coral on the Great Barrier Reef in AustraliaA scuba diver surveying damaged coral on the Great Barrier ReefArc Centre of Excellence

Where do they go exactly?

Bleached coral on the Great Barrier Reef in AustraliaArc Centre of Excellence

"That's a very good question," Eakin said. "Some of them die ... through starvation or predation. They stop behaving the way they normally do, and they're more susceptible to predators."

Bleached coral on the Great Barrier Reef in AustraliaArc Centre of Excellence

Other animals may try and move elsewhere, but this isn't always the best option.

"When the reef dies and gets covered in algae, they leave and move elsewhere to other nearby habitats," Eakin said. "But they won't be doing as well, and may be subject to greater predation."

Bleached coral on the Great Barrier Reef in AustraliaA fish swimming next to bleached, dead coralArc Centre of Excellence

The issue of coral bleaching may seem impossible to fix, but there are things we can do. To help ocean animals keep their coral reef homes, you can make a donation to the Great Barrier Reef Foundation.