If you’ve ever seen “Finding Nemo,” you’re familiar with the special relationship between clown fish and anemones, the stinging spaghetti rugs of the reef that Nemo calls home.
Normally, algae inside the anemone convert sunlight into energy to feed the anemone, which provides protection for the fish and its eggs. That’s why the clown fish chooses to live in the anemone. It’s not quite clear what the clown fish does for its host, but those stinging tentacles may save the fish from becoming someone else’s dinner.
But when warming temperatures or another environmental disturbance cause coral and anemones to bleach, the situation changes. Algae die. Anemones shrink. And clown fish, most of which remain in their bleached home, don’t reproduce like they used to. They’re not so abundant.
Although a bleached anemone is deprived of algae, it’s still alive and may one day recover, said Tommy Norin, an ecological physiologist at the University of Glasgow in Scotland. There’s no reason the fish, which doesn’t even eat the algae, should be suffering. But it does.
To try to get a handle on this mystery, Dr. Norin and a team of collaborators looked at physiological indicators of stress in clown fish living around bleached anemones. Their work, published on Tuesday in Proceedings of the Royal Society B, suggests the higher cost of living in a stressful environment may partly explain these clown fish struggles.
Researchers already knew that when corals and anemones bleach, clown fish and other animals living among them produce hormones that may help them cope with a changing environment. But scientists hadn’t found a way to show that the fish’s whole body could be stressed out.
To do this, the researchers collected 16 fish from a healthy coral reef near French Polynesia in the Pacific Ocean and took them back to the lab. They put half in tanks with healthy anemones, and half in tanks with anemones that had been bleached with hot water. The anemones were still living and had not yet shrunk, but their algal companions were gone.
After two weeks, the researchers moved the clown fish into tanks pumping out oxygen every few minutes and measured how much oxygen the fish consumed during their most sedentary times.
This gave them an idea of the fish’s basal metabolic rate — that is, how much energy the animals needed to sit around and do nothing. (That’s akin to a human “lying motionless on the couch doing nothing but still being awake,” Dr. Norin said.)
The researchers found that the clown fish living with the bleached anemones had a higher basal metabolic rate than the fish living around healthy anemones. That is, the fish needed more energy to just stay alive around bleached anemones.
You can think about basal metabolic rate as a kind of cost-of-living measure, like the minimum salary you need to get by in New York City. And just as you need to budget for emergencies, social or entertainment expenses, the fish need additional energy for behaviors like risk-taking and foraging that may lead to reproducing and surviving longer.
Researchers have found that in some places with warming water, fish with increased basal metabolic rates can’t get enough energy to adapt and also to conduct important survival tasks, like swimming. And Dr. Norin warns that as oceans continue to warm, clown fish could face a double whammy from bleached anemones and rising temperatures that increase the energy needed to survive.
More research is needed to determine whether the metabolic stress is just temporary. With more time, the anemones might recover or the clown fish could acclimate to conditions.
The tale of the stressed out clown fish is a reminder of the myriad ways in which humans are damaging the ocean, Dr. Norin said: “At some point, we will open our eyes and see that we really need to do something.”