Photograph by Andy Turko, University of Guelph
Read Caption

Humans trying to cool down might jump into water. But a new study shows that when an amphibious fish called the mangrove rivulus (Kryptolebias marmoratus) feels warm, it does the opposite—it jumps out.  

Photograph by Andy Turko, University of Guelph

To Cool Off, This Fish Leaves the Water

Researchers say the species reacts to warm water and lowers its body temperature quickly while on land.

The mangrove rivulus, a drab, elusive fish found from Florida to Brazil, has been observed flipping out of hot water and onto solid ground. In half a minute or less, its body temperature falls to match its new surroundings. 

This is the first time a fish has been shown to air-chill itself, scientists reported Tuesday in the journal Biology Letters. And it must come in handy, since the rivulus’s home waters have been reported to reach a sweltering 100°F (38°C).

Here, see if you can spot it jumping and cooling off in this video (if not, we'll slow it down later):

Did you catch it? Watch now:

(There are more examples below.)

“These fish always have this escape route,” says study co-author Patricia Wright of the University of Guelph in Ontario, Canada. “If the water starts to warm up, off they go.”

A number of amphibious fish have the ability to catapult onto dry land, but the rivulus lives in the tropics and subtropics, where humidity is high and water and air temperatures are roughly the same. Researchers wondered why an overheated fish would jump from warm water to warm, humid air.

To find out, the scientists heated water and then filmed the fish with a camera that measures body temperature. The fish began flinging themselves out of the water at about 97°F (36°C), almost certainly in response to the heat, the researchers say. Within 30 seconds, the fish were as cool as the damp filter paper on which they landed. Within 60 seconds, they were actually slightly cooler than the paper, a phenomenon that still has the scientists scratching their heads. Other amphibious fish have been suspected of doing the same, but none have been caught doing it until now. 

The researchers haven’t repeated the experiments in the wild, but they think it’s likely that the fish hurl themselves out of the water to beat the heat there too. Ordinary fish die if overheated, says comparative physiologist John Eme of California State University, San Marcos, who was not associated with the research. So it makes sense for the rivulus and other amphibious fish to find cooler ground, he says.

The rivulus has also been found to leave the water for other reasons. The fish, which dwell in puddles and pools and even old logs among mangrove trees, strike out for land when the water is too acidic, too high in carbon dioxide, or too low in oxygen. And, they may leave the water to pursue prey or escape combat with a rival. Other species of fish may head for dry ground to avoid low-oxygen water or to eat, and some “jump onto lily pads to escape predatory sunfish,” Wright says via email.

“They bend their tails up to their bodies so they’re really curved, and then there’s this explosive burst,” says study co-author Andy Turko of the University of Guelph.

Once out of water, they can wriggle to their desired locations and even navigate barriers.

“I love these fish,” Wright says. “They’re very boring looking, but they’re hiding all the remarkable things about themselves inside.”