The mangrove rivulus, Kryptolebias marmoratus, is a plain little fish that spends most of its life in small pools or crab burrows in mangrove forests. The fish's range stretches from Florida to Brazil, where temperatures of the shallow, brackish waters in which it lives can reach 38 degrees Celsius (100 degrees Fahrenheit), which may cause heat stress and even death. Luckily, this fish is an amphibious one that can respire on land. And according to findings published in Biology Letters this week, when it finds itself in hot water, the fish escapes by jumping out – and cools off within seconds.
University of Guelph’s Patricia Wright and colleagues reared dozens of these amphibious fish for a year at either 25 or 30 degrees Celsius (77 or 86 degrees Fahrenheit), and then later acclimated the adults for a week to varying temperatures before conducting a series of experiments. First, the team observed the threshold temperature for “emersion” (or leaving water, the opposite of immersion) by warming the water in their containers by 1 degree Celsius a minute (1.8 degrees Fahrenheit). Then, using a thermal imaging camera, the team recorded the body temperature of the fish leaping out onto moistened filter paper.
Raising water temperatures caused emersion in all of the fish tested. That’s 65 out of 65. Control fish reared at and acclimated to 25 degrees Celsius emersed between 28 and 39.5 degrees Celsius (82.4 and 103.1 degrees Fahrenheit); but fish exposed to elevated temperatures for a week tolerated warmer water better and emersed at higher temperatures. Rearing temperature didn’t seem to have an effect.
In the wild, the mean relative humidity right above crab burrows occupied by the fish was nearly 84%, and the temperature of the air right above them is roughly the same as that of the water. So why jump in and out? Turns out, once the fish leap out, water evaporates off and cools down their body temperature almost immediately. In just 30 seconds, the fish were barely distinguishable from the filter paper in the thermal images; in one minute, they were slightly cooler than their background. This rapid heat loss effect was more pronounced when it was less humid – though evaporative cooling worked even when the relative humidity was 95%.
When a mesh barrier prevented them from jumping out, the fish began to lose their equilibrium as temperatures reached 4 to 5 degrees Celsius (7.2 to 9 degrees Fahrenheit) above their emersion threshold. That’s a sign of neurological and muscular problems, Wright explained to IFLScience, but once the fish were placed back into cooler waters, they all recovered fully.
"Thermal flexibility in adult fish provides an advantage in a rapidly changing environment," Wright said in a statement. "As climate change continues, and temperatures in their habitats continue to increase, we could potentially see them jumping more." Though this mitigation strategy is unavailable to fully aquatic species.
Here’s a video of mangrove rivulus jumping in and out of a crab burrow in Belize: