While most fish are well suited to living in water, some fish also spend part of their time on land. For these amphibious fish, such as the mangrove killifish (Kryptolebias marmoratus), the transition from water to land is taxing. When out of the water, these fish breathe more oxygen and, no longer able to swim, they also get creative with how they move about, sometimes flinging themselves through the air to reach a new location. When the killifish are on land, a stress hormone called cortisol spikes within the first hour but returns to normal after a week out of water. For water-dwelling fishes, long-term increases in this stress hormone help increase how much oxygen a fish breathes in for an extra energy boost during stressful situations. But what if short-term increases in cortisol can help the trying transition from water to land? Sarah Young from the University of Guelph, Canada, and a team of scientists from other Canadian universities turned to the small but mighty mangrove killifish to test this idea.
To understand how cortisol helps their transition from water to air, Young and the team blocked cortisol from being produced in the fish's body, then transitioned them to air for 7 days. They measured how much oxygen the fish breathed in and their cortisol levels immediately after their transition to air, 1 h after the transition, and again at the end of the 7 days. Fish given the cortisol blocker didn't increase their cortisol levels much, but fish that weren't given the cortisol blocker increased their cortisol levels ∼10× after just 1 h in air. This temporary increase in the stress hormone returned to normal after 7 days, but the spike coincided with fish breathing in more oxygen. However, when their cortisol production was blocked, fish didn't breathe in nearly as much oxygen. Together, these findings show that the cortisol blocker worked, and the natural spike in the stress hormone helps the killifish fuel their transition from water to air. Without cortisol and the extra energy boost from breathing in more oxygen, the team thought that fish would probably be less lively on land.
So, the scientists then measured how far a fish could leap, when given a little motivation (a gentle tap on the nose or tail with the tip of a pen). Mangrove killifish have a particularly acrobatic method of jumping, sometimes propelling themselves over 15 body lengths away – the equivalent of a human jumping lengthwise across an entire tennis court. However, when the researchers stopped the fish from producing the stress hormone, the killifish jumped ∼2 body lengths shorter and generally moved around less than fish producing normal amounts of cortisol. This means that the fish aren't as athletic without cortisol, which may be costly when trying to avoid predators.
Young and the team discovered that without cortisol, these amphibious fish can't adjust their bodies and behaviour to land quite as well. More research is needed to discover exactly how cortisol is helping, but it may work together with other stress hormones to tap into the body's extra energy stores, giving killifish a boost when they take on the costly task of transitioning from water to air. With rapid changes to how much oxygen a fish breathes in and how much they jump about within just 1 h in air, the team found just how quickly cortisol can help their life on land. Perhaps unsurprisingly, without cortisol, the mangrove killifish acts like, well, a fish out of water.
