Fat Burning Mode: How one fish survives out of water

Reference: Rossi, G. S., & Wright, P. A. (2020). Hypoxia-seeking behavior, metabolic depression and skeletal muscle function in an amphibious fish out of water. Journal of Experimental Biology, 223(2). DOI: 10.1242/jeb.213355

The word “dormancy” often brings to mind thoughts of volcanoes, or perhaps even a bear hibernating through the winter. But how many would imagine a fish, smaller than a crayon, laying in the shelter of a rotting log? The mangrove rivulus, Kryptolebias marmoratus, is not only dormant for periods of time, but is such on land – an unusual habitat for most finned creatures. These fish may not lay still for the length of a winter, but they can live successfully on land for up to 60 days – an impressive feat, considering that the average human can only survive underwater for several minutes with no diving equipment. The mangrove rivulus can end up on land involuntarily, due to stranding when the tide lowers in its mangrove habitat, or voluntarily, to avoid a predator or poor water conditions.

The mangrove rivulus, Kryptolebias maramoratus, in aquaria. Image courtesy of Jean-Paul Cicéron via Wikimedia Commons.

It is generally thought that when on land, K. marmoratus does not eat at all. So, how is it able to survive on land for over two months while taking in no additional energy? Giulia Rossi and her team at the University of Guelph sought to shed some light on this question by examining a few different aspects of K. marmoratus with respect to oxygen conditions on land. Two different strains of mangrove rivulus, one from Florida and one from Belize, were studied, to see if differences in habitat play a role in oxygen use.

What did they do?

The researchers sought to examine if K. marmoratus exhibits a preference for low oxygen conditions when on land. Low oxygen concentrations decrease K. marmoratus‘s resting metabolic rate – a measure of energy use per unit time when a body is at rest – after three weeks in air. Because of this, it makes sense that if a fish is stranded on land for a long period of time and can’t obtain food, it would seek to find shelter in an area of low oxygen concentration, to survive on its available energy stores for as long as possible.

Secondly, researchers examined how the fish were using the energy stores in their body during prolonged air exposure in both normal and low oxygen conditions. They looked at the usage of carbohydrates and fat, which are the primary sources of stored energy. They also examined muscle, because loss of muscle area or tissue could indicate that fish are burning through protein, their last energy resource after using up carbs and fat. Fish should try to avoid this outcome, as a decreased muscle mass is dangerous when escaping predators or searching for food.

To examine how differences in muscle mass may affect survival, the scientists tested the mangrove rivulus for their terrestrial locomotor performance (yes, these fish can effectively jump on land!) after air acclimation to normal and low oxygen conditions.

Kryptolebias marmoratus performing a typical movement on land. This movement is called a “tail flip”. A shows the usual starting position on the ground, B shows the fish bending its head over its tail and pushing off the ground, and C shows the fish once airborne. Images courtesy of Francesca Giammona.

What did they find?

In the search to discover if K. marmoratus preferred a particular oxygen concentration on land, fish were given the choice of entering a low, normal, or high oxygen concentration container, based on typical concentrations in their native habitats. Fish from Belize preferred a low oxygen environment while Florida fish did not show a preference.

Examining energy usage, both mangrove rivulus strains used carb energy more when in air than in water, and oxygen concentration had no effect. The fish broke down more fat when placed in air, though only in normal oxygen conditions, which makes sense because fats are broken down for energy quickly in many dormant animals.

As for muscle, changes were only found in the normal oxygen condition. Fish from Belize had lower muscle area compared to those from Florida and fish in low oxygen. This is evidence that Belize fish are burning protein in normal oxygen conditions. This hurt performance, with Belize fish not jumping as often as fish from Florida. Both strains couldn’t jump as far under normal oxygen concentrations compared to low oxygen. The authors suggest this could be evidence that lower oxygen concentrations may allow fish to better move on land, increasing survival.

What does it mean?

Lower oxygen concentrations on land offer huge benefits to K. marmoratus. Through lowered resting metabolic rate, lower oxygen concentrations allow fish to use less energy when dormant on land, where fish must rely on their own energy stores of fats and carbohydrates to survive.

When in lower oxygen concentrations, fish used less of these energy stores, allowing them to retain muscle function and locomotor performance. Mangrove rivulus from Belize preferred lower oxygen concentrations and burned more protein in normal oxygen compared to Florida fish, suggesting that oxygen concentration is more important to this strain. Since Belize and Florida have slightly different weather patterns, currents, and habitats for the mangrove rivulus, it makes sense that individuals from these regions would behave differently. After all, not every bear from every location will hibernate the same way, much like all people don’t sleep the same way.

Why does this matter?

The mangrove rivulus is an unusual fish that spends a lot of time out of water. By studying how these tiny fish come onto land and how they survive there, the mangrove rivulus can help us understand how the first creatures crawled from water onto land millions of years ago. For now, who knows, maybe the next time you think of an animal hibernating or at rest, perhaps the second or third thought to come to mind will be of a small fish, taking shelter from the harsh sun under a shady, moist log.

2 thoughts on “Fat Burning Mode: How one fish survives out of water

    1. The maximum amount of time they’ve been documented out of the water is 66 days, which is pretty impressive!

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