How Does a Mudskipper, a Fish That Spends Time on Land, Keep From Drying Out?

The Mudskipper Goes Back to the Water Within a Minute of Eating on Land

On a tidal flat, mudskippers eat a mix of small sea animals and insects, and hunting on the mud looks like a bend of the body followed by a bite [1]. A Japanese research team (Katayama and colleagues, 2022) watched mudskippers on a tidal flat and found that they moved into water within a minute after eating on land [1].

A mammal can feel thirsty before it is short of water, because the mouth and gut send early signals to the brain. Humans do this too. The researchers asked whether a fish goes to the water before it is dehydrated. In aquarium tests, the fish went to the water to drink within a few minutes after they began eating, even though their bodies were not yet dehydrated [1]. The authors read this as an “anticipatory” response: the fish acts ahead of the problem instead of waiting for it. The paper adds that these fish carry a store of water in their mouth and gill chambers while on land, and that, unlike amphibians, they cannot take in water through their skin [1].

Rolling on Its Back and Flapping a Fin Wet the Mudskipper's Skin Before It Dries

The same study looked at a stranger habit: the full-body “roll,” in which a fish flips over onto its back in the mud, often together with movements of the pectoral fins, the fins just behind the gills [1]. Nobody had treated these behaviors as ways of managing water. The team exposed fish to low humidity, and the rolling and fin movements became more than five times as frequent, which the authors interpret as moistening the skin before the whole body has lost water [1].

So there are two moves, and both come early. Drink soon after eating, and wet the skin as the air dries it. One caution: the study used P. modestus and its results may not hold for every mudskipper species.

Another study (Sakamoto and colleagues, 2015) injected two brain hormones, arginine vasotocin and isotocin, into the fish. The fish then stayed in the water longer, for 1 to 8 hours, and left it less often. In fish kept out of water, the brain made about 3 and 1.5 times more of the instructions for those two hormones [5]. The authors suggest these hormones may be involved in the fish’s preference for water. This links hormones to behavior. It does not tell us how a mudskipper “feels” about the shore.

A Father Mudskipper Fills an Egg Chamber With Air Because the Burrow Water Has Almost No Oxygen

Staying alive is one job. Raising young is harder. Mudskippers dig burrows in the mud and lay eggs in them, but the water in those burrows is extremely low in oxygen, and eggs could not survive in it [2] [3].

In 2007 a team led by Atsushi Ishimatsu at Nagasaki University reported what the species does about this. It lays eggs on the walls of a chamber inside the burrow that is filled with air. The male guards the burrow and, at each low tide, carries mouthfuls of fresh air down into the chamber. He does this more when oxygen in the chamber is low [2].

When the eggs are ready to hatch, the chamber floods on a rising tide at night, and the flooding sets off hatching [2]. A later review by Ishimatsu and Jeffrey Graham adds that hatching has a window of only about 5 to 6 days. That makes it matter where in the tidal zone the male digs and when the eggs are laid, because the tide has to reach the burrow at the right time [3]. In short, the father’s job is to match the tide to the egg’s clock.

Young Mudskippers Leave the Water at About 30 Days Old With Little Change to Their Bodies

A team led by Shoichi Inaba at Nagasaki University raised P. modestus in the laboratory and watched the young come ashore. At about 30 days after hatching, the fish gradually shifted from preferring water to preferring an artificial shore and then land. After about five days of coming out by their own choice, they could move on land with their pectoral fins. Thirteen days after that, they began to feed on land [4].

The surprise is how little the body changes. The head changes shape in ways that suit life on land, but the abstract says the change is small compared with what some other animals go through. When the researchers kept late-stage larvae under water for 40 to 42 days, the abstract reports no statistically significant difference in survival or in the overall form of the body and gills [4]. These were laboratory results, so the days in the wild may differ.

Scientists are interested because early vertebrates once moved from water to land, and fossils tell us little about their soft bodies and behavior. Mudskippers are living fish that make a similar move, so they may offer clues [4]. A clue is not a copy of that history.

So How Does a Mudskipper Keep From Drying Out? Early Moves, Water Carried Along, and Air in the Nursery

Putting it together: the mudskipper does not wait until it is dry. It goes back to the water soon after eating, wets its skin by rolling and fin movements when the air is dry, and carries a little water in its mouth and gill chambers. Its young do not need a big body rebuild to leave the water, and its eggs grow in a chamber of air, tended by a parent who tracks the tide.

What is still open? The authors of these papers describe these behaviors, but how the fish senses that it is drying, or how the hormone signals above turn into choices, is still being worked out.

If You Visit a Mudflat, Watch From a Distance

If you ever visit a muddy shore where mudskippers live, stay on firm ground with an adult and follow local safety advice. Check the tide times first and leave well before the tide comes back in. Watch from a distance, because the fish retreat when you come close. Count how often one rolls in the mud or goes back to the water edge. Do not enter soft mud, approach burrows, or catch the fish. It is easy to sink in.

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