While being a "fish out of water" sounds like a bad thing for most of us, there are actually a surprising number of fish that have evolved to emerge from their subaqueous homes and onto land. Much like frogs, who live a life of half-and-half, these fish are amphibious.
Take the lungfish, a family of ancient fish from about 400 million years ago, which do indeed have lungs, and will drown if they spend too much time underwater. Eel catfish, meanwhile, can propel themselves out of the muddy swamps they inhabit to lunge at their favorite terrestrial snacks, like beetles. Then there are grunions, sardine-sized fish that leave the water during mating season and lay their eggs in the sand across beaches in California.
But among the amphibious fish, it is the mudskipper, arguably, that has best perfected its ability to thrive on land.
What Is a Mudskipper?
Mudskippers — of which there are 25 species, inhabiting mudflats, swamps, and mangrove forests from Africa to South Asia to South America — spend more than half of their life on land, doing everything from eating to mating out of the water.
Learning more about mudskippers helps scientists tackle a central evolutionary question about how vertebrates made the transition from being aquatic animals to being terrestrial organisms also, says Malcolm S. Gordon, a professor emeritus of biology at the University of California, Los Angeles and author of the book Invasions of the Land: The Transitions of Organisms from Aquatic to Terrestrial Life.
In fact, although no two species of mudskippers are identical, or function exactly the same, there are a couple of neat features that distinguish most of them from the rest of the fish we’re used to hearing about.
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How Do Mudskippers Breathe?
If fish have gills to extrapolate oxygen from the water even at depth, how do mudskippers breathe in the open air?
There are two ways that they can go about this. In the first, they breathe through cutaneous respiration, so the amphibious fish are constantly capturing oxygen from the air through their soft skin — as well as the lining in their mouth and throat — which is filled with capillaries that can absorb oxygen.
In order for this to work, however, they need to be wet, — and that’s why they spend most of their time out of the water. Still, in moist environments like mudflats, mudskippers can be seen rolling around in the mud to keep a sheen on their skin, flipping side to side in puddles to cool down and freshen up.
The second method is more direct: A mudskipper’s gills trap water in their cavities and store it for longer-term oxygen retrieval. This is why some species can often be observed making their way to a body of water, opening their mouth and taking gigantic gulps of it.
“They will put their jaws in the water and you can see them pumping water or sucking water in," says Gordon. "And then they end up puffing out their gill covers, and they have a mouth full of water that they walk around with,” says Gordon.
How Do Mudskippers Walk?
To trudge around on land, mudskippers use their pectoral fins like two tiny arms. Their fins, in fact, have mini-joints similar to the elbow and shoulder, allowing them to fold their flippers for pushups off the land, propelling them forward one step at a time.
This isn’t necessarily unique to mudskippers, though. There are lots of aquatic fishes that use their fins in similar ways to the mud skippers on the land, says Gordon, like gobies, a group of more than 2,200 species of fishes to which mudskippers are related.
“Chances are very good, from an evolutionary point of view, that the way in which the mudskippers move using their paired pectoral fins and their pelvic fins evolved in the water,” he says.
As the name suggests, mudskippers also literally skip on mud. But that’s not thanks to their pectorals as much as it is thanks to their tails. By bending their tail rapidly, they're able to use it as a lever and launch themselves into the air, a behavior called a C-start. This, too, evolved in the water.
“C-starts are ubiquitous among fishes,” says Gordon. “That's how fishes in general try and get away from predators.”
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When they’re not out hunting for food — snails, insects, small plants, anything the detritus of the midlands has to offer — mudskippers, especially the smaller species, build long, complicated burrows underground in between tides. To excavate their hideaways, they shovel long tube-like chunks of mud into their mouth, regurgitating them outside of their tunnel once they’ve reached the open air. These are great for hiding from predators, resting, or aiding their breathing and thermoregulating abilities.
For instance, when it’s hot and dry out, they hide in there to stay away from the sun — although they have special compounds in their skin that act as sunblock in the meantime — and to cool down. On the flip side, when the tide is high and they’re underwater, they hide bubbles of air in the tunnels to store some oxygen for themselves.
“The species of mudskippers that make raised walls around their burrow entrances often like to rest on top of them observing the mud flat around them,” says Gordon. “They clearly are observation posts for watching for possible food items, predators, potential mates and other mudskippers invading their territories.”
What’s more, to protect the patches of land they’ve burrowed in, male mudskippers engage in theatrical fights between themselves, with their mouths gaping and painted dorsal fins splayed out in a display of power, and flop onto each other violently.
Why Do Mudskippers Have Eyes on Top of Their Heads?
Unlike most fish, the mudskipper’s eyes are on top of their heads like periscopes for greater visibility, they can move independently of each other, and they can… blink.
Blinking is typically reserved for limbed vertebrates. But because mudskippers evolved to thrive outside of water, they also developed this attribute. Blinking helps mudskippers moisturize and keep their eyeballs wet, clean them from anything that might obstruct their vision, like mud, and reflexively protect them from, say, objects flying their way, according to a paper published in 2023 in the Proceedings of the National Academy of Sciences. Their blinking method is also highly sophisticated, as they retract their eyeballs inwards into a skull cavity replete with water.
But mudfish didn't require a lot of changes to their eye apparatus to evolve this behavior, says Brett Aiello, a biologist at Seton Hill University and one of the study's authors. They're relying on the same six extra-ocular eye muscles to retract the eye into the skull, so they took the structures they already had and they started using them in a completely different way.
“So it really demonstrates that you can produce a very complex multifunctional behavior using this really rudimentary or basic set of anatomy,” says Aiello.
Plus, in this transition to life on land, mudskippers are able to deploy a single behavior to produce three completely different unique, complex functions — in other words, the evolution of a multifunctional behavior.
“This really helps us reimagine how those early tetrapods, the lineage that led to us, underwent this same transition and may have had to evolve these same types of adaptations,” Aiello says.
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What Can Walking Fish Teach Us About Evolution?
Hypothetically, we can look at walking fish like mudskippers as nature’s example of an ongoing evolutionary experiment, says Gordon.
If the human race does not get itself promptly organized to halt current global climate change at some point, he continues, there is the remote possibility that a lot of important ecological niches and environments in the terrestrial world will become vacant.
“If that happens, and there happen to be some amphibious fishes that survive and are in the right places, there is a very small chance that a new invasion of the land could begin,” says Gordon. “I hope this is a very far-out scenario.”