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The Mangrove rivulus, a species of Caribbean killifish, can survive out of water for months, hiding inside damp mangrove logs. Photo: <a href="">USGS</a>
The Mangrove rivulus, a species of Caribbean killifish, can survive out of water for months, hiding inside damp mangrove logs. Photo: USGS

What can fish do when the water goes away?

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From walking catfish, to snakeheads, to species of killifish, some fish actually survive outside of water for a surprising length of time.

Martha Foley and Dr. Curt Stager discuss the old cliche "like a fish out of water," and about the strategies some fish use to do just fine out there in the air.

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Martha Foley: Here’s our opportunity to add some meaning to the phrase “fish out of water,” right? You know that old cliché.

There are fish out of water. I think we’re learning more about more fish being out of the water; there’s some horrible snake-head fish that’s an invasive species, and then there’s the catfish that we know “walk” between little wet spots. Are they out of the water by accident and just happen to be able to survive out there for a little while?

Curt Stager: For most of those kinds of species you could say it’s by accident. They’re not really out there because they love it. A lot of them will live in a habitat that’s sort of ephemeral; it’s going to dry up or just get really hot and nasty and they’re sort of driven out. But because they can survive out of the water they are able to get to another place and take up where they left off.

MF: So it’s an advantage for a little while?

CS: Yes. It turns out there are some that take this to the extreme. They almost seem like you could technically call them amphibians, in a way. Not in the sense like a salamander or a frog, but just in the literal sense of the word amphi-bio: two lives, living in the water and the land. There are some fish that can spend days or even weeks out of the water and still do just fine.

MF: Describe one to me. Like what would it do out of the water?

CS: I think the world champion of this is something that looks just exactly like what you think of as a normal little fish, two or three inches long. Technically, it’s a killifish, which is a minnow-looking thing. You can find versions of them everywhere in the world, but these are in the Caribbean area, Brazil, the South American area, down in there. These can live for up to two months, the record so far, on the land.

MF: What do they do there? Are they just flopping around?

CS: The way people find them is you go up to a log, usually in mangrove forests, which are on the coast where there’s salty water and it’s muddy. If a mangrove tree gets rotten and the termites dig these tunnels in it, it falls over and lays there on the forest floor. If you go up to that log and roll it over, the fish come flopping out underneath it. If you break the log open to the termite tunnels the fish are in there, lined up nose to tail sharing the tunnel. And when it is moist enough outside the log, they’ll come flopping out and kind of wriggle around on the forest floor. They’ll eat ants so they’re getting food. They don’t have any competition from other fish, that’s for sure. And at the same time they can live in the water. In which case they have totally different behaviors and you’d never know anything was unusual about them.

MF: What this brings to mind, obviously, is evolution. Fish, amphibians, us.

CS: It sounds so amazing, how could that have ever possibly happened? Why would a fish ever come out of water?

MF: It just happens, I guess.

CS: It happens all the time. It’s happening now; they’re out there doing it. Not turning into frogs or anything, but it does make the point there are ecological niches on land for fish and these things don’t look unusual at all.

MF: Are there any of these fish, out of the water fish, in North America? Here in the Northeast, would we find one of these fish? Do any of our fish do this kind of thing?

CS: I’m not aware of any of them. There could well be. In fact, that gets back to the fish in the Caribbean, these little killifish. They are called Mangrove rivulus and specialists thought they were rare down there too. They didn’t even really notice them, but because they can breathe through their skin in the air they don’t rely on their gills so much. The normal way to collect fish in these streams and habitats was to throw a fish poison like rotenone in the water which blocks their gills and stuns the fish. The other fish would float up while these guys would just swim away no problem. So people didn’t even know how common they were. As far as I know, not being a real fish specialist, there might be something in North America. But all the ones I know of offhand are actually invasive exotic species brought in from mostly Southeast Asia.

MF: Well, we’ll keep our eyes open.

CS: Now that we’re on the topic and I think back to my childhood, I remember when I was in Maine and I caught a big eel. I brought it home proudly in a bucket of water and left it on the porch of the house about 200 yards from the lake. The next morning it was gone. I went looking for it and about 200 yards away, right on the edge of the lake, there it was slithering towards the water.

MF: So you actually were an eyewitness to eel going over land?

CS: To walking eels.

MF: Okay, well I’ll keep my eye out for that. I’ve heard jokes about eels going across lots, but never seen one.

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