Mrs. TweedieOctober 2010

Swamp Scramblers

Running across shoreline sludge, sometimes leaping high into the air, the mudskipper is a fish out of water. But it's definitely not out of place.

Though they're truly fish—with fins and gills to prove it—mudskippers are so at home on land, they can not only walk and breathe air, they can also climb trees.

Sound weird? Maybe, but then the world they live in—the mangrove swamps and mud flats of Asia, Africa, and the South Pacific—is a pretty unusual one as well.

Covered with water at high tide and a lot of oozing muck when the water goes back down, these areas can seem inhospitable. Yet mangrove trees flourish here.

Propped up by their long, stiltlike roots, these trees are able to both rise above the waves and stand firm in the shifting mud.

They also provide the climbing mudskippers with the perfect place to snatch a meal of insects or spiders and a safe refuge at high tide, when larger predatory fish patrol the water below.

How do mudskippers move on land? The answer lies in their fins. The sturdy fins near the front of the fish are not only extremely strong, they're also bent to look a lot like arms. Supporting its weight on these fins, the mudskipper lifts its body off the ground and pushes itself forward at the same time, a bit like a seal.

The fins at the back of its body are the ones the mudskipper uses when climbing. Shaped like small suckers, they allow the fish to cling to mangrove roots while their front fins find a good grip even farther up the limb.

When frightened, the mudskipper can also scurry across the mud or leap into the air with a few powerful lashes from its tail.

Of course, being able to move on land is no good if you can't also breathe. Mudskippers have that problem licked too. Amazingly, they have been known to survive out of water for two and a half days. Most other fish would die in just a fraction of that time.

That's because all fish, even mudskippers, use their gills to get oxygen from water. The gills are packed with tiny blood vessels, and, as the water passes over them, the oxygen in the water is absorbed into the fish's bloodstream. Normally when fish leave the water, their gills dry out and become useless.

Not a mudskipper's gills, though—they're always moist!

Before leaving the water, mudskippers soak up water into special gill chambers, which are storage pockets located around each gill. Once on land, the fish can absorb oxygen from this stored water by rolling its large, movable eyes back into its eye sockets.

This movement swirls the stored water around the pouch and remoistens the gills. It's a great trick, but there's one small catch: eating.

As mudskippers swallow insects and other tasty treats, the water stored in the gill chambers floods out, leaving the mudskipper high and dry. Then the fish has to quickly scoot back to a nearby puddle for a refill.

But moist gills are only part of the answer for the land-loving mudskipper. Mudskippers can also soak up oxygen through their skin, as long as it is kept wet.

Some scientists think that's why mudskippers roll in mud puddles or lounge on the sun-drenched flats with their tails in the water.