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Mummichogs: A Fish that Opened an Evolutionary Window

Imagine a small, slender, slippery fish swimming through the water. Now flip to an image of animals on land; creatures like you and me. Is it difficult to imagine that land animals evolved from aquatic creatures over 395 million years ago? Probably!

Fortunately, there’s a special kind of fish alive today that can help us understand how fishes began the transition to living on land: the mummichog. Mummichogs—known as Fundulus heteroclitus to scientists—are a small, silvery green and white fish. It lives within the intertidal zone of North America’s Atlantic coast.

In the intertidal zone, the sea’s water level fluctuates up and down with the cycles of the tides. As the tides rise, seawater floods small depressions on the shore. It fills these tidepools with aquatic plants, crabs, barnacles, and fish, and mummichogs, too! But soon, the tide starts to recede. The water drains away. The salt in the water doesn’t evaporate, so the water becomes saltier and saltier. The scorching sun rapidly warms the tidepool. To the stranded fish who need cool ocean water, the tidepool becomes unbearably hot. And if the tides don’t rise and refill the tidepool soon enough, the waters evaporate or drain away entirely. The plants and fish which rely on water to breathe begin to drown in the air and die . . . unless they begin a fight for survival.

This is exactly what a type of fish known as killifish do: they fight to survive in the intertidal zone. In fact, the harsh conditions of the ocean’s intertidal zone encourage many organisms to evolve unique adaptations. Many killifish have evolved to be amphibious; that is, they can live both in and out of the water. Mummichogs and the mangrove rivulus (also a killifish) can breathe oxygen from the air with their specialized gills and skin.

The Gulf of Maine Intertidal Zone, where the mummichog research took place.

That’s not all killifish can do. If the tidepool’s water drops too low or becomes uninhabitable, the mummichog has a unique trick up its sleeve. It locates where the ocean is, then jumps and flops its way back into the safe sea.

Imagine a typical fishing experience: you reeled in a fish, grabbed it in one hand, and took it off the hook. But the fish was wriggling in your hand. You lost your grip; the fish was slippery and worked itself free. The fish fell to the ground and started flopping on its side.

But, if you’d caught a mummichog and dropped it on the ground, the mummichog would’ve done something very unique. In a 2016 study, scientists used a high-speed camera to record mummichogs jumping towards water. They found that, for a brief moment before jumping back in the water, the mummichog positions its body into an upright position.

This behavior—righting oneself into an upright position—is quite like what land animals do! As with land animals, the scientists theorized that an upright position allows mummichogs to gather more visual information than they could by laying on their side. By being upright, a mummichog can improve its visual field and search for prey or predators. The scientists then hypothesized that mummichogs use their upright position to gather visual cues, like where light is coming from, to navigate.

By placing mummichogs on a wet table, the researchers found that mummichogs preferred to jump towards either a lit shimmering container of water or a container of water covered with shiny foil. This experiment suggested that muscling themselves into an upright position may help mummichogs look for shimmering, watery light before they somersault into the water.

Watch exactly how a mummichog fish jumps in the video above. Via CornellCast.

Thanks to this scientific research, we have a unique insight into how swimming fish evolved into walking land animals. By investigating how the humble mummichog leaps into the water, it’s easier to understand the fascinating link between fish and people.

* This post’s featured image comes from the Chesepeake Bay Program on Flickr and is licensed via CC BY 2.0