Evolution was halfway through making a four-legged terrestrials when it decided to take a break and forgot to pick up where it left off. The result? A fish that can walk on land.
The evolution of limbs was a crucial plot point in the story of how vertebrates colonized dry land, marked by the fateful divergence between ray-finned fishes and lobe-finned fishes over 400 million years ago. Most of the spiny, scaled fish we know today descend from those ancestral ray-finned fishes, while the lobe fins of the second category of fish eventually evolved into the multitude of limb shapes seen among “tetrapods” (four-legged animals).
As always, nature refuses to color inside the lines of classification we try to draw around everything. In particular, the line between fishes and amphibians is much blurrier than you might think. This ambiguity is epitomized by the mudskipper, a ray-finned fish that enjoys long walks on the beach. Mudskippers gained their terrestrial adaptations in a wholly independent manner from the branch of tetrapod evolution — rather remarkable, considering the similarity in adaptations between these distant groups.
For one thing, mudskippers can breathe through their skin just like amphibians. This mode of breathing, called cutaneous respiration, enables mudskippers to traipse about on land for up to 3 days at a time, as long as they keep their skin and mucous membranes moist. Their gills also help them survive on land by operating like a scuba diver’s oxygen tank in reverse. They retain bubbles of water inside their gill chambers to supply oxygen and keep their gills wet and functioning.
This bubble-carrying ability is what makes mudskippers ultra-attentive parents. After a female mudskipper lays her eggs in an air-filled chamber under the sand, the eggs quickly consume what little oxygen is trapped in the air pocket. In order to maintain oxygen-levels, the male mudskipper gulps air at the tunnel entrance, swims down the tunnel, and releases the air in the egg chamber. Then he swims back the entrance and repeats the process almost continually until the eggs hatch. Someone needs to get that guy a “World’s Best Dad” mug!
Despite having the same bony fins as their fully aquatic cousins, mudskippers use highly-developed muscles to employ these fins like limbs. Although it’s hard to imagine dragging ourselves around with our arms all day, that’s exactly what mudskippers do to move about on land. Their pectoral fins have a unique morphology with strong rays and joints, equipping the mudskippers with greater control and force. Swinging and rotating these pectoral fins propels the mudskipper forward in hops and jumps, hence their common name. Meanwhile, their lower fins are modified to form a sucker that can attach to rocks and tree roots.
The advantages of an amphibious lifestyle are clear: access to the delicious morsels living in the mudflats exposed by low tide. But in order to ingest terrestrial food, mudskippers had to further adapt their feeding mechanisms beyond the simple suction method used underwater. Before surfacing, they fill their mouths with water to form a sort of hydrodynamic tongue. When the mudskipper finds prey, it spits out its mouthful of water and then immediately sucks it back in, taking the prey along with it.
You can watch the mudskippers in action in this footage by BBC Life: