True legs with digits belong to the tetrapod lineage (amphibians, reptiles, birds, and mammals), making the idea of a fish with legs biologically misleading. However, the phrase captures the extraordinary adaptations found in certain aquatic species that have evolved specialized fins to mimic terrestrial locomotion. These fish use their modified appendages for supporting their body weight and moving across a solid substrate, whether a muddy riverbank or the deep-sea floor.
How Fins Become Limbs: The Mechanics of Aquatic Walking
The transition from a fin used for propulsion to an appendage used for support requires significant structural change. A standard fish fin is a flexible, fan-like membrane supported by bony rays, designed to generate thrust and lift in water. For a fin to function like a limb, it must become load-bearing, achieved through a reorganized internal skeleton and musculature. The paired fins, specifically the pectoral and pelvic fins, are the ones that undergo this transformation.
The primary mechanical challenge for a walking fish is counteracting gravity and friction without the support of water’s buoyancy. This adaptation involves strengthening the skeletal connection between the fin and the main body axis. In typical fish, the pelvic fins have no direct connection to the spine. However, in species adapted for walking, like the cave angel fish (Cryptotora thamicola), the pelvic girdle becomes robust and fuses with the vertebral column, providing a rigid anchor for movement.
Specialized musculature is developed around the pectoral and pelvic girdles to allow for a wider range of motion than is needed for simple swimming. These fins become fleshy, with internal bones that are longer and more robust, acting as levers to lift and push the body forward. This robust structure allows the fish to execute a distinct gait, often described as a “crutching” motion, where the fins are planted on the ground to push the body.
Terrestrial Movers: Fish That Walk on Land
Fish that walk on land are typically found in intertidal zones or shallow, oxygen-poor freshwater habitats, driven by the need to find new resources or escape poor water quality.
Mudskipper
The mudskipper, an amphibious goby native to the Indo-Pacific, uses its highly muscular pectoral fins, which have an elbow-like joint, to execute a unique “crutching” gait across mudflats. The movement involves planting both pectoral fins simultaneously, using them as rigid supports to swing its body forward. This locomotion is supported by anatomical changes, including enlarged fin muscles and the replacement of some bone-connecting tendons with fascia tissue, which provides the necessary stability for pushing off the substrate. To survive out of water, mudskippers breathe air by absorbing oxygen through their moist skin and the lining of their mouth and throat.
Walking Catfish
A different approach is seen in the Walking Catfish (Clarias batrachus), a freshwater species known to traverse short distances overland. This catfish uses stiff, spine-like elements within its pectoral fins to anchor itself to the ground. Forward propulsion is primarily generated by a vigorous, snake-like flexing of its body and tail, pushing the fish over the pectoral spines. The ability to move from pond to pond is enabled by its accessory breathing organ, which functions like a lung and allows the fish to survive out of water.
Deep-Sea Strollers: Fish That Walk the Ocean Floor
Not all walking fish leave the water; some use modified fins for slow, deliberate movement along the benthic substrate, often in deep-sea environments. This behavior is common among ambush predators that conserve energy by waiting for prey rather than actively swimming. The frogfish and batfish, both belonging to the anglerfish order, are prime examples of this deep-sea strolling.
Frogfish
Frogfish have pectoral fins that are highly modified into fleshy, limb-like structures with an internal bone arrangement that mimics a shoulder and elbow. They use these limbs, along with their pelvic fins, to move across the seafloor in a slow, alternating, tetrapod-like gait or a crutching motion. This slow movement is ideal for their ambush predator lifestyle, as they rely on camouflage and a worm-like lure to attract prey.
Batfish
Batfish, such as the Red-lipped Batfish, are poor swimmers and instead use their highly adapted pectoral and pelvic fins to “walk” or shuffle along the ocean floor. Their flattened, disc-shaped bodies are suited for this benthic existence, and their robust fins allow them to prop themselves up on the substrate.
Gurnard
The gurnard, or sea robin, possesses three specialized, detached rays on the lower part of each pectoral fin. These rays are used like individual fingers or legs to “walk” along the seabed. Uniquely, these rays are highly sensitive and contain chemoreceptors, allowing the gurnard to “taste” the sediment and locate hidden prey as it walks.