It is a common belief that fish are confined to aquatic environments. However, some remarkable fish species challenge this perception by demonstrating the ability to move and survive on land. These creatures represent extraordinary examples of adaptation, showcasing how life can evolve unique strategies to thrive. Their terrestrial excursions highlight the diverse paths evolution can take, blurring the lines between aquatic and terrestrial life.
Fish That Traverse Land
Several fish species can move across land. Mudskippers, found in mangrove swamps and tidal flats, are well-known amphibious fish. They use their muscular pectoral fins, much like crutches, to move across muddy surfaces and climb onto roots and low branches.
The walking catfish (Clarias batrachus), native to Southeast Asia, is another example. This fish uses its strong pectoral spines and a wriggling, snake-like body motion to propel itself across land. They possess specialized accessory breathing organs, enabling them to survive out of water for extended periods.
Lungfish, such as the West African lungfish, are also capable of terrestrial movement. They use their strong, thread-like pectoral and pelvic fins in a limb-like fashion, allowing them to crawl.
The epaulette shark (Hemiscyllium ocellatum), a small reef-dwelling shark, is known for its ability to walk across coral reefs and out of water. It moves by bending its body and pushing off the substrate with its paddle-shaped, muscular paired fins. This behavior helps them navigate shallow reef systems.
How Fish Move Beyond Water
Fish that move on land have specific anatomical adaptations for terrestrial locomotion. Their fins are modified; pectoral and pelvic fins are often more muscular, robust, and sometimes jointed, allowing them to bear weight and provide propulsion. Mudskippers, for example, have pectoral fins with strengthened shoulder girdles for leverage. Epaulette sharks possess paddle-shaped fins with modified musculoskeletal attachments for their walking gait.
Skeletal support is important for these fish to counteract gravity outside of water. Some species, like hillstream loaches, have robust pelvic girdles connected to their spine, providing body support.
These fish have evolved specialized respiratory systems for air breathing. Unlike most fish, terrestrial fish often have modified gill structures, highly vascularized skin, or rudimentary lungs. Walking catfish, for example, possess a labyrinth organ, an accessory breathing organ, which allows them to extract oxygen directly from the air. Mudskippers can absorb oxygen through their moist skin and the lining of their mouth and throat. These adaptations allow them to survive in oxygen-poor aquatic conditions or completely out of water.
Why Fish Evolved to Walk
The evolution of terrestrial locomotion in fish is driven by ecological pressures and offers survival advantages. One primary reason is the need to seek new, more favorable habitats. When aquatic environments dry up or become overcrowded, moving across land allows fish to escape deteriorating conditions and find new bodies of water. This dispersal is important for species in environments prone to seasonal droughts.
Accessing new food sources is another evolutionary driver. Terrestrial excursions provide opportunities to prey on insects, crustaceans, or other small organisms found on land or in very shallow waters. This expands their dietary options and reduces competition for aquatic resources.
Oxygen deprivation in water is a factor contributing to this adaptation. In stagnant ponds or during hot periods, water can become depleted of dissolved oxygen. Air-breathing fish can leave these oxygen-poor waters to access atmospheric oxygen. Moving onto land also serves as a strategy to escape aquatic predators. For some species, land excursions may also play a role in reproduction, allowing them to lay eggs away from aquatic threats or to find mates.