While fish do not possess true “legs” like mammals or reptiles, some species have evolved remarkable adaptations for movement on land. These unique abilities allow them to navigate out of water for various reasons, offering insight into the broader evolutionary processes that shaped the transition of life from aquatic to terrestrial habitats.
Understanding Fish Movement
Fish typically navigate aquatic environments using fins, specialized appendages for propulsion, steering, and maintaining balance. The caudal fin, or tail fin, primarily generates forward thrust. Pectoral and pelvic fins, usually paired, aid in braking, turning, and precise maneuvering. Dorsal and anal fins provide stability, preventing rolling or yawing.
Unlike terrestrial limbs, most fish fins are composed of bony or cartilaginous rays covered by skin, supported by muscles and ligaments. This structure makes them highly flexible and efficient for movement in water.
Fish with Terrestrial Movement Adaptations
A number of fish species exhibit adaptations for movement outside of water, often utilizing modified fins or body movements.
Mudskippers are highly amphibious fish that employ their pectoral fins like crutches to “walk” or “skip” across muddy terrains. Their pectoral fins have internal skeletal structures and musculature for a “crutching” motion, swinging forward simultaneously to propel the fish. These fish can absorb oxygen through their skin and mouth, and retain water in their large gill chambers to breathe on land. Mudskippers frequently venture onto land to forage, seek mates, or escape aquatic predators, often spending a significant portion of their lives, sometimes up to three-quarters, out of water.
Lungfish are another group with notable terrestrial abilities, known for breathing air using lung-like organs. When their aquatic habitats dry up, some lungfish species can burrow into the mud and secrete a mucus cocoon, entering a state of dormancy called estivation, to survive for months or even years until water returns. For terrestrial locomotion, lungfish use their fleshy, muscular fins and powerful tails to wriggle across land. Some species use a distinctive “flicking” motion, planting their head in the mud and then propelling their body forward. Their ability to use pelvic fins to elevate their bodies and propel themselves forward has also been observed.
Walking catfish are also known for terrestrial movements, often moving across land during wet weather to find new water sources or food. These fish do not truly “walk” but use a wiggling, snakelike motion, aided by their pectoral fins for upright stability. They possess specialized gill structures that allow them to breathe atmospheric air, enabling them to survive out of water for extended periods, sometimes up to 18 hours, and cover distances of over a kilometer. This adaptation provides a survival advantage, particularly in environments prone to drying out or where new feeding grounds can be accessed overland.
The Evolutionary Link: From Fins to Limbs
The ability of some fish to move on land offers insight into the water-to-land transition, a significant event in evolutionary history that gave rise to all terrestrial vertebrates. The fins of fish and the limbs of four-legged animals, known as tetrapods, are considered homologous structures, sharing a common evolutionary origin despite their different forms and functions. This deep connection is evident in the skeletal similarities found in the pectoral and pelvic fins of ancient lobe-finned fish, the ancestors of tetrapods.
Fossil discoveries, particularly of species like Tiktaalik roseae, provide compelling evidence of this evolutionary bridge. Dating back approximately 375 million years, Tiktaalik possessed both fish-like features, such as scales and gills, and characteristics resembling early tetrapods, including a mobile neck and robust ribcage. Its pectoral fins contained sturdy internal bones that allowed it to prop itself up in shallow water, hinting at the development of shoulder, elbow, and wrist-like joints.
The transition from water to land was influenced by environmental pressures, such as fluctuating oxygen levels in shallow waters, the search for new food sources on land, and escaping aquatic predators. The development of primitive lungs in fish, allowing them to breathe air, preceded the full move to land, serving as a pre-adaptation for terrestrial life. The fossil record reveals a gradual modification of fin structures into the foundational bone patterns seen in modern limbs, a process driven by natural selection favoring adaptations for movement and survival in a new terrestrial environment.