While most fish swim using flexible fins, some remarkable species possess adaptations that allow them to move in ways reminiscent of land animals. These specialized fins function to propel them across solid surfaces, whether on the seabed or on land. This offers insights into the ancient evolutionary journey transforming fins into the limbs of terrestrial vertebrates.
Fish That Use Fins Like Legs
Several contemporary fish species exhibit “leg-like” locomotion using modified fins. Mudskippers are amphibious fish that spend considerable time out of water in mangrove environments. They use strong, muscular pectoral fins to “crutch” or “walk” across mudflats, moving both fins synchronously. These fins have robust internal structures that provide mechanical stability, distinguishing them from the more flexible fins of most fish. Mudskippers can also breathe through their skin and mouth lining, enabling prolonged terrestrial excursions.
Another example is the walking catfish, native to Southeast Asia, which can traverse dry land. It does not truly walk like a quadruped but uses its pectoral fin spines as anchors while wiggling its body forward. This fish has specialized gill structures, allowing it to breathe atmospheric air and survive out of water for up to 18 hours.
On the ocean floor, frogfish and other anglerfish species use modified pectoral and pelvic fins to “walk” or “clamber” along the seabed. This slow movement assists them in ambushing prey, as it disturbs the surrounding water less than swimming. Sea robins also use specialized pelvic fin rays for walking.
Lobe-Finned Fish: Evolutionary Significance
The evolution of limbs involves a specific group of fish known as lobe-finned fish. These fish are distinguished by their fleshy, paired fins, supported by a central axis of bones and muscles. This structure contrasts with ray-finned fish, whose fins are supported by slender, bony rays. The internal bone structure of lobe-finned fish fins resembles the basic skeletal pattern of tetrapod limbs.
Coelacanths, often called “living fossils” and once thought extinct until their rediscovery in 1938, are a prime example of lobe-finned fish. Their paired fins extend from the body on fleshy stalks and move in an alternating pattern, similar to how a land animal trots. Lungfish, another group of lobe-finned fish, possess the ability to breathe air using lung-like organs and can use their fins to move across land, often with a belly crawl or an alternating gait. These features and behaviors highlight the importance of lobe-finned fish as the ancestral lineage from which all land vertebrates, known as tetrapods, evolved.
The Transition from Fins to Limbs
The transformation of fish fins into the weight-bearing limbs of land animals represents a major evolutionary event. This gradual process unfolded over millions of years. The fossil record provides crucial evidence of this transition, showcasing intermediate forms that bridge the gap between fully aquatic fish and early terrestrial vertebrates.
A transitional fossil is Tiktaalik roseae, often called a “fishapod,” discovered in rocks dating back about 375 million years. Tiktaalik possessed a mosaic of features, including fish-like characteristics such as scales, gills, and fins, alongside tetrapod-like traits like a mobile neck, robust ribcage, and primitive lungs. Its large forefins contained bones homologous to the shoulders, elbows, and partial wrists of land animals, suggesting they could support its body weight and allow it to prop itself up. This indicates that the evolution of hind limbs and a shift towards hind-limb propelled locomotion began in fish before vertebrates fully colonized land.
Research into genetic and developmental mechanisms further illuminates this transition, showing that the underlying genetic pathways for limb development were present in the common ancestor of fish and tetrapods. Studies on modern fish like zebrafish have demonstrated that specific genetic changes can lead to the formation of more limb-like fin structures. This suggests that the capacity for limb development was retained in a dormant state within some fish lineages, ready to be activated by evolutionary pressures.