While primarily aquatic, some fish species possess the ability to venture onto land. This capacity challenges the traditional understanding of fish, showcasing a biological adaptation that allows them to navigate both terrestrial and aquatic environments.
Physical Adaptations for Land Travel
Certain fish have developed anatomical modifications for movement and survival outside of water. Their fins, particularly the pectoral and pelvic fins, are often adapted to function like limbs, enabling them to crawl, wiggle, or “walk” on solid surfaces. These fins are more robust than those of purely aquatic fish, sometimes featuring joint-like structures or specialized fascia tissue that provides strength and leverage for propulsion. Some species, like the walking catfish, employ a wiggling, serpentine motion, using their stiff pectoral spines to propel themselves forward. Mudskippers use a “crutching” movement, swinging their pectoral fins forward to move across mudflats.
Respiration outside water requires specialized mechanisms beyond traditional gills, which can collapse in air. Many walking fish utilize cutaneous respiration, absorbing oxygen through their skin, mouth lining, or throat, provided these surfaces remain moist. Others possess dedicated air-breathing organs, such as labyrinth organs found in climbing perch and walking catfish, which extract oxygen from the atmosphere. Lungfish have lung-like sacs that allow them to breathe atmospheric oxygen in addition to using their gills.
To counter desiccation, many amphibious fish secrete a protective mucus coating that reduces evaporation and maintains skin moisture. Their eyes are often positioned on top of their heads, offering a wider field of vision for navigating terrestrial environments and spotting predators.
Remarkable Species of Walking Fish
Among the most well-known examples of walking fish are mudskippers, commonly found in mangrove environments and mudflats. They use their pectoral and pelvic fins to “crutch” or skip across the mud, spending significant time out of water. These fish can absorb oxygen through their skin and mouth lining, and they retain water in their gill chambers to keep their gills moist while on land. Mudskippers’ eyes protrude from the top of their heads, providing vision in their semi-terrestrial habitat.
Walking catfish, native to Southeast Asia, traverse land using a wiggling motion. They propel themselves by flexing their bodies and pushing against the ground with stiff pectoral spines. These catfish possess specialized suprabranchial organs that allow them to breathe atmospheric oxygen, enabling them to cross short distances between water bodies. This adaptation has contributed to their success as an invasive species in some regions.
Snakehead fish move across land by propelling themselves forward using their head and back fin. They have a specialized chamber near their gills that allows them to take gulps of air, enabling them to survive out of water for several days. These predatory fish utilize this terrestrial mobility to search for new water sources or prey.
Lungfish survive prolonged dry periods. When their aquatic habitats dry up, they burrow into the mud and secrete a thick mucus cocoon. This cocoon hardens and seals in moisture, allowing the fish to enter a dormant state called aestivation, where they breathe air using their lung-like organs and survive until water returns.
Survival Strategies on Land
Fish move onto land due to ecological and evolutionary pressures. Environmental changes, such as droughts, low oxygen levels, or poor water quality, can force them to seek more favorable conditions or escape inhospitable habitats.
Land excursions also provide opportunities for resource acquisition. Fish venture onto land to find new food sources, such as insects or small invertebrates, unavailable in aquatic environments. This can lead them to unexplored territories with abundant resources.
Terrestrial movement serves as a strategy for predator avoidance, allowing fish to escape aquatic predators. It facilitates dispersal, enabling fish to colonize new areas and expand their geographical range, particularly when moving between isolated water bodies. Terrestrial movement plays a role in reproduction, such as mudskippers building burrows on land for nesting, or other species moving to land to find mates or egg-laying sites.