Some fish species can venture out of water and navigate terrestrial environments. This unique biological adaptation offers insights into diverse evolutionary paths organisms take when faced with environmental pressures. Their capacity to survive and move outside aquatic habitats highlights vertebrate evolution.
Reasons for Land Excursions
Fish venture onto land for various reasons, driven by environmental pressures and survival advantages. They escape unfavorable aquatic conditions like oxygen-depleted waters or overcrowding. Shallow, stagnant environments often have low oxygen, making terrestrial forays a means to access more oxygen-rich air. Air breathing provides an advantage when water oxygen levels are insufficient for gill-based respiration.
Another reason for land excursions is to evade predators in their aquatic homes. Terrestrial environments offer temporary refuge from larger aquatic hunters, reducing predation risk. Additionally, some fish move onto land to seek new food sources unavailable in their aquatic surroundings, such as abundant plant and insect life in early terrestrial landscapes.
Migration between water bodies also drives terrestrial movement, especially during droughts or when habitats dry up. The ability to “walk” or “slither” across land allows these fish to disperse to new, more hospitable aquatic environments, ensuring their survival and reproductive success.
Physical Adaptations for Moving Ashore
Fish move and survive on land due to specialized physical adaptations. Respiration out of water requires modifications beyond typical gill function, as gills collapse in air, reducing their surface area. Many amphibious fish developed accessory air-breathing organs, like modified swim bladders (lungs) or specialized structures in their mouths, pharynx, or gill chambers. These highly vascularized organs allow efficient oxygen absorption from the atmosphere. Some species also use cutaneous respiration, absorbing oxygen directly through their moist, vascularized skin.
Terrestrial locomotion requires anatomical changes for movement against gravity. Many walking fish have strong, muscular pectoral fins modified as limbs, enabling them to “crutch” or push across surfaces. The walking catfish uses pectoral fins for upright posture while wiggling its body to traverse land. Other elongated species rely on axial-based locomotion, generating undulations to propel forward. Some combine axial movements with fins to navigate terrestrial environments.
Sensory perception also adapts for life out of water. Mudskippers have high-set eyes for aerial vision to spot predators and prey. Their eyes can retract and moisten, akin to blinking, to prevent dehydration. These adaptations enable fish to overcome non-aquatic challenges, allowing temporary or extended terrestrial existence.
Diverse Examples of Walking Fish
Several fish species showcase terrestrial capabilities. Mudskippers, found in mangrove and intertidal zones, are well-known amphibious fish. They use modified, muscular pectoral fins to “crutch” across mudflats and climb mangrove roots. They breathe through skin and mouth/throat lining when out of water, and hold air in gill chambers. Their high-set eyes provide vision above water, essential for navigating and spotting prey or predators.
Walking Catfish (Clarias batrachus), native to Southeast Asia, move between water bodies. They “walk” by wiggling their bodies and using stiff pectoral fins for support and propulsion. These catfish have a specialized “suprabranchial arborescent organ” above their gills, extracting oxygen directly from air. This enables them to survive out of water for up to 18 hours, especially when dispersing or seeking food.
Lungfish are an ancient group with well-developed lungs, homologous to terrestrial vertebrates. African lungfish can burrow into mud and form a mucus cocoon during dry seasons, aestivating for months or years without food or water, breathing solely through their lungs. While not “walkers,” some lungfish use strong, limb-like pectoral and pelvic fins to propel themselves, resembling early tetrapod locomotion.
The Climbing Perch (Anabas testudineus), native to Asia and Africa, is also capable of terrestrial excursions. They have an accessory air-breathing labyrinth organ above their gills, allowing survival out of water for several days in moist conditions. They move across land using pectoral fins and body wriggling; despite their name, they do not climb trees. Their land excursions are often driven by the search for new habitats or food, particularly during droughts.