Fish swim. But they also walk.
One might wonder what use an aquatic organism would have for walking. Water exerts a buoyant force that, for many organisms, is enough to counter gravity and allow them to essentially levitate, whereas the far lower density of air grants few terrestrial beings this gift. Combined with any amount of propulsive force, touching the bottom becomes essentially optional. Flight, by contrast, requires near-total specialization of those beings that would attempt it, because they must counter the lack of meaningful buoyancy with other forces. But just as crabs and crayfish spend most of their time walking along underwater surfaces, so too do some fish walk instead of swim. And they’re all so charming about it.
The best-known walkers are those fish that venture onto land. Many freshwater fish live in floodplains, ephemeral ponds, and rivers whose courses shift over time. These fish can be stranded when their home waters recede. Those that can maneuver on land have an advantage over those that cannot, and numerous lineages have developed this ability. Bichirs (pronounced “bee-sheers”), family Polypteridae, are an ancient lineage with this capability and combine the same kind of undulating motion that characterizes most fishes’ swimming with strong pectoral fins to walk with some adroitness on land.
Perhaps the most famous fish with this ability is the walking catfish, genus Clarias, which has become invasive in Florida in part thanks to its ability to migrate between isolated water sources under its own power.
Lungfish, particularly the African genus Protopterus, similarly, are famous for their ability to aestivate underground in mucous sacs for months or years at a time, emerging from seemingly nothing once water returns to their seasonal homes and quite able to migrate elsewhere overland if it suits them.
Seaside conditions are another ideal case for fish walking on land. Estuaries and intertidal zones experience the marine analogue of flood and recession multiple times per day. Fish that can maneuver between the resulting tide pools, mudflats, and other water sources can avoid those that are running low on oxygen, seek out less maneuverable prey, and otherwise gain advantage over less mobile competition. The most famous fish with this ability are the mudskippers, several genera of goby-like fish from Indian and Pacific Ocean coastlines that spend up to three quarters of their life on land. Mudskippers have joints similar in function to elbow joints in their strongly muscled pectoral fins, making them perhaps the most capable terrestrial walkers of all walking fish. Unlike many others, which venture onto land primarily to seek out new water, mudskippers feed, mate, and more while out of the water.
A personal favorite among these marine fish that use the ability to walk to venture between tide pools is the epaulette shark, Hemiscyllium ocellatum. Where many terrestrial fish make little use of their rear fins to travel on land, H. ocellatum comes close to imitating a quadrupedal land animal in its movements, using both pectoral and pelvic fins to maneuver.
A key specialization that comes along for the walking ride for these fish is the ability to breathe air. To a one, walking fish have some ability to extract oxygen from the air, whether that involves accessory breathing organs, functional lungs, or the ability to keep their gills wet even when out of water. Some, such as the lungfish, are so dependent on air that they drown when kept from it. Most of the situations that encourage fish to venture onto land include reduced aquatic oxygen, and even the ones that do not require a fish to be able to function while exposed to air for at least a short while. If this makes fish that venture onto land sound a lot like modern amphibians, that is not a coincidence.
But it is not only fish with an eye toward terrestrial locomotion that try their fins at walking. There are walking fish underwater, too. Most of the fish mentioned in the previous sections will use their walking motions, or similar movements, to maneuver underwater, and that shows a key use case for this ability: maneuverability on complex surfaces. Steering a swimming body, especially one partially adapted for walking on land, can be difficult, and being able to latch onto and push off of a solid surface can be useful—so useful that fish with no special facility for moving about on land also pursue this ability.
Perhaps the best known of these are various anglerfish lineages. Other than the deep-dwelling ceratioid lineage, anglerfish are benthic predators that specialize in luring prey toward them, and that benefits from being able to hold very still. Anglerfish fins are thus specialized for grasping and maneuvering on surfaces, including rocks and sand, and some can maintain this while moving. Antenarioid anglerfish often spend more time clambering about their rocky and weedy homes than they do swimming, and one species, the sargassum fish Histrio histrio, is often found climbing within and clinging to bits of formerly floating seaweed on warm Atlantic shorelines.
Some of the strangest aquatic walkers are the sea robins, family Triglidae, whose main concession to walking is gently pulling themselves along the seafloor on a few fin rays while feeling for food beneath the sand.
But none are stranger than the tripod fish, Bathypterois grallator, which balances on three fin rays to hold a steady position above the deep seafloor. It might be a bit of a stretch to call this creature a “walker,” though.
Relatively few walking fish are common in the pet trade. Most of them are predators and most of them are adapted for specialized niches that are difficult to emulate in home aquaria, making them poor pets. Many are threatened by human activities, including habitat destruction and overfishing, and one, the Tasmanian smooth handfish Sympterichthys unipennis, was declared extinct in 2020. Epaulette sharks are available for experienced marine aquarists to keep in large tanks and several bichir species are commonly spotted in pet stores, giving many people their first experience with fish that walk.
It is a delightful reality we inhabit, that such beings can find homes in it. I hope we get to enjoy them a long time yet.