Walking Catfish

It can travel over land, though walk is a generous description of the movement. The fish holds its body rigid, plants the stiff, spine-tipped front edges of its pectoral fins against the ground on each side, and throws its body from side to side in a serpentine flexing motion, which levers it forward in a series of lurches. On a wet surface, grass, mud, a rain-slicked road, it can cover a surprising distance this way, tens of metres or more, and it uses this ability to escape a drying or overcrowded pool and find fresh water. It cannot move on dry ground and it needs to stay moist, so overland trips happen on rainy nights or through morning dew.

With an accessory breathing organ. In addition to normal gills, the walking catfish has a pair of branched, spongy structures in chambers above the gills, richly supplied with blood, that absorb oxygen from air the fish gulps at the surface or holds while on land. This lets it live in water so stagnant and oxygen-poor that other fish suffocate, and it lets it stay alive for many hours, sometimes more than a day, in damp conditions out of the water. The same organ means the fish must have access to the surface; kept in a sealed, fully submerged container it will actually drown.

Because its toughness and mobility remove the natural limits that keep most introduced fish contained. It can live in warm, muddy, low-oxygen water; it breeds readily; it eats almost anything, including the eggs and young of native fish and amphibians; and, crucially, it walks between water bodies, so it colonises isolated ponds, borrow pits, and drainage ditches that a purely aquatic invader could never reach, and it has repeatedly walked into commercial fish farms and eaten the stock. It arrived in the 1960s through the aquarium trade, spread across much of the southern half of the state within a decade, and has never been controllable.

It belongs to a family, the Clariidae or airbreathing catfishes, in which nearly all members have a version of the same accessory breathing organ, including the large African sharptooth catfish widely farmed across Africa. Air-breathing has evolved separately in a number of other fish that live in oxygen-poor tropical fresh water, such as the lungfishes, the bettas and gouramis with their labyrinth organ, the snakeheads, and the climbing perch, several of which, like the walking catfish, are also both valuable food fish and troublesome invaders. It is a common solution to the same problem, namely shallow, warm, still water with very little dissolved oxygen.