Fish
Climbing Perch
Anabas testudineus · also called climbing perch, climbing gourami
The climbing perch is a plain, tough little fish from the ponds and paddies of Asia that behaves like it did not get the memo about being a fish. It breathes air, it survives for days out of water, it can sit out a drought buried in mud, and when its pool starts to dry it hauls itself overland to the next one on its gill covers. In Australia, where it has not yet arrived, it is treated as one of the most alarming fish in the world.
Overview
The climbing perch is native across South and Southeast Asia, in warm fresh and brackish water — ponds, ditches, canals, and rice fields. It grows to about 10–25 cm, dull greenish-grey, with rough scales and strongly spined gill covers and fins. It is a hardy omnivore that thrives in muddy, low-oxygen water.
Air, land, and mud
Above each gill chamber sits a labyrinth organ that lets the fish breathe atmospheric air — so completely that it will drown if kept from the surface. Kept moist, it can live several days on land and can wait out a dry season buried in damp mud.
Travelling overland
To reach new water it props on its spiny gill covers, pushes with its tail, and lurches forward in jerks, usually at night or after rain. Its name comes from an old report of one found up a palm tree — probably carried there rather than climbed.
A feared invader
Established in New Guinea and spreading toward northern Australia, it is a major biosecurity concern. A bird, snake, or fish that swallows one can be choked or killed when the fish flares its razor-sharp gill covers in its throat.
Status
The climbing perch is Least Concern, abundant, farmed, and increasing where introduced.
Climbing Perch: common questions
Can the climbing perch really walk on land?
It can move across land, though walk is a generous word for how it does it. The fish has no legs, so it travels by bracing its stiff, spined gill covers against the ground on each side, arching its body, and thrusting with its tail and fins to jerk itself forward, tipping from side to side as it goes. On wet grass or mud after heavy rain it can keep this up for many metres and reportedly for hundreds of metres over a night, moving between drying pools and into fresh water. It is slow and clumsy on the surface but effective, and it is one of a small number of fish that routinely make overland journeys.
How does it breathe air?
Behind its normal gills, in a chamber above each gill arch, the climbing perch has a labyrinth organ, a maze of thin, folded bony plates covered in a membrane packed with blood vessels. The fish gulps air at the surface, holds it in this chamber, and absorbs oxygen from it directly, the way a lung works. This lets it survive in stagnant, warm, oxygen-poor water and out of water altogether as long as the organ stays moist. The trade-off is that the fish becomes dependent on air-breathing and will suffocate if it is prevented from reaching the surface, even in clean, well-oxygenated water.
Why is the climbing perch a problem in Australia?
Australia does not have the climbing perch yet, but it has spread through New Guinea and reached islands in the Torres Strait, putting it on the doorstep of Cape York. Biosecurity agencies treat it as a high priority because it is so hard to contain. It tolerates a huge range of conditions, walks between water bodies, survives being dried out or frozen for a time, and can be moved accidentally in bait buckets or floodwater. Worse, native predators that try to eat it are often harmed — the fish erects its razor-sharp gill covers, which can lodge in and puncture the throat of a bird, snake, or larger fish, so the invader can kill the animals that would normally control it.
Is the climbing perch related to other air-breathing fish?
It belongs to the labyrinth fishes, or anabantoids, the same broad group as the Siamese fighting fish, gouramis, and the paradise fish, all of which have a version of the labyrinth organ and can breathe air. It is not closely related to lungfishes, which use true lungs, or to the walking catfishes and snakeheads, which have their own separate air-breathing structures. These are all examples of fish independently evolving ways to get oxygen from the atmosphere, usually as an adaptation to warm, still, poorly oxygenated tropical water.