Mudskipper

It breathes in more than one way. Like other fish it has gills, but a mudskipper holds a reserve of water in enlarged chambers around the gills so they stay wet and working while it is on land. On top of that, it takes in a large share of its oxygen straight through its skin and through the thin, blood-rich lining of its mouth and throat, in the same way an amphibian does, which is why it must keep its skin damp by rolling in wet mud or returning briefly to a puddle. It also cannot let its body dry out, so at low tide it stays in the humid mangrove shade or near its burrow, and at high tide, oddly, it often climbs onto a root or stem to stay out of the deeper water.

It uses its front fins as legs and its body as a spring. The two pectoral fins sit on short, muscular, arm-like stalks, and the fish plants them and drags itself forward in a rowing crutch-walk, sometimes with the pelvic fins helping to grip. To move quickly it curls its body to one side and flicks it straight, flinging itself forward in a skip or a leap that can clear a good distance across the mud or over a pool. The same flick lets it jump vertically during displays. In water a mudskipper swims normally, but on the flat it is one of the most mobile fish out of its element.

The burrow is central to a mudskipper's life. Dug into the soft mangrove mud, often to depths of half a metre or more with one or more entrances, it provides refuge from herons, kingfishers, snakes, and large crabs, a cool, moist retreat from the midday sun, and a place to shelter through the high tide. It is also the nursery. The female lays her eggs on the roof of a chamber inside the burrow, and because the surrounding mud contains almost no oxygen, the male keeps the chamber supplied with air by carrying mouthfuls down from the surface and releasing them into an air pocket, guarding and tending the eggs until they hatch.

It is a true fish, one of the gobies, and it is not on any evolutionary path to becoming an amphibian. It simply shows how far a fish can go in adapting to life at the water's edge. Mudskippers have fins rather than limbs, they breed in water, their eggs and young are fully aquatic, and their body plan is that of a goby. What they have evolved is a set of add-ons, air-holding gill chambers, skin and mouth breathing, propping fins, raised eyes, and burrowing behaviour, that let an adult spend most of its active life out of the sea on the mud. They are a striking example of convergence, arriving at an amphibious lifestyle from the fish side.