Bichir

It is often called one, and it does belong to a very old and isolated branch of the ray-finned fishes — bichirs (family Polypteridae) split off before the ancestors of almost all other modern bony fish, and fossils of the group go back to the Cretaceous, with the lineage older still. They retain a striking set of ancient features — a pair of functional lungs, thick enamel-like ganoid scales like those of gars, fleshy-based (lobe-like) pectoral fins, a spiral valve in the gut, and feathery external gills in the larvae. But "living fossil" overstates the case; bichirs are a modestly diverse, successful modern group that has continued to evolve. What they offer is a window onto body features that were probably present in early bony fish and that most living fish have since lost.

To a limited but real extent, yes. Its stiff, armoured body and muscular, lobe-based pectoral fins let it prop itself up and lever forward across wet ground or mud, and wild bichirs use this to cross between drying pools and swamps, helped by their ability to breathe air. This makes the bichir a favourite model for studying how the first vertebrates moved onto land. In a well-known 2014 study, researchers raised young bichirs entirely on land for eight months; those fish developed a more efficient walking gait, held their fins closer to the body, and showed changes in the bones and muscles of the shoulder girdle compared with water-raised siblings — an example of developmental plasticity that could have helped ancient fish begin the transition to terrestrial life.

Bichirs live in warm, still, swampy African waters where dissolved oxygen is often very low, and their solution is a pair of true lungs — connected outgrowths of the gut, ventilated by gulping air at the surface, and used alongside the gills. In fact a bichir relies on air breathing enough that if it is kept in deep water with no way to reach the surface, particularly warm water, it can suffocate and drown despite having gills. Conversely, because its lungs and moist, armoured body let it tolerate air, a bichir can survive for hours or, in cool damp conditions, longer out of water, which is why they sometimes turn up having crawled out of an aquarium.

They are popular with keepers of large predatory fish and are hardy and long-lived, but they need appropriate setup. A single bichir needs a long tank (many species need 4 feet or more as adults), a tight-fitting lid with no escape gaps because they are determined climbers and jumpers and will breathe air at the surface, a soft substrate, hiding places, and a meaty diet of pieces of fish, prawn, earthworms, and pellets. They have poor eyesight and hunt by smell, so they can be outcompeted at feeding time by fast tankmates, and they may eat any fish small enough to swallow. Most species are peaceful toward fish too large to be prey. The "reedfish" or "ropefish" is a closely related, eel-like member of the same family kept the same way.