All living lungfish possess a single central fin that extends two-thirds down the body.
Unlike other fish they do not have paired fins at the sides, rather they have spindly ‘lobes’ that they use to 'walk' along the river or lake bed.
Instead of marginal teeth, living lungfish have two opposing pairs of tooth-plates that are ideal for crushing food before eating.
Lungfish are of particular evolutionary interest as they and coelacanths are the nearest living relatives to the land-dwelling vertebrates (tetrapods).
There are 6 living species: one from South America, one from Australia and four from Africa. The South American and African species are most closely related.
The oldest lungfish fossils date back approximately 400 million years.
The group is an excellent example of how the rate of evolution can slow down dramatically. Fossils very similar to the living species are known from 100 million years ago, indicating relatively little evolution over a long time period.
Protopterus annectens looks rather like an eel and is very tricky to distinguish from other Protopterus species.