The most important features of lobe-finned fish is the lobe in their fins. Unlike other fish, Lobe-finned fish have a central appendage in their fins containing many bones and muscles. The fins are very flexible and potentially useful for supporting the body on land, as in lungfish and tetrapods (vertebrates with four limbs). Tetrapods are thoughto to have evolved from primitive lobe-finned fish.

Coelacanth

The coelacanth was thought to have become extinct 65 million years ago until its capture in 1938 by a South African museum curator on a local fishing trawler. There are only two known species of coelacanths: one that lives near the Comoros Islands off the east coast of Africa, and one found in the waters off Sulawesi, Indonesia.

Coelacanth
Image © Arne Kuilman Flickr

Coelacanths are hard to catch, deep-sea creatures, living in depths up to 700 metres. They can grow to two metres in length or more and weigh up to 90 kilograms. It is estimated that they can live up to 60 years or more.

The most striking feature of this "living fossil" is its paired lobe fins that extend away from its body like legs and move in an alternating pattern, like a trotting horse.

Other unique characteristics include a hinged joint in the skull which allows the fish to widen its mouth for large prey; an oil-filled tube, called a notochord, which serves as a backbone; thick scales common only to extinct fish. It also has an rostral organ which is a large gel-filled cavity in the snout which appears to be used to detect prey in the dark by detecting electric currents. It could also help the balance of the fish.

Coelacanths have a tiny heart that is basically a straight tube and a brain that takes up only 1.5% of the braincase; the rest of the cavity is filled with fat.

Their population numbers are not well known but studies in the Comoros Islandssuggest only about 1,000 remain there. They are considered an endangered species.

Lungfish

Lungfish are freshwater fish, normally found in still or slow flowing pools, living only in Africa, South America, and Australia. The Australian lungfish grows to about 1.5 m in length and over 40 kg. It occurs naturally in the Burnett and Mary River systems in Queensland and has been introduced into other rivers and dams in south-eastern Queensland and north-eastern New South Wales.

The Australian lungfish is a carnivore eating frogs, tadpoles, small fishes, snails, shrimp and earthworms. They use use electroreception to locate hidden prey. Unlike the African and South American species the Australian lungfish has a single lung compared to paired lungs. Usually they breathe using gills but in drought conditions they can come to the surface and breathe air. Only the Australian lungfish can breathe using its gills.

The Australian lungfish spawns at night from August to December. Fertilized eggs are attached to aquatic plants and hatching takes about three weeks. Growth is very slow, with young reaching 6 cm in length after 8 months and 12 cm after two years.

The Australian Lungfish is a protected species and may not be captured without a special permit.

African and South American lungfish are capable of surviving seasonal drying out of their habitats by burrowing into mud and aestivating (similar to hibernation taking place during times of heat and dryness) throughout the dry season. Changes in physiology allow the lungfish to slow its metabolism to as little as 1/60th of the normal metabolic rate.

Lungfish can be extremely long-lived, one kept in captivity is around 80 years old.

Australian lungfish
Image © Ron DeCloux Flickr

Australian lungfish
Image © Ron DeCloux Flickr

http://animals.nationalgeographic.com/animals/fish/coelacanth/
http://en.wikipedia.org/wiki/Coelacanth
http://en.wikipedia.org/wiki/Lungfish
http://australianmuseum.net.au/Australian-Lungfish-Neoceratodus-forsteri-Krefft-1870