What is a coelacanth?
Coelacanths (pronounced ‘seel-uh-kanth’) are large, marine, lobe-finned fishes belonging to the Order Coelacanthiformes, a group of primitive bony fishes.
In 1839 the Swiss scientist Louis Agassiz described a fossil fish in his book Poissons Fossiles and called it Coelacanthus because the spines that project from the back bone to the caudal fin rays were hollow (from the Greek koilos, meaning  cavity or space, and akantha, thorn). Prof. Agassiz’s fossil fish Coelacanthus granulosus from Europe at about 260 million years ago was actually the fourth fossil coelacanth to be described.

What does the coelacanth look like?
The living coelacanth can reach about 2 metres in length and weigh over 95 kg, but most extinct species were much smaller. However, Megalocoelacanthus dobiei from the eastern USA at about 75 million years ago reached lengths of around 3.5 metres, the largest species known. The two living species are dark blue to brownish and each fish has a distinctive pattern of pinkish white blotches that enables scientists to distinguish one individual from another.
 
What are its anatomical features?
The coelacanth has several very distinctive anatomical features.
The head - The skull has two parts, a front and rear section, with an intracranial joint which allows up and down movement between them. A strong pair of muscles beneath the skull-base lowers the front half of the skull, giving the coelacanth a powerful bite. The coelacanth is the only living animal with that structure. The eyes and olfactory organs are in the front part of the skull and the tiny brain and inner ear are in the rear.
In the middle of the snout is a large cavity filled with a jelly-like sac which opens to the outside through three pores. This sac is called the rostral organ. It may be used to detect weak electric currents and help the coelacanth find hidden prey.
The fins - Coelacanths have 8 fins (2 dorsals, 2 pectorals, 2 pelvics, 1 anal and 1 caudal). Ray-finned fishes such as pilchards or kob (in fact most fishes except sharks and rays) usually have only one dorsal fin. In ray-finned fishes the fins have a basic structure of bony, flexible fin rays with a web of skin stretched across them. The fin rays can flex slightly so that the fish can ‘fan’ or ‘scull’ its fins. The first dorsal fin of the coelacanth is much like that of other fishes and can be folded down or erected. The other fins have a well-developed, muscular, limb-like basal lobe projecting from the body wall, and a fringe of unbranched rays like a fan attached to the outer end of the base. The fleshy scale-covered lobe can be bent or rotated so that each fin can work like a paddle or sculling oar. The caudal fin (tail) has three divisions: a characteristic, small, projecting middle lobe between the longer upper and lower lobes of the fin.
The skeleton - Most of the skeleton is made of cartilage. In place of the vertebral column, a large notochord extends from the skull to the tip of the caudal fin. The notochord is a thick-walled cartilaginous tube filled with an oil-like fluid which is under slight pressure; it is tough and elastic and does the job of a backbone, since no complete vertebrae are developed around it. In higher vertebrates the notochord is replaced by vertebrae in the embryonic stage of development.
The scales - The body is covered with hard scales with small tooth-like growths called denticles on the outer surface which protect the coelacanth. The scales of the Indonesian species have a beautiful gold-coloured flecking. 
The swimbladder - Unlike most bony fishes which have a gas-filled swimbladder, the coelacanth has a large swimbladder that is filled with fat. Being lighter than sea water, the fat provides buoyancy.

How does the coelacanth swim?
The lobed fins are extremely mobile and can be rotated through 100 degrees. The coelacanth can swim fast for short periods, but it usually swims around slowly using its paired and median fins as sculls to push itself through the water.  When cruising, the fish works the right pectoral fin in tandem with the left pelvic fin and vice versa.  The coelacanth likes to hover near the sea bottom with all fins fully extended - a beautiful sight to see.  The paired, lobed fins are not used to ‘walk’ on the sea bed or, as with lungfish, to prop the fish up off the bottom or push against it.  It seems, rather, to actively avoid touching the sea floor. When startled, it darts forward at speed using the large caudal fin.  Sometimes it performs a head stand keeping its snout to the sea floor and rotating its rigid body to an upright position.  It may then make use of the electro-receptive rostral organ to find prey on the bottom.

Why is the coelacanth important to science?
The study of plants and animals of the ancient world is called palaeontology. Fossils are the remnants or impressions of organisms that lived in those times. The original meaning of the word ‘fossil’ is derived from the Latin verb ‘fodere’, to dig, as fossils are usually encountered as petrified artefacts preserved in rocks. Charles Darwin coined the phrase ‘living fossil’ to describe the East Asian ginkgo tree, but what was not meant was a fossil that had somehow come back to life. Rather he meant a life form that had evolved very little for millions of years - a case of arrested evolution.  Examples are cycads, elephant shrews and broad-nosed crocodiles.  The closest relatives of ‘living fossils’ have been extinct for a long time.
The scientific discovery of a living coelacanth in the sea off East London in 1938, named Latimeria chalumnae, caused a sensation in the scientific world because it was then the only living member of a very old group of fishes, the Coelacanthiformes (or Actinistia in the old literature). Just over 90 species of coelacanths are positively identifiable from fossil remains, with about another 30 species possible from bone fragments too questionable to be identified with certainty. The early coelacanths were predominantly small marine fish (though some lived in freshwater) which were thought to have died out at the end of the Mesozoic era more than 65 million years ago. They flourished in the Triassic Period (208-251 million years ago). A Triassic fossil coelacanth named Whiteia africana was discovered in the Free State, and the original specimen is on public display at the South African Museum in Cape Town. The Coelacanthiformes and lungfishes are separate side-branches of the primitive fish group that gave rise to the amphibians, reptiles, birds and mammals.  Although Latimeria is not a ‘missing link’ in the story of evolution, it is a survivor of a line of development that otherwise became extinct. From its anatomy, biochemistry, physiology and behaviour scientists can learn much about the processes of evolution