Coelacanth (Latimeria chalumnae): The Living Fossil Fish

What does a coelacanth look like?

A living coelacanth is a deep steel blue, covered in irregular pale spots. The pattern of spots is unique to each fish, which lets researchers recognize individuals from photographs year after year. After death, the blue quickly fades to gray or brown, which is why museum specimens look dull.

The body is heavy and covered in thick, overlapping scales with tiny tooth-like bumps. The most striking features are the fins. A coelacanth has eight of them: two dorsal fins, a pair of pectoral fins, a pair of pelvic fins, an anal fin and a broad tail. The four paired fins, plus the second dorsal and anal fins, grow from fleshy, muscular lobes that contain bones, a little like stubby limbs. The tail has three lobes, with a small extra tuft sticking out in the middle.

Inside, the coelacanth is just as unusual. Instead of a backbone made of solid vertebrae, it has a thick, oil-filled tube called a notochord. In place of a swim bladder it has a fat-filled organ, with a tiny lung that no longer works. Its skull has a hinge that lets the front part tilt up when it feeds, and an organ in the snout, called the rostral organ, is thought to detect the weak electric fields of prey.

How big is a coelacanth?

Coelacanths are among the larger bony fishes of the deep reefs. Females grow considerably bigger and heavier than males.

Where do coelacanths live?

The West Indian Ocean coelacanth lives along the steep underwater slopes of East Africa and nearby islands. The best-known population lives off Grande Comore and Anjouan in the Comoros, where local fishermen had long known the fish as gombessa. Coelacanths have also been found off South Africa, Mozambique, Madagascar, Tanzania and Kenya. Genetic studies suggest that the fish off southern Tanzania split from the Comoros population some 200,000 years ago.

Coelacanths need cool, dim, well-oxygenated water of about 14–22 °C (57–72 °F). By day they shelter in lava caves and overhangs, mostly about 100–250 m (330–820 ft) deep. They have been recorded from about 54 m (177 ft) in South African canyons to roughly 700 m (2,300 ft). In South Africa they rest in canyons at about 90–150 m (300–490 ft), within the iSimangaliso Wetland Park.

A second living species, the Indonesian coelacanth (Latimeria menadoensis), was discovered in 1997–1998 off Sulawesi, more than 9,000 km (5,600 mi) away. It looks very similar but is brownish gray rather than blue, and genetic studies suggest the two species split roughly 30–40 million years ago. It has since been recorded off Papua, North Maluku and other parts of eastern Indonesia, and the IUCN lists it as Vulnerable.

How do coelacanths sense their world?

Coelacanths live where very little sunlight reaches, and their senses are tuned to it. Their large eyes are packed with rod cells, which work in dim light, and have a reflective layer behind the retina, the tapetum lucidum, that sends light back through the eye a second time. It is the same kind of layer that makes a cat’s eyes shine at night. Their visual pigments are most sensitive to blue light of about 480 nanometers, the color that travels deepest in clear seawater.

The rostral organ in the snout gives coelacanths an electrical sense, similar in purpose to the one sharks use. It probably helps them pinpoint prey at very close range in the dark, just before the final strike. A lateral line system on the head and body also picks up movements in the water.

What do coelacanths eat?

Coelacanths are carnivores that hunt at night. Rather than chasing prey, they drift slowly with the currents a few meters above the slope, using their fins to steer. When a suitable animal passes close to the mouth, they snap it up with a quick suck. Prey includes deep-reef fish such as cardinalfish, alfonsinos, grenadiers and cusk eels, as well as squid, cuttlefish and small sharks.

They are sometimes seen in a “headstand” posture, hanging head-down near the bottom. This may help them search crevices for food, perhaps using the electric sense in the snout.

Behavior: slow drifters of the deep

Coelacanths save energy wherever they can. They have a slow metabolism and spend the day resting in caves, often several fish sharing the same cave without any obvious social contact. If two touch, they simply move apart. At night they leave the caves and drift along the slope, sometimes covering several kilometers before returning to a cave, not always the same one, by morning.

When swimming, a coelacanth moves its paired fins in a diagonal rhythm: the left front fin moves with the right rear fin, like the legs of a trotting animal. It can also swim backward, upside down or head-down with ease. During 21 years of submersible studies off Grande Comore, researchers recognized the same individuals by their spot patterns again and again, some for more than two decades.

Reproduction and lifespan

Coelacanths give birth to live young. The female carries large eggs, up to about 9 cm (3.5 in) across, which hatch inside her body. Pregnant females have been found with 5 to 26 well-developed pups, each up to about 35 cm (14 in) long and able to fend for itself at birth. Live birth was confirmed in 1975, when scientists at the American Museum of Natural History dissected a female and found five large embryos inside. Newborn and young coelacanths are very rarely seen, so where the pups spend their first years is still a mystery.

Everything about the coelacanth’s life is slow. A 2021 study of growth rings on the scales suggested that the species lives to about 100 years, does not mature until roughly 40–69 years of age, and has a pregnancy of around five years, one of the longest known in any animal. The details are explained in how long do coelacanths live?

Predators and threats

Large sharks are the most likely natural predators, and some coelacanths carry shark bite marks. The main threat today is accidental capture by people. In the Comoros, coelacanths are sometimes hooked at night by fishermen fishing deep for oilfish, which live in the same places. Since 2003, dozens have been caught in deep-set gillnets off Tanzania.

Because the population is small, scattered and reproduces extremely slowly, the loss of even a few adults matters. Surveys off Grande Comore suggest only a few hundred adults live there, and the total population of the species was estimated at no more than about 500 in the late 1990s, although new finds off Tanzania and South Africa show it is more widespread than once thought.

Is the coelacanth endangered?

The IUCN Red List classifies the West Indian Ocean coelacanth as Critically Endangered, the highest category before extinction in the wild. That assessment dates from 2000, and the population trend is listed as unknown. Both coelacanth species are on Appendix I of CITES, which bans commercial international trade in them.

Conservation work includes protected areas, such as the marine part of South Africa’s iSimangaliso Wetland Park, awareness programs with fishing communities, and weighted “deep release” hooks that let fishermen return an accidentally caught coelacanth to deep water.

The 1938 discovery and the living fossil

In December 1938, museum curator Marjorie Courtenay-Latimer spotted a strange blue fish in a trawler’s catch at East London, South Africa. Chemist and fish expert J. L. B. Smith recognized it as a coelacanth and named it after her and the Chalumna River, near where it was caught. A second specimen did not turn up until December 1952, in the Comoros, home of the main population.

In the late 1980s, a team led by the German biologist Hans Fricke became the first to film and photograph living coelacanths in their natural habitat, from a small submersible off Grande Comore. In 2000, deep divers found a group in the canyons of Sodwana Bay, South Africa. The full story is told in how was the coelacanth discovered?

Coelacanths belong to an ancient lineage whose fossils go back more than 400 million years, and they are more closely related to lungfish and four-legged animals than to most other fish. Why they are called living fossils, and why scientists question the label, is explained in why is the coelacanth a living fossil?

Coelacanths and people

The coelacanth has no value as food: its flesh is oily and unpleasant, and Comorian fishermen avoided it long before scientists became interested. Instead it became a national symbol of the Comoros, appearing on banknotes, coins and many postage stamps. Museums around the world display preserved specimens, and the original 1938 fish is still on show in East London.