What Is the Coelacanth? A Fish That Was Extinct for Sixty-Six Million Years

Marjorie Courtenay-Latimer had an arrangement with the trawler captains of the port to be shown anything unusual, and on 22 December 1938 she was shown a fish nobody recognised. She could not preserve it whole in the summer heat, had it mounted by a taxidermist after the organs had spoiled, and sent her sketch to J. L. B. Smith at Rhodes University, whose reply, when it came, was that it was a coelacanth and that the loss of the soft parts was a catastrophe. Smith named it Latimeria chalumnae for her and the river mouth where it was caught, and spent fourteen years searching for a second, offering a reward in leaflets printed in three languages along the East African coast, until one was brought to him from the Comoros in 1952, where fishermen had been catching them occasionally for generations and calling them gombessa.

Coelacanths belong to the lobe-finned fish, the sarcopterygians, a group whose paired fins are carried on fleshy stalks with a skeleton of bones arranged like a limb's, rather than the fans of rays that the ordinary bony fish have. That group produced the animals that walked onto land, so the coelacanth's nearest relatives, after the lungfish, are amphibians, reptiles, birds and mammals rather than the fish it swims among. Its other features are equally archaic:

• •A hinged skull, unique among living animals, which lets it swing the front of its head upward to widen its gape
• •A notochord, a fluid-filled tube, in place of a backbone of vertebrae
• •A rostral organ in the snout that detects the electric fields of prey
• •Thick, overlapping scales with a rough surface, which Comorian fishermen used as sandpaper
• •Live birth: the females carry eggs the size of oranges and give birth to pups after a gestation that a 2021 study put at five years, the longest known of any vertebrate

Submersibles have filmed them since 1987, and the animal turned out to do almost nothing that had been guessed. It lives at 150 to 250 metres, spending the day in caves in volcanic slopes and drifting out at night to feed, hanging almost motionless in the current and taking fish and squid by suction; it swims by sculling with its paired fins in the alternating pattern of a trotting horse, which was taken as a clue to how the first walking animals moved, and it can stand on its head, apparently using the electric organ to sense prey in the sediment. It does not walk on the seabed, which the early reconstructions assumed. It grows slowly, matures at around fifty-five years by the 2021 estimate, and may live a century, which makes it one of the slowest-living fish known and a species that cannot absorb much fishing.

Two things are often said about the coelacanth that are not right. It is not a living fossil in the sense of an unchanged animal: the modern genus is not the same as the Devonian ones, the family has changed over time, and a 2013 genome paper found that its genes evolve slowly but not uniquely so. And it is not the ancestor of land animals; the lineage that walked ashore was a different branch of the lobe-fins, closer to the lungfish, and the coelacanth is a cousin that stayed in deep water. What it does provide is a living example of the body plan from which limbs came, with the fins, the hinged skull and the notochord available to be watched rather than inferred, which is why its discovery mattered more than the discovery of an ordinary new fish.

A second species, Latimeria menadoensis, was found in an Indonesian market in 1997 by a biologist on honeymoon who noticed a coelacanth being wheeled past on a cart, and it lives off Sulawesi, ten thousand kilometres from the first. The African species is known from the Comoros, Madagascar, Mozambique, Tanzania, Kenya and a population off South Africa found by divers in 2000, and the total is thought to be in the low thousands, perhaps a few hundred around the Comoros. They are caught accidentally by fishermen going deeper for oilfish, they cannot be eaten, and a species that takes half a century to mature and five years to gestate recovers from nothing. Both are protected by international trade rules, and the practical conservation is teaching fishermen to release them and to fish shallower.

The coelacanth is a deep-water lobe-finned fish known only from fossils older than 66 million years until one was recognised on a South African dockside in 1938, with a hinged skull, a notochord instead of a spine, an electric sense organ and fins on limb-like stalks. It is a cousin rather than an ancestor of the animals that walked ashore and it has changed since the Devonian, and with two species, a few thousand individuals, fifty years to maturity and a five-year pregnancy, it is extremely vulnerable.