Fish
Coelacanth
Latimeria chalumnae · also called African coelacanth, West Indian Ocean coelacanth, gombessa
The coelacanth is the most famous “back from the dead” animal in science. Known only from rocks up to hundreds of millions of years old, and believed extinct since the age of dinosaurs, it turned up alive in a fishing net in 1938 — and it carries in its fins a clue to how vertebrates first walked onto land.
Overview
The African coelacanth lives on steep deep-water slopes and in submarine lava caves in the western Indian Ocean, at depths of roughly 150 to 250 metres. It is nocturnal, resting in caves by day in loose groups and drifting out at night to hunt. A second, separate species, Latimeria menadoensis, was discovered off Sulawesi, Indonesia, in 1997–98.
The 1938 discovery
In December 1938 a South African museum curator, Marjorie Courtenay-Latimer, examined the catch of a trawler at East London and picked out a large, steel-blue fish with strange limb-like fins and hard scales. Unable to preserve it fully or identify it, she sketched it and alerted the ichthyologist J. L. B. Smith, who recognised it as a living coelacanth. It took another 14 years to find a second specimen, in the Comoros, where local fishers had long known the fish as the gombessa.
A window on our own history
Coelacanths are lobe-finned fish (sarcopterygians), the branch of the fish family tree that gave rise to the first four-limbed land animals. Their paired fins are not simple flaps but fleshy, muscular lobes supported by a bone arrangement homologous to the upper arm, forearm, and hand — and the coelacanth swings them in the same diagonal, alternating pattern a walking tetrapod uses its legs. Studying the living animal has helped biologists understand the transition from fin to limb.
Other archaic features include a notochord (a fluid-filled tube) in place of a backbone, a hinged skull that lets the fish widen its gape, a small vestigial lung, and a rostral organ in the snout that senses the weak electric fields of nearby prey.
Reproduction
Coelacanths are ovoviviparous — the female retains the eggs inside her body, where the embryos develop and are nourished, and gives birth to live, fully formed pups. The gestation period is now estimated at around five years, the longest known for any vertebrate, and females may not reach breeding age until close to 60. Litters are small, a handful to a few dozen pups.
Threats and conservation
The African coelacanth is Critically Endangered, with a total population that may be only a few hundred animals. The species is protected internationally, cannot legally be traded, and is not deliberately fished, but it is vulnerable to accidental capture in deep gillnets and longlines set for other species, and to disturbance of its limited cave habitat. Its extreme life history — slow growth, very late maturity, tiny litters after a five-year pregnancy — means any added mortality is serious.
Coelacanth: common questions
Why is the coelacanth called a living fossil?
Coelacanths were known only from fossils, the youngest around 66 million years old, and were assumed to have gone extinct alongside the non-bird dinosaurs. Then in 1938 a living one was hauled up off South Africa. The modern coelacanth is not identical to its fossil relatives, but it belongs to a very old lineage that has changed relatively little in body plan for hundreds of millions of years, so the "living fossil" label stuck.
How is a coelacanth related to humans?
Distantly, but importantly. Coelacanths are lobe-finned fish, the group that also contains lungfish and the tetrapods — every amphibian, reptile, bird, and mammal, including us. Their paired fins have a fleshy, bony, muscled base built on the same limb skeleton pattern as our arms and legs, and they move them in the alternating "walking" rhythm of a four-legged animal. The coelacanth itself is not our ancestor, but it is one of the closest living relatives of that ancestor.
Does the coelacanth really "walk"?
Not on the seafloor, but it moves its four lobed fins in the diagonal sequence a trotting tetrapod uses — front-left with back-right, then front-right with back-left. Divers have filmed coelacanths drifting head-down, doing handstands, and even swimming belly-up, using the mobile fins for fine control. It also has a fluid-filled electroreceptive organ in its snout to detect prey in the dark.
Why is the coelacanth endangered?
It was never common, it lives in a small number of deep sites, it grows slowly, matures extremely late, and has a five-year pregnancy — so the population cannot absorb losses. Coelacanths are not eaten (the flesh is oily and unpalatable) but they are caught accidentally by deep-set fishing gear meant for oilfish and sharks, and that bycatch, together with habitat disturbance, is enough to threaten such a tiny, slow-breeding population.