NOW YOU SEE ME

Naturalist Georgina Jones sheds light on the enigmatic coelacanth.

AMOST TANTALISING FISH, the coelacanth. Popping up unexpectedly in time and place, perplexing in reproduction, behaviourally enigmatic, a bony fish more closely related to mammals and reptiles than to other bony fish; coelacanths are fascinating.

The ancient history of the coelacanth begins about 400 million years ago, with Miguashaia. The first coelacanth described by science, Coelacanthus, lived about 260 million years ago. Its name was derived from the hollow spines supporting the fish’s powerful tail fin. The line diversified in shape, size and habitat, exploiting all aquatic environments around the world. But by the time of the dinosaurs, it had dwindled to the most recent coelacanth fossil known, Swenzia, when, so far as anyone knew, they became extinct.

They rose to prominence again in the great controversies surrounding evolution, being suggested as the possible ancestor of landdwelling vertebrates. Their eight powerful lobed fins and heavy spiny scales hinted at a possible link to the earliest land-dwelling vertebrates, a link that was later disproved as other, more likely, ancestors were discovered.

The recent chapters in the coelacanth story began just before the start of World War II, offshore of the Chalumna River in the Eastern Cape. A trawl in 40-70m of water produced, along with the normal load of ‘edibles’ (hake, kingklip, seabream), a large deep blue fish with white markings and a huge lamp-like eye. It looked very different to the other fishes: as well as its size, it had an unusual number of fins and a very strange-looking tail.

The trawler captain was accustomed to keeping aside any unusual finds for the local museum curator. Which was how Marjorie Courtenay-Latimer found herself loading 2m of already smelly fish into a wheelbarrow several hours later. No taxi driver could be found to transport the carcase, nor could she find refrigeration for it. She tried to communicate with JLB Smith, then an amateur ichthyologist and on holiday at the time, but eventually in desperation, thinking the fish must somehow be preserved, she took it to the local taxidermist. In the process of stuffing the fish, the internal organs were discarded but enough remained for Smith to know it for what it was. A living representative of a family of fishes last seen 65 million years before. The find created a worldwide sensation.

Smith concluded it was a stray from a distant population, named it Latimeria chalumnae, and immediately posted a reward for the capture of another. Silence for 14 years until Smith received notification that a second fish had been found, this time off the Comores islands. Undeterred by the fact that the islands were French possessions, Smith got hold of DF Malan, then South African prime minister, and asked for help. Which resulted in a South African military plane arriving back in South Africa, along with a second coelacanth, an exultant JLB Smith and, astonishingly, an absence of international incidents, though the French took the lead in coelacanth studies for a while after that.

It turned out that Comoro fishers occasionally caught coelacanths. They didn’t target them because their flesh is oily, unpalatable from urea and tends to cause diarrhoea.

Work then began on their biology. Unlike all other vertebrates, coelacanths have a joint between their upper jaws and the rest of their skulls, which may aid them in engulfing large prey fishes. Their brains are small, taking up less than 2% of the skull, and at the front of the skull is a mysterious organ, possibly an electroreceptor, with three external openings. The eyes have many rods which aid vision in their low light environment.

Their skeletons are mostly made of cartilage like those of sharks and rays, and instead of a vertebral column, they have a thick cartilaginous oil-filled tube, which is flexible and supports the spinal cord. They have a large fat-filled swim- bladder for buoyancy regulation.

Reproductively, coelacanths have internal fertilization, but how sperm is transferred is still unknown since the males have only a cloacal opening. Their eggs are huge, about the size of oranges, and at over 300g, are the biggest eggs known of all the fishes. The pups are born at about 36cm after a gestation period of perhaps three years.

Most biological understanding came from the study of dead or dying specimens until German-led submersible work began in the 1980s. Working in depths of 120-400m, the coelacanth was established to be a generally slow-moving fish which rests in caves in groups during the day, possibly for protection from predators. They emerge at night to drift above the ocean floor, presumably in search of prey.

Coelacanths shelter in caves and under overhangs during the day.

They are capable of sudden bursts of speed, using their powerful tail fin, and seem to use their pectoral fins mainly for turning and stability. Since their discovery, coelacanths had been caught off East Africa and Madagascar, but in 1998, another strange fish was spotted in a fish market in Indonesia This turned out to be another species of coelacanth, Latimeria menadoensis. Meanwhile, unconvinced by Smith’s ‘stray’ designation, the search for coelacanths continued in South Africa. Similarities between the underwater topography of the Comores and the slopes and overhangs offshore of Sodwana suggested coelacanth habitats. In 2000, divers saw three fish with huge eyes under an overhang at 104m. A second expedition confirmed three coelacanths and since then a total of 33 individual fish have been found in depths ranging from 54 to 133m. This is much shallower than those off the Comores, possibly because the cooler water off Sodwana, being richer in oxygen, supports the coelacanths’ inefficient gills. But perhaps coelacanths have an even greater range around our coast. The offshore topography is suggestive of coelacanth habitat and as the water gets colder and more oxygen-rich further south, who knows? There’s an unsubstantiated tale from a Wild Coast spearfisherman in the 70s of a coelacanth in 27m of water. Also, divers exploring a reef off southern KwaZulu-Natal in 2019 came across a huge blue fish with a large lamp-like eye... A tantalising fish indeed.