A Remarkable Fish is the Coelacanth

“It must be a pretty important fish if the Prime Minister is prepared to give an aircraft and crew to some hair-brained scientist to fetch it.” – Commandant J. P. D. Blaauw (in command of the air force Dakota sent to fetch a dead fish in the Comoros.)

It was around Christmas of 1952.

Needless to say, the media were in a frenzy over the discovery of a second fish of a species that was thought to have been extinct for 260 million years. And the fact that the Prime Minister Dr D.F. Malan commandeered an aircraft for Prof Smith to fetch it in a foreign territory, was the most exciting news of the year. Of course, every citizen with access to newspapers and radio was enthralled by the tale – including my parents. I was old enough to understand that something pretty remarkable had happened.

The world’s first living coelacanth had been found off the coast of East London in 1938 (also at Christmas time) and was recognised as an important specimen by Marjory Courtney-Latimer of the local museum. She sent a drawing to Prof J. L. B. Smith who positively identified the specimen as a coelacanth.

That discovery of a living coelacanth off South Africa is regarded as one of the greatest biological discoveries of the 20th century. What makes this fish so special?

Fossil records of coelacanths go back more than 420 million years. They were presumed to have gone extinct in the late Cretaceous period, over 65 million years ago. So you can imagine the surprise and excitement of the scientific world when a real live specimen turned up in South Africa.

In an evolutionary sense, Coelacanths were once (erroneously) thought to be the ancestors of tetrapods (four-legged, land-living animals) because they have leg-like fins that stick out from the body. The first coelacanth was, in fact, nicknamed “Old Fourlegs”. They have a unique combination of the anatomical and physiological traits of both bony fishes and cartilaginous fishes.

African coelacanths (Latimeria chalumnae) are large fishes, reaching close to 2m in length and 100kg in weight. They are bright blue in colour with patterns of white spots. The arrangement of the spots is unique for each individual fish, so each one can be identified.

Coelacanth reproduction is fascinating. They are ovoviviparous, meaning that they retain eggs in the body during gestation and give birth to live offspring (like some sharks). They produce the largest eggs of any fish (the size of a grapefruit) and one of the smallest broods of any fish (usually fewer than 30 live-born juveniles). The juveniles are large at birth at about 33cm and 500g. This is not surprising, considering that they have the longest gestation period of any animal at 5 years (60 months) – more than twice as long as the African elephant. Coelacanths are estimated to live to 60 years or more.

They have tiny brains. A coelacanth’s brain occupies only 1.5 percent of its cranial cavity. The rest of the braincase is filled with fat. Perhaps a better nickname would be “Old Fathead”.

They swim like no other fish with alternate sculling movements of their paired fins. The large tail fin is only used for ambushing prey or to escape predators (or divers). Coelacanths are passive drift feeders, moving lethargically near the ocean bottom and using the current and their flexible lobed fins to move about.

They detect their prey – mostly fish and cephalopods – using an electro-receptive organ, called a rostral organ, in their snouts. They do a headstand when searching for prey in the sand and bend the middle lobe of their tail fin back and forth to stabilise them while hunting. A coelacanth can take very large prey thanks to a hinge in its skull that allows it to open both top and bottom jaws to give an extremely wide gape.

Coelacanth have the lowest values for resting oxygen consumption of any fish and the slowest metabolic rate of any known vertebrate. They also have the lowest haemoglobin count in their blood of any vertebrate and a very small gill surface area. They are therefore very sensitive to low oxygen concentrations in the water and prefer deep cool water with a high oxygen saturation. When they are pulled to the surface by fishermen they die of asphyxiation in the warm, relatively oxygen-poor surface water.

During the day they hang out, sometimes with several other coelacanths, in caves and under overhangs along the steep sides of underwater cliffs.

Coelacanths have survived five great extinction events but they are now threatened with extinction by the sixth extinction event that is being caused by humans.

If coelacanths have been around for such a long time, why was the first one only found in 1938? What is their conservation status? How many are left in the world? Where are they found? Are they good to eat?

You are invited to the Whale Coast Conservation AGM on Thursday 22 September at 17h30 to hear the answers to all these questions. The guest speaker after the AGM is mixed-gas diver Bruce Henderson who has actually been face to face with a live coelacanth – and amid the excitement he remembered to take some videos of it that he will show at his talk.

The KZN coelacanth – Image: Bruce Henderson