Imagery Reveals Remarkable Secrets of Ancient Fish


“This fish is iconic, extremely rare, and remains shrouded in mystery. It is difficult to observe alive because it lives in underwater caves at depths of 150-200 meters. And, like many of the few specimens collected at over the years have been cut into pieces, new methods were needed to learn more about its way of life. Now we know a little more,” said Peter Rask Møller, associate professor and curator at the Museum of Natural History in Denmark from the University of Copenhagen.

Overview of Coelacanth CT Volume Rendering

CT overview in volume rendering of the coelacanth of Copenhagen. Video courtesy of Dr Henrik Lauridsen and Dr Peter Rask Møller.

There are two living species of coelacanth: Latimeria chalumnaewho lives off the coast of East Africa, and Latimeria menadoensis, which lives off Indonesia. The oldest known coelacanth fossils date back more than 410 million years. Just over 300 living coelacanth specimens have been captured worldwide. Coelacanths are pregnant for five years and give birth to live young. The coelacanth can measure up to two meters in length and weigh up to 100 kg.

The Danish coelacanth specimen was caught in the Comoros archipelago in the Indian Ocean in 1960. At the time, Comoros was a French colony. Danish oceanographer Anton Bruun persuaded France to donate the specimen to the Zoological Museum in 1962.

The donation came with strict requirements that the specimen was only to be used as an exhibit and could not be subjected to dissection under any circumstances. This requirement was unofficially lifted in 1975, when the Comoros gained independence from France, Copenhagen opted to honor the original agreement, so the specimen was kept intact. Over the years, the specimen has been exhibited in the Zoological Museum and the Old Aquarium in Denmark.

Since its discovery in South Africa in 1938, many other coelacanth specimens have been dissected, so its anatomy is no secret, but very little is known about the physiology of the fish.

Peter Rask Møller and Henrik Lauridsen with the Copenhagen coelacanth on the CT scanner, preparing the specimen for high-resolution imaging covering an entire night. Image courtesy of Henrik Lauridsen.

By placing the fish in CT and MRI scanners at Aarhus University Hospital in Skejby, the researchers were able to model the species more accurately than ever before, without destroying the fish. The models show the exact distribution of bone minerals and fats in his body. Among other things, the models help explain the coelacanth’s unique “headspinning drift hunt” technique, whereby it drifts slowly along a seabed vertically with its head and snout down, as it uses an electro-sensitive organ to scan the bottom for cephalopods and fish to eat. .

“We found that the coelacanth has a special skeleton with a lot of bone mass in the head and tail, while there are almost no vertebrae. It’s quite unique. The heaviest parts are in each end of the fish, making it easier for the fish to stand on its head. The balance point is an advantageous mechanism for its way of life,” says Associate Professor Henrik Lauridsen from the University of Arhus.

The researchers also discovered the precise distribution of fatty tissue in the fish’s body, including the amount in its fatty bladder, because the coelacanth does not have a regular gas bladder like modern fish. The figures show that the fat content correlates with the depths at which the fish live, where the fat allows the fish to have neutral buoyancy and expend virtually no energy to stay hundreds of meters deep.

Prey in the gastrointestinal tract and prey with a swim bladder. (ad) Mineralized remains of prey in the distal part of the coelacanth digestive tract imaged by CT and MRI. The dashed gray square in (a) is magnified in (b) and shown in a similar view plane using a T2-weighted MRI (c). A 3D surface rendering of the fecal matter is shown in (d). Sagittal T2-weighted MRI slice (e) and three-component model (skin, bone, swimbladder) (f) made from CT scan and MRI of Beryx decadactylus, a known coelacanth prey. This species contains an air-filled swim bladder (light blue segment in f) which, assuming neutral buoyancy, displaces a volume of seawater of the same mass as the net weight of the fish (at the exclusion of the swim bladder) in seawater. Image courtesy of Dr. Henrik Lauridsen and BMC Biology.

A special characteristic of the coelacanth is that the females gestate for five years before giving birth to live young. One of the great mysteries of the coelacanth among researchers is: Where does the coelacanth give birth? Danish researchers hope to shed light on this question soon.

“We still have no idea where their fry are born. By analyzing the distribution of bone and fat in a fetus, we can probably tell how deep the fry are adapted to live. This knowledge is also important. for the preservation of this critically endangered species. because when we don’t know where they are, we can’t know where to protect them. And there is cause for concern. Coelacanths have a reproductive rate incredibly slow, which makes them even more vulnerable,” Lauridsen said.

The researchers point out that their models can be applied to all other aquatic organisms and used to determine, among other things, whether the organisms are adapted to the ocean depths in which they live. This is relevant knowledge at a time when climate change could lead to altered ocean currents and therefore impact marine life.

They now plan to use the imagery on preserved specimens of juvenile coelacanths.

“They are born alive after five years of gestation (the longest in the animal kingdom), but no one knows how deep,” Lauridsen said. “By having the content and distribution of bone minerals (CT) and adipose tissues (MRI), we might be able to model at what depth they are adapted to have neutral buoyancy, at which would be the expected depth of birth because a small fish can’t afford to fight gravity all the time.”

Full details on the imaging technique used by the authors are available in the open access article, published August 19 in BMC Biology.

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