The secret to swimming upside down — ScienceDaily

An after-hours trip to the radiology department at Aarhus Skejby University Hospital brought to light a mysterious and ancient fish, one of the rarest in the world, the coelacanth. Researchers from the University of Copenhagen and the University of Aarhus have removed the only Danish specimen of this primordial fish from its pot of alcohol and gained new insights into how it works. The new knowledge could help save this critically endangered deep sea dweller.

When a South African fisherman stumbled upon a coelacanth in his net in 1938, it was like finding a living dinosaur – a catch that sent shockwaves through the entire scientific community. Until then, the coelacanth (SEE-l-kanth) was thought to have been extinct for 66 million years. Since then, only about 300 specimens of this living fossil have been captured worldwide. In Denmark, a single fish, “specimen number 23”, has been immersed in alcohol at the Zoological Museum in Copenhagen for 60 years.

Like many other coelacanth specimens have been dissected, its anatomy is no secret. But very little is known about the physiology of the fish – the way it works. Now researchers from the University of Copenhagen and Aarhus University have discovered new things about this extraordinarily rare and elusive deep-dweller.

“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 over years have been cut to pieces, new methods were needed to learn more about its way of life. Now we know a little more,” says Peter Rask Møller, associate professor and curator at the Natural History Museum of Denmark in the University of Copenhagen He is co-author of the research article published in the journal BMC Biology.

Together with Henrik Lauridsen from Aarhus University and a group of colleagues, Møller is the first person to carry out research on the Danish specimen, which has been kept intact and has only been used for display until now. .

The secret to swimming upside down

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 Department of Clinical Medicine at 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, as 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.

Five years pregnant and young live births

A special feature 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,” says Henrik Lauridsen.

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.

“Instead of going out and catching new coelacanths, which are both rare and protected, modern scanning techniques have allowed us to perform exciting new analyses, despite the animal having been immersed in alcohol for decades. Having museum collections of rare animals is like entering the best thrift store in the world, with the craziest range of recycled items that can no longer be found new,” concludes Peter Rask Moller.


  • The Danish coelacanth specimen — specimen no. 23 — was captured in the Comoros archipelago in the Indian Ocean in 1960. At the time, the Comoros was a French colony.
  • Danish oceanographer Anton Bruun, leader of the Galathea-2 expedition among others, 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.
  • While this requirement was unofficially lifted in 1975 when the Comoros gained independence from France, Copenhagen opted to honor the original agreement. Therefore, specimen 23 was kept intact.
  • Over the years, the specimen has been exhibited in the Zoological Museum and the Old Aquarium in Denmark.


  • There are two living species of coelacanth – Latimeria chalumnae, which 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 are born young.
  • The coelacanth can measure up to 2 meters in length and weigh up to 100 kilograms.
  • The IUCN has listed the East African coelacanth as critically endangered and the Indonesian species as vulnerable.