Scanning ‘living fossil’ fish explains its odd swimming

Above, Latimeria chalumnae specimen in the Natural History Museum, Vienna, Austria. (Credit: Wikimedia Commons)

Researchers have removed Denmark’s only specimen of a primordial fish, a coelacanth, from its jar of alcohol and gained new insight into how it functions.

The new knowledge could contribute to saving this critically endangered deep-sea dweller.

When a South African fisherman came across a coelacanth in his net in 1938, it was like finding a living dinosaur—a catch that sent shockwaves throughout the scientific community. Until then, the coelacanth (SEE-lə-kanth) was believed to have been extinct for 66 million years.

Since then, only about 300 specimens of this living fossil have been caught worldwide. In Denmark, a single fish, “specimen number 23,” has been submersed in alcohol at the Zoological Museum in Copenhagen for 60 years. The Danish coelacanth specimen was caught in the Indian Ocean’s Comoros archipelago in 1960.

As many other coelacanth specimens have been dissected, its anatomy is no secret. But very little is known about the fish’s physiology—the way it functions. Now, researchers have discovered new things about this extraordinarily rare and elusive deep-sea dweller.

two adult men hold large pale fish at entrance to CT scanner
Peter Rask Møller and Henrik Lauridsen with the Copenhagen coelacanth at the CT scanner preparing the specimen for high resolution imaging spanning an entire night. (Credit: Henrik Lauridsen)

“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 (roughly 500-650 feet). And, since a large portion of the few specimens collected over the years have been cut to pieces, new methods were required to find out more about how it lives. We now know a bit more,” says Peter Rask Møller, an associate professor and curator at the University of Copenhagen’s Natural History Museum of Denmark. He is a coauthor of the paper in the journal BMC Biology.

Along with Henrik Lauridsen of Aarhus University and a group of colleagues, Møller is the first person ever to conduct research on the Danish specimen, which has been preserved intact and only been used for display until now.

By putting the fish in CT and MRI scanners at Aarhus University Hospital in Skejby, the research scientists could model the species with more precision than ever while not destroying it.

Head-down hunting

The models show the exact distribution of bone mineral and fat in its body. Among other things, the models help explain the unique “headstand drift hunting” technique of the coelacanth, whereby it slowly drifts along a seabed vertically, head and snout downwards, as it uses an electrosensitive organ to scan the bottom for cephalopods and fish to eat.

“We discovered 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 at either end of the fish, which makes it easy for the fish to stand itself on its head. The balance point is an advantageous mechanism for its way of life,” explains Lauridsen, associate professor at Aarhus University’s department of clinical medicine.

red, orange, and purple are darkest at fish's head in scan image
This CT image shows the bone mineral density of the coelacanth (Credit: Henrik Lauridsen)

The researchers also discovered the precise distribution of fatty tissue in the body of the fish, including the amount in its fatty bladder, as coelacanth don’t have a regular gas swim bladder like modern fish. The numbers show that the fat content correlates with the depths at which the fish live, where fat allows the fish to be neutrally buoyant and spend hardly any energy to remain hundreds of meters deep.

Coelacanth mystery

One peculiar feature about the coelacanth is that females gestate for a full five years before birthing live young. One of the great coelacanth mysteries among researchers is: Where do coelacanth give birth? The 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 find out at what depth fry are adapted to live. This knowledge is also important in terms of preserving 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. Coelacanth have an incredibly slow reproduction rate, which makes them extra vulnerable,” says Lauridsen.

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

“Instead of going out and catching new coelacanth, which are both rare and protected, the modern scanning techniques have allowed us to perform exciting new analyses, despite the animal having been submersed in alcohol for decades. Having museum collections of rare animals is like entering the world’s best secondhand store, with the wildest array of recycled things that can no longer be found new,” concludes Møller.

Source: University of Copenhagen