These primitive, deep-sea fish live to 100, surprising scientists

Coelacanths may also carry their babies for five years, according to a study that raises concern for the future of this critically endangered species.

Published 18 Jun 2021, 09:34 BST, Updated 18 Jun 2021, 11:03 BST
01-coelacanth-fish

A coelacanth swims in South Africa's Sodwana Bay. Long thought extinct, the animal was rediscovered in 1938.

Photograph by Laurent Ballesta, Nat Geo Image Collection

Coelacanths—primitive, deep-sea fish once thought to have died out with the dinosaurs—have astonished scientists yet again. These six-foot-long “living fossils” can live up to a hundred years, five times longer than thought, a new study says.

New analysis of the scales of African coelacanths—one of two known species—reveals their life span had been erroneously estimated at only 20 years.

Not only that, female coelacanths likely carry their young for five years, three years longer than thought—and don’t reach adulthood for 55 years, says study leader Kelig Mahe of IFREMER, France’s ocean research institute. (Read how the coelacanth evolved slowly over time.)

The coelacanth’s presumed short life span had never matched with its low reproductive rates, slow metabolism, and low oxygen absorption—all qualities of slow-maturing marine animals, such as deep-sea sharks

“It’s been an enigma this whole time,” says Selena Heppell, head of the Department of Fisheries Wildlife and Conservation Sciences at Oregon State University, who wasn’t involved in the study. “So [now] it makes a lot more sense.”

At the same time, the results, published in the journal Current Biology, mean the critically endangered fish are even more vulnerable to threats such as accidental bycatch or extreme climate events.

Scaling up

In 1977 the first study on coelacanth ageing focused on calcified structures called macro-circuli on the scales of 12 African coelacanth museum specimens. These macro-circuli were thought to be a bit like tree rings or ice cores, marks that record intervals of time.

Based on the regular increments of macro-circuli growth on the scales, scientists at the time concluded macro-circuli were likely deposited two times a year—later revised to once a year, a result that equated to a life span of 20 years.

Curious if there was more to the story, Mahe and his team recently examined 27 African coelacanth specimens, stored at the National Museum of Natural History in Paris and caught between 1954 and 1991, including one juvenile and two embryos.

When they looked at macro-circuli with a transmitted light microscope, as the previous teams had, they found the same result as the scientists in the 1970s.

Hidden clues

But when the team examined the coelacanth scales again using polarised-light microscopy—a more advanced technique that reduces glare and sharpens images—it was a game-changer. The polarised light revealed the presence of numerous smaller micro-calculi among the macro-circuli. Analysis of these micro-calculi also revealed there are deposited on an annual basis.

Counting these smaller circuli as annual deposits put the museum specimens between five and 84 years old, much more in line with the coelacanth’s known biology.

Further analyses using a standardised measurements for ageing animal specimens confirmed their results, meaning the coelacanth has one of the most sluggish metabolisms in the sea. (See incredible photos of coelacanths up close.)

To figure out the coelacanth’s gestational period, the team measured the scales of the embryonic specimens using the same methods for the adult fish, which showed that a coelacanth pregnancy lasts five years.

Though the scientists didn't study the related Indonesian coelacanth, which is also critically endangered, they hope to apply the same technique to that species.

Convincing results

The study’s results are convincing, says Brian Sidlauskas, an ichthyologist at Oregon State University who wasn’t involved in the paper.

The study has a thorough statistical approach, he says, with the scientists “asking themselves whether they believe this data.” 

Mark Terwilliger, an expert in fish growth at Oregon’s Department of Fish and Wildlife, notes that since fish scales can slough off and reabsorb into the body as an animal ages, they’re not a reliable estimate of the ages of long-lived fish.

What’s more, micro-circuli in the scales of other bony fish species have not been shown to correspond to an animal's age. (Read about an extinct, tuna-like coelacanth that hunted fast prey.)

“That being said, their results are what I would expect for a slow-growing, deep-water fish,” Terwillger said by email.

‘Particularly sensitive’

The results translate into concerning news for this rare fish, which typically lives at depths of 2,300 feet off Africa’s eastern coast.

If a coelacanth dies before it can mate, that animal can’t contribute to the species’ already declining populations, Mahe says. African coelacanths likely produce a litter of three to 30 pups every five years. 

This means African coelacanths are “particularly sensitive to any perturbation,” says Mahe, who’s interested in studying next whether warming seas due to climate change may impact coelacanths.

The study, Heppells adds, “reminds us that there's a lot we still don't know about weird things in the sea.”

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