Episode 83 – Coelacanths

Listen to Episode 83 on PodBean, iTunes, YouTube, or your favorite podcast place!

In 1938, a fishing crew pulled up a fish that everyone thought had been extinct for 70 million years. Since then, these fish have become famous not just for their strangely incomplete fossil record, but also for their unique anatomy and their close relationship to land vertebrates. In this episode, we discuss the strange and incredible story of Coelacanths.

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Coelacanths

Coelacanths are a small group of fish, with only two living species: the West Indian Ocean coelacanth (Latimeria chalumnae) found near Madagascar and Africa, and the Indonesian coelacanth (Latimeria menadoensis). They are slow-moving predators, often found deep underwater along the slopes of volcanic islands. At night, they go out in search of food, and many rest in caves during the day.

West Indian Ocean coelacanths first became known to science when one was fished up off the coast of South Africa in 1938. It was much later, in 1997, that scientists first identified the Indonesian coelacanth.

Latimeria_chalumnae_replica
The West Indian Ocean coelacanth (pictured here) is the more common of the two species and can be identified by its royal blue coloration, compared to the brown colors of the Indonesian coelacanth. Both can grow to 2 m (6.5 ft) in length. (Citron / CC BY-SA 3.0)

Coelacanths are part of a group called the sarcopterygians, or lobe-finned fish, a group that also includes lungfish and tetrapods. This makes coelacanths one of our closest living fish relatives.

Like other sarcopterygians, coelacanths have fleshy lobes in their fins. They also have their own set of unique features, including a “fatty lung” in place of an air bladder, an electrosensitive rostral organ, a notochord that persists into adulthood, and a habit of bearing live young.

In 2013, researchers sequenced the genome of the West Indian Ocean coelacanth. Genetic studies on these fish have helped scientists understand where coelacanths fit in the sarcopterygiian tree of life (close to us, but not as close as lungfish); recognize that coelacanths have a genetic code for limb growth very similar to ours; and realize that coelacanths show an unusually slow rate of evolutionary change.

Fossil Coelacanths

The fossil record of coelacanths extends back about 408 million years, and molecular (genetic) estimates indicate that coelacanths diverged from lungfish not long before that.

Coelacanth_indet
Most fossil coelacanths, like this one from the Museum Bergér, closely resemble their living relatives. (Ghedoghedo / CC BY-SA 4.0)
Rebellatrix_divaricerca
Rebellatrix divaricerca, or the “rebel coelacanth (with a) forked tail,” found in Lower Triassic sediment of British Columbia, was a 1.3-meter (4 ft 3 in) fish with a tuna-like forked tail, and was likely a fast swimmer. . (Apokryltaros / CC BY-SA 4.0)
MawsoniaDB16
Mawsonia, a genus from Cretaceous Africa, includes the largest coelacanths ever discovered, growing up to 4 meters (13 feet) long. This group was not the only one to contain giant coelacanths. (DiBgd / CC BY-SA 4.0)

The latest coelacanths in the fossil record date to around 70 million years ago. For a long time, paleontologists inferred that these fish had gone extinct during the end-Cretaceous extinction – that is, until the living species were discovered!

“Living Fossils”

Because of their close resemblance to their fossil relatives, coelacanths have been called living fossils. This is a term originally introduced by Charles Darwin, and used by many since to refer to organisms that are similar to fossil relatives. It’s a popular term, but can also be misleading, since it’s often used to suggest that these organisms have somehow avoided evolution.

This, of course, isn’t true. Coelacanths are still evolving today just like all species. And indeed, a recent study found evidence for a diverging Tanzanian population of the West Indian Ocean species.

 

Other links:
The Dramatic Early Research on the Indonesian Coelacanth:
Dispute Over a Legendary Fish
Tangled tale of a lost, stolen and disputed coelacanth

 

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