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Thoughtomics

Thoughtomics

Exploring evolution through genes, computers and history

Coelacanths are not living fossils. Like the rest of us, they evolve

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This stuffed coelacanth, described by Smith in 1939, achieved worldwide fame. Source.

It was supposed to be extinct. Yet here it lay, with fins round and fleshy, scales as hard as bone and a tail unlike any living fish. "Lass, this discovery will be on the lips of every scientist in the world", James Smith said to Marjorie Courtenay-Latimer, curator of the East London Museum. Smith had good reasons to make such a grand claim. This was a coelacanth.

Naturalists had known about coelacanths for a long time - but only as fossils. It was Louis Agassiz who first described the group in 1839. Paleontologists had found dozens of different coelacanth species since then, but always in rocks older than 70 million years. The lack of coelacanth fossils in younger strata led them to conclude that coelacanths had gone extinct a long time ago. But the fish that now lay before Smith was no fossil. This creature had been caught only weeks ago, as bycatch by a fishing trawler off the coast of South Africa.

It was 1939, and the discovery of the first living coelacanth was on the lips of scientists around the world. The press heralded the fish as a 'missing link', 'prehistoric fish' and 'living fossil'. In doing so, they branded the coelacanths as a backwards fish for years to come.

The same stereotypes have haunted the coelacanth to this very day. In most popular accounts, the coelacanth is portrayed as a a forgotten survivor that has been left at the evolutionary wayside. In this modern fable, coelacanths had been trapped in a private bubble of time for millions of years until they re-emerged in the 20th century.

Axelrodichthys araripensis, an extinct coelacanth from South America. Photo by Ghedoghedo.

But the truth is that evolution leaves no fish behind. Coelacanths are as much affected by evolution as finches, ferns and flying lemurs. They have their own evolutionary history - we only need to look for it. This is what Japanese and African coelacanth researchers did not long ago when they took stock of the genetic diversity amongst coelacanths in the Indian Ocean. Through their research, they uncovered a small part of the coelacanth's history of change.

Coelacanths have popped up in many places along the East African coast. Figure from first reference.

Coelacanths have popped up in several places in the Indian Ocean over the years, but the majority of them has been found in the Comoros archipelago. In the late eighties, Hans Fricke filmed how coelacanths inhabit rocky crevices and caves around the Comoros. At night he saw them drifting along the up- and downwelling currents, using their fins as stabilizers, to sneak up on unsuspecting fish. A short stroke with its fan-like tail, a sudden and forceful bite and its prey is gone.

Other coelacanths have been captured off the coast of Madagascar, South Africa, Mozambique and Kenya, but these fish have been dismissed as strays. Biologists reasoned that coelacanths would not be able to survive on the flat and sandy sea floors near Mozambique and South Africa. They presumed that strong ocean currents had swept the creatures away from the Comoros. These dead-end drifters were destined for death.

But there's evidence that these stragglers represent distinct coelacanth populations. Geologists have identified several marine canyons near South Africa and Mozambique in which coelacanths could live. A dozen coelacanths have been caught near Tanzania every year since 2003. It's unlikely that these are all strays. Indeed - when marine biologists let a remotely operated submersible descend in Tanzanian waters, they were able to capture footage of nine living coelacanths. Could this be the second home of coelacanths in the Indian Ocean?

In the paper that was published a few months ago, researchers have compare the DNA of Tanzanian and Comoran coelacanths. They found that some Tanzanian fish carry unique genetic variants. These variants were not found in any Comoran fish or anywhere else. This was especially true for coelacanths captured off northern Tanzania. The team believe their results indicate that coelacanths from northern Tanzania form a separate breeding population from the coelacanths from the South and the Comoros. These last two populations are much closer to each other genetically.

The researchers think the last common ancestor of the Tanzanian and Comaran coelacanths lived at least 200,000 years ago. For your sense of time: this was around the same time when the first modern human walked the earth. The researchers arrived at this estimate with a simple technique, known as the molecular clock: the more genetic differences exist between two lineages, the longer ago they diverged. But calibrating the clock can be tricky. Using a different calibration point, the researchers dated the split between the two populations to a few millions years ago.

Whatever the exact figure is, fact is that the Indian Ocean harbours distinct populations of coelacanths. If the Comoros Archipelago is the ancestral home of coelacanths, some fish have packed their things and settled somewhere else. Given enough time these populations might evolve into distinct species. We know this has happened in the past, for there are two species of coelacanth alive today. Aside from the West Indian Coelacanth, there exists a second species of coelacanth that was discovered at a local fish market two decades ago, near Indonesia.

Scientists have just started to collect and sequence coelacanth DNA. The amount of DNA analyzed in genetic studies (including this one) has been tiny so far. As more sequences will become available, more evidence of the continued evolution of the coelacanth will come to light.

Let's leave the silly concept of 'living fossils' behind. Watch the movie above, and see the coelacanth sail the currents with subtle movements of its fins. Marvel at the mysterious headstand these creatures perform. Peer into its eyes, and see how the light is reflected back at you. These creatures are no fossils. They are very much alive.

As Smith wrote in the paper that announced the discovery of a second specimen:

"Numbers of successful modern fishes appear less well equipped for survival than the coelacanth. [..] Coelacanths can scarcely be regarded as degenerate fish. They are apparently full of vigour."


References:

Nikaido, M., Sasaki, T., Emerson, J., Aibara, M., Mzighani, S., Budeba, Y., Ngatunga, B., Iwata, M., Abe, Y., Li, W., & Okada, N. (2011). Genetically distinct coelacanth population off the northern Tanzanian coast Proceedings of the National Academy of Sciences, 108 (44), 18009-18013 DOI: 10.1073/pnas.1115675108

Schartl, M., Hornung, U., Hissmann, K., Schauer, J., & Fricke, H. (2005). Genetics: Relatedness among east African coelacanths Nature, 435 (7044), 901-901 DOI: 10.1038/435901a

Fricke, H., Hissmann, K., Schauer, J., Reinicke, O., Kasang, L., & Plante, R. (1991). Habitat and population size of the coelacanth Latimeria chalumnae at Grand Comoro Environmental Biology of Fishes, 32 (1-4), 287-300 DOI: 10.1007/BF00007462

Fricke, H., Reinicke, O., Hofer, H., & Nachtigall, W. (1987). Locomotion of the coelacanth Latimeria chalumnae in its natural environment Nature, 329 (6137), 331-333 DOI: 10.1038/329331a0

SMITH, J. (1939). A Living Fish of Mesozoic Type Nature, 143 (3620), 455-456 DOI: 10.1038/143455a0

Fricke, H., Hissmann, K., Froese, R., Schauer, J., Plante, R., & Fricke, S. (2011). The population biology of the living coelacanth studied over 21 years Marine Biology, 158 (7), 1511-1522 DOI: 10.1007/s00227-011-1667-x

The views expressed are those of the author and are not necessarily those of Scientific American.

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