Coelacanths are not living fossils

ResearchBlogging.org

The term ‘living fossil’ is often misleadingly used in the popular press to describe species which have, supposedly, stopped evolving. Commonly cited examples include horseshoe crabs, Ginkgo trees, hagfish and, perhaps the most famous of all, the coelacanths, a group of lobe finned fish with a very long evolutionary history of which two species still survive in the deep waters of the West Indian Ocean.

A modern day coelacanth (Latimeria chalumnae)
A modern-day coelacanth (Latimeria chalumnae)

Coelacanths have long been known from the fossil record with the oldest specimen dating back to the Devonian period, some 400 million years ago. They were however thought to have gone extinct, along with many other animals, in the end Cretaceous mass extinction event. That all changed one day in 1938 when a South African museum curator named Marjorie Courtenay-Latimer discovered a coelacanth amongst the catch of a local fisherman. The discovery was a sensation, a fish that had been thought to have been extinct had been rediscovered 65 million years later, it was not extinct! It was alive! It was amazing!

That’s how the story goes at least, and ever since it’s discovery journalists have talked about the fish that has been “left behind by evolution”. But is this really true? Can a species really exist for a span of time so great that it will have seen ice ages come and go, mountain ranges form and the great super-continent of Gondwana break apart, and through all this not change at all? Over recent years a mountain of evidence has been steadily growing showing that this is in fact not the case, coelacanths, like any other species, are constantly evolving to adapt to changing conditions.

A comparison of the living coelacanth (Latimeria) with some of it's extinct relative. The morphological differences are striking
A comparison of the living coelacanths (genus Latimeria) with some of its extinct relatives. The morphological differences are striking. Image from Casane and Laurenti.

It is sometimes claimed that there is a low rate of change in coelacanth DNA and that this leads slow evolution. However, this idea is now being challenged by systematic studies of the coelacanth genome which do not detect slow rates of genetic change. In one study forty-four genes were analysed and no dramatic decrease in the rate of change compared to other species was detected. Furthermore, there is no known reason why coelacanths should have slowly evolving genomes. Their environment in the deep ocean, while relatively stable, is not particularly unusual and is inhabited by other species which are not considered living fossils. Another factor that may lead to a slow rate of evolution is a slow generation time, however, the reproductive rates of coelacanths are not thought to be particularly long. Finally, coelacanth populations are small, and small population size is known to increase the rate of genetic change within a species. We might therefore expect these species to be evolving rapidly, not standing still.

Probably the most widely held belief about coelacanths is that, even if they are genetically different, they look exactly the same now as they did millions of years ago. This belief is mistaken. No fossils are known for either species of surviving coelacanth or even for members of its genus, Latimeria. This suggests that the scientists responsible for classifying the fossil and living species consider the morphological differences so great the they should be placed in widely separated groups. In fact, there are significant differences in the body shape and structure of modern and extinct coelacanth species. These include changes in the number of vertebral arches and substantial differences in skull morphology. The swim bladder of coelacanths has also changed from being filled with oil in the extinct genus Macropoma, to being ossified in modern species, suggesting that the two groups lived in very different environments. Lastly, there are substantial differences in size, with modern coelacanths being three and a half times larger than their closest extinct relative (one and a half vs half a metre).

Comparison of the skeleton of modern and extinct coelacanths. A) Latimeria chalumnae (a modern species), B) Macropoma lewesiensis (extinct), C) L. chalumnae skull D) M. lewesiensis skull, E) Pectoral fins of L. chalumnae (above) and Shoshonia actopteryx (another extinct relative) (below). Image from Casane and Laurenti.
Comparison of the skeleton of modern and extinct coelacanths. A) Latimeria chalumnae (a modern species), B) Macropoma lewesiensis (extinct), C) L. chalumnae skull D) M. lewesiensis skull, E) Pectoral fins of L. chalumnae (above) and Shoshonia actopteryx (another extinct relative) (below). Image from Casane and Laurenti.

The view that coelacanths are ancient prehistoric fish which have stopped evolving has been around for a very long time. However, the evidence is now in and it shows that it is time to put this mistaken idea to bed.

                                                                         

For a comprehensive review of the evidence showing that coelacanths are not living fossils see: –

Casane D, & Laurenti P (2013). Why coelacanths are not ‘living fossils’: a review of molecular and morphological data. BioEssays : news and reviews in molecular, cellular and developmental biology, 35 (4), 332-8 PMID: 23382020

For the study analysing forty-four coelacanth genes see: –

Takezaki N, Figueroa F, Zaleska-Rutczynska Z, Takahata N, & Klein J (2004). The phylogenetic relationship of tetrapod, coelacanth, and lungfish revealed by the sequences of forty-four nuclear genes. Molecular biology and evolution, 21 (8), 1512-24 PMID: 15128875

For a contrasting study claiming slow molecular evolution in these species see: –

Amemiya CT, Powers TP, Prohaska SJ, Grimwood J, Schmutz J, Dickson M, Miyake T, Schoenborn MA, Myers RM, Ruddle FH, & Stadler PF (2010). Complete HOX cluster characterization of the coelacanth provides further evidence for slow evolution of its genome. Proceedings of the National Academy of Sciences of the United States of America, 107 (8), 3622-7 PMID: 20139301

Two new species profiles on ARKive

Just over a year ago I was asked if I’d like to write some species profiles for the website ARKive as part of their ARKive and universities scheme. Well, it’s been a long time coming but my two profiles have finally been published! You can now go read all about Allen’s rainbowfish of northern Papua, a species that was only discovered in 1997. What ARKive doesn’t tell you is that this species has not been seen since its initial discovery and no one knows exactly where it was originally found. Thankfully several type specimens have been preserved as a record, although without a known location its conservation status is not clear.

Allen's rainbowfish
Allen’s rainbowfish (Chilatherina alleni)

You can also read about the Southern blue-ringed octopus of Australia, a beautiful animal but also one of the most deadly. At less than 15cm across its bite, containing the venom tetrodotoxin, has been known to kill humans, luckily for us they are not aggressive and generally shy away from people.

The southern blue ringed octopus
The southern blue ringed octopus (Hapalochlaena maculosa)