The coelacanth is an airhead

Evolution normally hates waste. If something isn’t needed, more often than not it will be selected out. This makes the ancient marine fish the coelacanth something of a puzzle.

After all these millions of years, why would Latimeria chalumnae still have a brain that occupies just 1% of the skull that surrounds it? It’s the equivalent of a cavity the size of a tennis ball housing a pea. 

It’s a mismatch unequalled among vertebrates, and understanding it may provide insights into how other marine and land animals evolved. The coelacanth is closely related to tetrapods – four-limbed vertebrates including amphibians, mammals and reptiles.

An international team has started down that path by presenting, in the journal Nature, the first observations of the development of the skull and brain in a living member of the species.

And that, of course, is a story in itself. Coelacanths were thought to have been extinct for 70 million years until the accidental capture of a living specimen by a South African fisherman in 1938.

In the latest study, researchers led by Hugo Dutel at the University of Bristol, UK, used imaging techniques to examine the fish’s brain cavity at different stages of development. In particular, they wanted to find out when its skull divides.

The coelacanth’s braincase is completely split into anterior and posterior portions by an “intracranial joint” – an unusual feature otherwise found only in fossil lobe-finned fishes from the Devonian period, which lasted from 410 to 360 million years ago.

Collaborator John Long, from Flinders University in Australia, says they now have a better understanding of why ancient fossil fish had hinged heads and maybe why four limbed animals later lost the joint between two parts of their skull.

“We think that formation of this special joint is probably caused by the unique development of the notochord, a tube extending below the brain and the spinal cord in the early stages of life,” he says.

“It usually degenerates into a small rod below the brain in some fishes. However, the notochord for coelacanth expands dramatically to become 50 times bigger than the brain in the adult fish.

“This process of brain growth is very unusual, especially compared to primates like us in which the brain expands dramatically. A mismatch between the brain and its cavity also exists in some other living and fossil fishes, but what is observed here is totally unequalled among vertebrates.”

It’s exciting, but it’s just a start.

“These are very unique observations, but they represent only a tiny step forward compared to the amount we know on the development of other species,” says Dutel. “There are still more questions than answers.”