Researchers at Tokyo Institute of Technology have undertaken what is believed to be the very first CT scan of eggs inside a coelacanth fish.

“I was surprised to see that all the eggs were the same size,” said Dr Norihiro Okada, a bioscience professor at the university and a member of the research team. “I hope to do research into why this is.”

Each coelacanth fish was roughly 170 cm (67 in) long and weighed about 70 kg (154 lbs). After being captured off the coast of Tanzania, both fishes were frozen and send to Japan where the CT scan showed how each fish contained roughly 40 eggs; each egg being about 7 cm (almost 2 ¾ in) in diameter.

The eggs of a coelacanth are never released into the water because the offspring hatch while still inside their mother. The young fish sometimes reach a length of 30 cm (12 in) before leaving their mother’s body.

Coelacanths were long believed to have gone extinct around the same time as the dinosaurs, until scientists realized that these fishes actually turn up in the nets of African and Asian fishermen now and then. The first confirmed finding is from 1938 when a specimen was captured in the Indian Ocean.

Coelacanths are of special interest to evolutionary biologists since they are thought to represent an early step in the evolution of fish to amphibians. You can read more about this in our coelacanth article.

According to a new study from Uppsala University, the origin of fingers and toes can be traced back to a type of fish that inhabited the ocean 380 million years ago. This new finding has overturned the prevailing theory on how and when digits appeared, since it has long been assumed that the very first creatures to develop primitive fingers were the early tetrapods, air-breathing amphibians that evolved from lobed-finned fish during the Devonian period and crawled up onto land about 365 million years ago.

Lead author Catherine Boisvert[1] and co-author Per Ahlberg[2], both of Uppsala University in Sweden, used a hospital CT scanner to investigate a fish fossil still embedded in clay. “We could see the internal skeleton very clearly, and were able to model it without ever physically touching the specimen,” says Ahlberg. The scan revealed four finger-like stubby bones at the end of the fin skeleton. The bones were quite short and without joints, but it was still very clear that they were primitive fingers. “This was the key piece of the puzzle that confirms that rudimentary fingers were already present in the ancestors of tetrapods,” Catherine Boisvert explains.

The scanned fossil was that of a meter-long Panderichthys, a shallow-water fish from the Devonian period. Panderichthys is an “intermediary” species famous for exhibiting transitional features between lobe-finned fishes and early tetrapods, while still clearly being a fish and not a tetrapod. The specimen used was not a new finding; it had just never been examined with a CT scan before.

So, why have researchers for so long assumed that digits were something that evolved in tetrapods without being present in their fishy ancestors? The main reason is the Zebra fish (Danio rerio), a commonly used model organism when vertebrate development and gene function is studied. If you examine a Zebra fish, you will find that genes necessary for finger development aren’t present in this animal. Researchers therefore assumed that fingers first appeared in tetrapods and not in fish.

It should be noted that similar rudimentary fingers were found two years ago in a Tiktaalik, an extinct lobe-finned fish that lived during the same period as Panderichthys. Tiktaalik is however more similar to tetrapods than Panderichthys.

The Panderichthys study was published in Nature on September 21.