Coelacanths are a group of primitive deep-dwelling fish closely related to tetrapods, four-limbed vertebrates including amphibians, mammals and reptiles.
They 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.
The skull of a coelacanth happens to be completely split in half by a special intracranial joint and its brain is so ridiculously small, it remains only 1% the size of the cavity that houses it, which makes its survival unique amongst all living vertebrates.
How the coelacanth skull grows and why the brain remains so small has puzzled scientists for years.
To answer these questions, University of Bristol’s Dr. Hugo Dutel, Flinders University’s Professor John Long and their colleagues from the United Kingdom and France studied the brain cavity of a coelacanth at different stages of development.
“Our discovery provides a better understanding of why ancient fossil fish had hinged heads and suggests why four limbed animals later lost this joint between two parts of their skull,” Professor Long said.
“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).”
“It usually degenerates into a small rod below the brain in some fishes. However, the notochord for coelacanths 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.”
Two species of coelacanths exist in the world today: the West Indian Ocean coelacanth (Latimeria chalumnae) and the Indonesian coelacanth (Latimeria menadoensis).
The West Indian Ocean coelacanth is a blue fish that has been sighted around Africa, off the coasts of South Africa, Mozambique, Kenya, Tanzania and Madagascar. The Indonesian coelacanth is a brown fish found recently in Indonesia.
Dr. Dutel, Professor Long and co-authors used state-of-the-art imaging techniques to visualize the internal anatomy of West Indian Ocean coelacanths without damaging them.
They digitalized a 2-inch (5 cm) long fetus, the earliest developmental stage available, with synchrotron X-ray at the European Synchrotron.
The data was used to generate detailed 3D models, which allowed the team to analyze how the form of the skull, the brain and the notochord changes from a fetus to an adult.
“Coelacanths are iconic animals thought to be on the line to the first land animals or tetrapods, because of their strange hinged head,” Professor Long said.
“This research shows the peculiar hinge in the skull was caused by persistence of the large cartilaginous rod, or notochord, preventing the skull form ossifying as one solid unit.”
The scientists also observed how these structures are positioned relative to each other at each stage, and compared their observations with what is known about the formation of the skull in other vertebrates.
“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,” Dr. Hutel said.
“There are still more questions than answers! Coelacanths still hold many clues for our understanding of vertebrate evolution, and it is important to protect these threatened species and their environment.”