The elephant sharks, “living fossils” that have not evolved at all for many hundreds of millions of years, may carry the secret to combat osteoporosis in their cartilage. Named for their long snouts that looks a bit like a trunk, these prehistoric fish dig around on the ocean floor for their diet of crustaceans. They can live at depths of 200 meters and grow to around 120 centimeters long. While changes and advances of all descriptions have been going on in the world above and beyond their watery domain, the elephant shark has remained robustly unchanged, right down to its molecular structure. In fact, it has evolved at an even slower pace than the famed coelanth, another “living fossil” carrying the same DNA as it did in days of very, very long ago.
Scientists writing in Nature journal (published January 9th) have now stated that the elephant shark is evolving “significantly slower” than the coelanth. Although it is called a shark, it is not a member of the shark family at all. It is, in fact, a chimera, a sub-sect that took a different evolutionary route from sharks, skates and rays (the elasmobranchs) around about 420 million years ago. Like sharks, and like us, they are vertebrates. Unlike us, their skeletons are made of cartilage, not bone. How and why that is could be fundamental to the advancement of the knowledge of how bones develop.
Wesley Warren, the associate professor of genetics who worked on the study from Washington University School of Medicine in St Louis, said they now had a genetic blueprint for understanding diversity in all bony vertebrates, including the human race. He called the species a “crucial outlier” for that understanding. One important discovery was that the elephant shark had a third less of the genetic code C.milii than humans. This genome in particular has the potential to increase awareness of the process in the formation of bones, and this in turn, will affect possible new treatments for osteoporosis. The genome name is allied to its classical nomenclature of Callorhinchus milii. These fish also have rudimental immune systems, with almost no defences against the sorts of infections, bacterial and viral that humans are prey to. In spite of this, they live long and healthy lives. Again, this is an exciting discovery from the elephant shark investigation.
Decoding the genome has offered original insights into the origins of bone formation, along with significant insights into the beginnings of adaptive immune systems. There are hitherto missing parts in the growth pattern of the evolutionary tree of life which can now be identified. There are slightly less than one billion DNA pairs in an elephant shark genome compared to around three billion in humans. They may have remained “simple,” but they have certainly survived well.
The leading author behind the work, Byrappa Venkatesh PhD, who works from A*STAR in Singapore, said that they now had a “cornerstone” for analyzing how both skeletal and immune systems evolved. There has never been a better proxy than the elephant shark when it comes to study of vertebrates with jaws. Many other international universities have been involved with the research which was primarily funded by the National Human Genome Research Institute.
Thankfully, the elephant shark is not on the endangered species list and belongs in the “least concern” category of conservationists. They are sometimes known by another name, Australian ghost sharks, and occasionally end up being caught commercially, but they are not at any risk and thrive in abundance. Their habitat is the waters that lie off New Zealand and the coastline of South Australia. The coelanth is much more rare, and inhabits the seas off South Africa in its own unique little niche.
In other shark news today, which will not go down well with surfers and water sports enthusiasts, it has been revealed that Great White Sharks live up to three times longer than was previously understood to be their lifespan. Far from expiring around the age of thirty, like a long-lived cat, they carry on, much like us, to more like ninety years.
While most humans live in fear of the Great White Sharks, we have nothing to fear from the elephant shark. In truth, it is not even a shark. Instead, it is a primodial key to unlocking ancient mysteries of creation and evolution. Scientists seeking cures and treatments for osteoporosis have much reason to be excited by the discovery of the “cornerstone” blueprint for bone development in the catilagineous skeleton of the elephant shark.
By Kate Henderson