Move over, coelacanth. No longer is this extremely rare order of ancient fish crowned the slowest evolving vertebrate animal in the world. That honor now goes to the elephant shark, whose freshly sequenced genome was described in Nature this week.
Known formally as Callorhinchus milii, the elephant shark boasts an incredibly compact genome -- about a billion DNA base pairs, roughly one-third the length of the human genome. And it could provide scientists with new insight into the evolution of their now very distant cousins -- the group of bony fishes called Osteichthyes, which gave rise to all terrestrial vertebrates, including humans.
The elephant shark, also known as the Australian ghost shark, can be found off the coast of southern Australia, and can stretch to about 4 feet long. They’re part of the group of cartilaginous fish known as Chondrichthyes, whose skeletons are made up mostly of cartilage instead of bone.
The cartilaginous fish separated from the bony-jawed fishes -- our ancestors -- around 450 million years ago. But together, these two groups make up about 99.9% of the living vertebrate species.
“Among the three living lineages of vertebrates [cyclostomes, cartilaginous fishes and bony vertebrates], bony vertebrates are the largest and most diverse group of vertebrates,” the authors wrote. “Because cartilaginous fishes are the sister group of bony vertebrates, they constitute a critical outgroup for understanding the evolution and diversity of bony vertebrates.”
So studying the elephant shark's genome, which hasn’t changed much, gives scientists deep insight into what such ocean-dwelling predecessors looked like. That’s why animals like the coelacanth and now the elephant shark are called "living fossils."
After sequencing the elephant shark’s genome, the researchers found that the elephant shark genetic code does hold most types of genes needed for bone formation -- except for the genes for secreted phosphoproteins, which could explain why their cartilage doesn’t turn to bone.
They also found that it lacks genes for many immune system cells and receptors that are typical of other jawed vertebrates. And yet, in spite of a relatively simple immune system, the elephant shark appears to be doing just fine.
“It thus presents a new model for understanding the origin of adaptive immunity,” the authors wrote.