GoKawiilThis great white shark (Carcharodon carcharias) is still a shark. Credit: Dave Fleetham/Design Pics Editorial/UIG via Getty
In a 1981 magazine essay, the evolutionary biologist Stephen Jay Gould let readers in one of his field’s counter-intuitive truths. Aquatic animals, including lungfish and coelacanths, are more closely related to tetrapods — four-limbed vertebrates — than to salmon, sticklebacks and many other things people call ‘fish’ — or, as Gould quipped, “there is surely no such thing as a fish”.
Sharks could be in a similar situation. A genomic study of dozens of shark species and their close relatives suggests that the ocean’s top predators might also not be a natural biological group, contrary to what studies using more-limited genetic data have suggested.
The analysis, posted last month to the bioRxiv preprint server, finds that, when researchers look at some ‘ultra-conserved’ parts of the genome, a peculiar family of sharks called Hexanchiformes might be part of an evolutionary lineage that is distinct from the group that includes all other sharks, as well as skates and rays1.
The results, which haven’t been peer reviewed, suggest that most animals that people call sharks are more closely related to rays and skates than to hexanchiform shark species — just as Gould pointed was the case for some species called fishes. Biologists call such groups paraphyletic.
Whether a grouping of animals is paraphyletic or not doesn’t keep most scientists awake at night. But accurate family trees, or phylogenies — including one for sharks — help researchers to chart the evolution of key traits. “Having an accurate phylogeny is a way forward for understanding the processes that have shaped life,” says Gavin Naylor, an evolutionary biologist at the Florida Museum of Natural History in Gainesville.
Shifting definitions
Sharks, along with rays, skates and other sea creatures with a cartilaginous skeleton, are part of a group called chondrichthyes, which shared a common ancestor with bony fish that lived more than 400 million years ago.
Animals resembling modern-day sharks have existed for much of this time, says Naylor. “These things have been around looking the way they look, or at least recognizable as sharks, for 330 million years.”
Yet researchers are unsure how the different members of Chondrichthyes — which is one of the major jawed vertebrate groups — are related to one another. Anatomical studies have concluded2 that rays and skates, together known as batoids, were either distinct from all sharks, or members of a shark subgroup.
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