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Shark species more diverse than thought

Daniel Cressey
Monday, July 02, 2012

A GENETIC study of thousands of specimens of sharks and rays has uncovered scores of potential new species and is fuelling biologists' debates over the organisation of the family tree of these animals.

The work also raises the possibility that some species are even more endangered than previously thought.

Sharks and rays are key predators in marine ecosystems, but the life cycles and population numbers of many species remain poorly understood.

The family tree of these animals ?" which are part of the elasmobranch subclass ?" has proved similarly opaque, with little agreement among researchers over their evolutionary relationships.

Gavin Naylor, a biologist at the College of Charleston in South Carolina, and his colleagues sequenced samples from 4283 specimens of sharks and rays as part of a major effort to fill the gaps.

The team found 574 species, of which 79 are potentially new, they report in the Bulletin of the American Museum of Natural History.

Naylor says that he was "flabbergasted" by the result, especially because the sequencing covered only around half of the roughly 1200 species thought to exist worldwide.

The huge number of new species found raises immediate conservation concerns ?" the reason that some of these purported new species have gone undetected is probably their close resemblance to already-identified species.

The populations of such species may, therefore, be even smaller than estimated, as what was thought to be one population may instead be several smaller populations of separate species.

For example, Naylor's work suggests that the endangered scalloped hammerhead (sphyrna lewini) is actually two separate species.

"Scalloped hammerheads in general have taken a huge hit, so it may be even worse than has been documented if there's more than one species out there," he says.

Naylor is now working on a project with the US National Science Foundation to catalogue the diversity of sharks and rays and is working to assist the International Union for Conservation of Nature to map which species are where in the world.

"This will have an impact on what is considered endangered and the fragility of different organisms," he says.

"These are sentinel species of all sorts of other organisms in the sea which are probably undergoing similar or worse kinds of impacts."

One of Naylor's collaborators, William White, an ichthyologist at the Australian Commonwealth Scientific and Industrial Research Organisation in Hobart, says that their work also highlights some of the problems with the use of genetic information in zoology.

The increasing use of molecular techniques provides a new means to scrutinise the relationship between sharks and rays, he says.

But it also shows that some of the existing ideas about their relationships are problematic.

"Part of the problem is some researchers mining data blindly without a good understanding of where that data has come from in the first place and presuming the names are correct which in many cases they are not."

White and Naylor stress that molecular techniques are only of one many available tools and should be combined with conventional taxonomic work.

Ximena Vélez-Zuazo is a biologist at the University of Puerto Rico in San Juan and the lead author of a 2011 paper that Naylor says suffers from some of the flaws that stem from relying solely on molecular information.

Though she strongly defends her group's work, she agrees that shark phylogeny is still a work in progress.

Vélez-Zuazo adds that most of the debate centres on two of the eight living orders of sharks and rays.

"The main challenge for resolving the phylogeny for sharks remains the limited amount of data," she says.

But both groups agree that having a proper phylogeny of these animals is invaluable.

"For an ancient and diverse group like sharks, having a comprehensive phylogeny ?" meaning including the majority of the more than 500 recognised species ?" will refine our current understanding of fundamental aspects about the evolution of life history traits of these animals, as well as bony fishes," says Vélez-Zuazo.

* Nature Magazine

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