From Nature magazine

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 4,283 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 1,200 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.”

Circling the answer
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 scrutinize 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 paper² 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.

4 Comments

1. 1. David Russell 01:36 AM 6/26/12

   I am sure that we have missed a logarithmic amount just in the so called industrial age. As a race if it can eat us or compete with us, we genocide it. It is amazing we are starting to put the brakes on and taking time to learn before burn.

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2. 2. Alenz 07:42 PM 6/26/12

   Don't tell this to Shark-eaters!

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3. 3. David Russell in reply to Alenz 09:53 PM 6/26/12

   Most shark eaters eat the fins. Means they catch the shark, cut off all the fins and throw the sharks back into the ocean. So far all the sharks are witches, they drowned.

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4. 4. Ed Greding 01:04 AM 6/29/12

   I would like to know the basis for the genetic analysis and conclusions. How are two or more species separated taxonomicaly using a molecular basis. That is, which specific genes are involved in the separation, and how are these sequences separated from the individual gene sequences that represent intraspecific variation but not interspecific difference? How does one know which nucleotide sequences to apply? How did the investigators decide the number of gene differences necessary to constitute a species? In the cited study, was the entire genome sequenced for each included individual(unlikely),or only part of the genome? Which part? Why? The only way I can see to answer these questions is to read the original paper (to which many have no ready access), and even then one would have to assume that the answers to these questions are incorporated into the paper. Perhaps this article should have included a more detailed part of this megthodology. As it is written in this essay, details of methods and criteria are not even mentioned.

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