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Shark tooth weapons lead to biodiversity discovery

April 03, 2013|By Geoffrey Mohan
  • Scientists studied teeth used in weaponry from the Gilbert Islands, uncovering two species of shark previously unreported in the island chain's surrounding waters.
Scientists studied teeth used in weaponry from the Gilbert Islands, uncovering… (The Field Museum )

The shark tooth weapons were the kind of cool stuff that drew marine conservation biologist Joshua Drew to the Field Museum of natural history in Chicago. The postdoctoral researcher was admittedly a bit burned out from a job search and the demands of a newborn child. 

So Drew and his colleagues went down a floor to check out the collection of menacing swords and knives laced with dozens of shark's teeth, which once were used to hack enemies to death in the Central Pacific's Gilbert Islands.

The trio, including Pacific Islands collections director Christopher Philipp and zoology curator Mark Westneat, were soon matching the teeth to known species of shark. They discovered two that had not been reported before in the reef areas around the island chain, part of Kiribati. Their findings of this "shadow biodiversity" that apparently went unrecorded for more than 100 years were published Wednesday in PLOS One.

"I just wanted to do something cool and different," said Drew, now at Columbia University. "I just wanted to go down and look at really cool stuff. We were just going to see what was there."

The researchers compared some 100 teeth to examples in catalogs, photographs and in real jaws, then searched the historical and contemporary record of species found in the Gilberts. There was no mention of spot-tail (Carcharhinus sorrah) and dusky (Carcharhinus obscurus) sharks.

"Honestly, the thing that helped us the most is we were at a natural history museum," said Drew. "If I wanted to know what a hammerhead jaw looked like I could go to the ichthyology department and pull out a hammerhead jaw."

Drew and others hope the discovery of a "shadow biodiversity" in the historic Gilbert Island waters will aid in conservation and restoration efforts for marine animals whose populations have been devastated by overfishing, including for fins sold as Asian culinary delicacies.

"You can tell what a reef used to be so we don't set our benchmarks too low" in conservation efforts, Drew said.

"If people made weapons of shark's teeth, then that species must have occurred within the reach of these people," said Daniel Pauly, a fisheries biologist at the University of British Columbia who pioneered the concept of "shifting the baseline" of biodiversity based on historical records.

Looking at weaponry or written accounts, Pauly said, is "as reliable as looking at DNA records." Similar work has been done looking at the historical record of pearls in the Persian Gulf and at the contents of middens in Hawaii, he noted.

"The presence of shark teeth in cultural artifacts permits a comparison that we often can't perform," said Tim Essington, a marine fisheries biologist at the University of Washington. He noted that the Field Museum team does not draw a conclusion about why the sharks are no longer present near the islands. But sharks have been vulnerable to overfishing, particularly as "by-catch" from long-line tuna fishing in that area of the Pacific, Pauly and Essington noted.

Dwindling fish populations are not the most pressing problem confronting residents of the Gilbert Islands. Rising sea levels spurred by climate change threaten to flood low-lying Kiribati, where saltwater has begun to infiltrate groundwater supplies. Residents are considering wholesale relocation, perhaps to nearby Fiji.

"They're really thinking about it," said Drew, who has never visited the Gilberts. "One hundred to 150 years from now, their island may be underwater."


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