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Ancient Insect Hails From Sunken Island

17 December 2008 (All day)
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Thomas Reischig

Rare breed. A male and a female (left) Lord Howe tree lobster.

Which came first: the tree lobster or the island? You may not have been asking that odd question, but researchers have nonetheless answered it with a report indicating that one species of this flightless insect is apparently older than its native home, Lord Howe Island off the coast of Australia. The find suggests that the bug originally evolved on an older island, one now submerged under the Pacific Ocean.

Stretching up to 13 centimeters long, tree lobsters look like a cross between a grasshopper and a cockroach. Unlike the other members of its group, the elegant stick insects, tree lobsters have a stocky build and live on the ground. The Lord Howe tree lobster (Dryococelus australis) was thought to have gone extinct in the 1960s after the arrival of black rats to the island. But the species hit the headlines in 2001 when researchers discovered a population of 20 to 30 individuals on Ball's Pyramid, a rocky islet off Lord Howe Island. It's currently known as one of the rarest insects in the world.

The Lord Howe tree lobster appears to be harboring even more surprises. As part of an analysis of the evolutionary origin of stick insects, biologist Thomas Buckley of Landcare Research, New Zealand's main research institute for environmental science, and colleagues collected DNA from three tree lobster groups, including D. australis, and about 70 other stick insect species. The team found that D. australis was more than 20 million years old, 13 million years older than the rocks on Lord Howe Island.

So where did this species evolve? Buckley thinks that the solution lies under the Pacific Ocean. Lord Howe Island is the youngest of an old chain of islands formed as the Indo-Australian tectonic plate travels north over a fixed volcanic center, or hot spot. Older islands are now submerged inactive volcanoes. The Lord Howe tree lobster may have evolved in one of these drowned islands and traveled south as its habitat eroded away, the team reported online 16 December in the Proceedings of the Royal Society B.

The DNA analysis revealed something else unexpected: The three tree lobster groups are not closely related. That means that their similar stocky body shape evolved separately as an adaptation to a ground-dwelling lifestyle, a process known as convergent evolution. (The wings of birds and bats are another example.) It's a "very interesting and new ... case of convergent evolution in an unusual animal system," says evolutionary biologist Marco Passamonti, an expert on stick insects based at the University of Bologna in Italy. "This research seems to me very well-supported and sound."

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