Tree snails traced back 3 million years
By Jan TenBruggencate
Advertiser Science Writer
Genetic research is providing new evidence suggesting that dozens of species of Hawaiian tree snails originated nearly 3 million years ago from a single ancestor.
Bruce Asato • The Honolulu Advertiser
Brenden Holland, a University of Hawai'i population geneticist, said that first snail, probably stuck onto the feathers of a migratory bird, almost certainly arrived on O'ahu, probably in the Ko'olau Mountains when they were still geologically young.
Brenden Holland, a population geneticist at the University of Hawai'i, studies Achatinella livida, a species of Hawaiian tree snail found in northern areas of the Ko'olau range.
Holland said that by comparing genetic samples from different snails, scientists can determine how closely different populations are related. By studying genetic material that naturally mutates at a standard rate, it is possible to determine when that first snail arrived and began spreading into separate species.
In the Wai'anae Mountains, Holland has sampled populations that show clear genetic breaks over very short distances — on adjacent ridges, for example. He thinks the snails date so far back that they originally were on northwestern O'ahu when the volcano had a shield shape resembling the slopes of Mauna Loa and Mauna Kea. Back then, it was easy for snails to move through the canopy of trees from one area to another.
Now, deep gulleys and valleys provide barriers, and the snails on one ridge have no way to interact with snails on the next. Slowly, they are evolving into unique forms of life.
But the tree snails of O'ahu and those on the other islands don't have much more time.
"They're all in trouble," Holland said.
Rats and an introduced cannibal snail are the main threats to their existence.
Snail shells are found scattered under trees where rats have been dining on them, said Mike Hadfield, a UH zoology professor who specializes in the invertebrates. Cannibal snails are worse, he said. While the rats overlook baby snails, the Euglandina cannibal snail will kill every tree snail in an area.
The tree snails were known to early Hawaiians as pupu kani oe, literally translated as "shell that sounds long," because of the belief that land shells sing. Once so common they could be shaken from the trees, the snails are now so rare that virtually every species is either endangered, threatened or considered a "species of concern."
They were collected because of their attractive coloring. Many have alternating stripes of cream and chocolate, some have gold tints and some green.
There are four genus groups in the tree snail subfamily achatinellinae: the O'ahu achatinella snails, the partulina snails from Maui County and the Big Island, the West Maui snail newcombia, and Moloka'i's perdicella snails.
But those groupings are turning out not to be entirely accurate. Holland said genetic studies show that some of the partulina snails actually are more closely related to the newcombia than they are to the other partulinas.
Scientists are taking a four-pronged approach to saving Hawai'i's tree snails. Two involve research to learn more about them: field studies of snails in the wild and Holland's genetic studies. Another is Hadfield's captive-rearing facility at UH-Manoa, where since the late 1980s he has built up a population of 1,200 snails of 15 species.
Twice, groups of captive-raised snails have been taken from the facility and established in the wild.
A final conservation technique is the development of two "exclosures" in the Wai'anae mountains. One is operated by the Army and the other by the state at Pahole. The exclosures use a metal fence topped by a bed of salt, which is backed up by an electric fence. Rats can't climb the fence, and the salt and electric wires are designed to prevent the cannibal snails from entering, Holland said.
"Populations inside are pretty good and they're increasing," Hadfield said.
Reach Jan TenBruggencate at jant@honoluluadvertiser.com or (808) 245-3074.
