They are only a few millimetres long, hard-shelled and humble. But the New Zealand mud snails have laid siege to four of the five Great Lakes and are threatening to invade rivers and streams, too.
A Penn State research team says these foreign-intruder species that have long been a problem in the western United States could have the ability to change ecosystems in the East.
"Invasive species in general often have substantial negative effects. ... My biggest concern is about them getting into the streams and rivers that are emptying into the Great Lakes - and they are there right now," Edward Levri, an associate professor of biology at Penn State's Altoona campus, said yesterday.
"There is potential for the populations to get very large and start causing ecological problems in those areas, similar to what we've seen out west in terms of changing food webs and out-competing native species and causing them some harm."
Dr. Levri is presenting his research today at the Ecological Society of America's annual meeting in Milwaukee.
He has found mud snails in all the lakes except Lake Huron, although he suspects they have made their way there, too. And he discovered them last year - and revisited them this year - in a stream in upstate New York, close to the Niagara River.
"It hasn't spread that rapidly yet. The problem is there's potential. And the reason that there is potential is that snail loves to hitchhike on humans," Dr. Levri said.
Already there are posted signs along rivers in the western U.S. warning anglers not to transport these snails, he said. In some places in streams in Yellowstone National Park, they reach population densities of 323 individuals per square inch.
While densities in the Great Lakes are not as great as in the West, Dr. Levri said the fact they've reached at least one stream is a cause for concern because he had believed the snails would be confined to deep waters.
While some foreign species have been relatively harmless, many have turned out to be insidious pests. In the late 1980s, zebra mussels, originally from the Black Sea, turned up in the Great Lakes, damaging harbours, boats and power plants. There are about 180 foreign species that have hitched their way to the Great Lakes. They reproduce rapidly, aided by the absence of the predators that kept them in check in their native habitats.
The mud snails, which grow to a maximum of 6.5 millimetres in length, reproduce asexually or sexually. The snails found their way into Idaho in 1987, but did not appear in the East until 1991 in Lake Ontario.
Not considered a problem in New Zealand because parasites keep their population down, they can spread rapidly in a foreign land. If this happens, native species can become extinct. Also, because these snails have a low nutritive value, organisms that feed on them will suffer, Dr. Levri said.
He advises recreational water users to be careful moving from one place to another, and anglers to freeze or clean their gear to kill the snails.
"The stream that it was in was a small stream and not fished. However, all it takes is for one person to go in there and walk into another stream to transport the snail. If that happens, and once it gets into a stream where fishermen like to fish in, it might start spreading very rapidly," Dr. Levri said.
Steps to control their spread are necessary, but "usually with these types of invasive species, once they get here they're here to stay," he added.
The Great Lakes invader
The New Zealand mud snail, Potamopyrgus antipodarum, is an invasive freshwater gastropod mollusk that occupies four of the five Great Lakes and is spreading through rivers.
Operculate snail
Uses a lid to seal its opening, preventing it from drying out and even protecting it from being digested if swallowed
Varieties
Can be brown or grey and typically have five to eight whorls coiling out clockwise
Numbers
A one metre square area of water can contain 500,000 individual mud snails
Breeding
Female mud snails are born with an embryo inside them and breed asexually
Worldwide spread
Mud snails spread to Britain by ship as early as 1850, then later that century to Europe
Unstoppable
New Zealand mud snails change the ecosystems they occupy outside of their native habitat because termatodes* that control their population in New Zealand are not found elsewhere
Eastern U.S. mud snails were most likely brought there by European ships
*A parasite that sterilizes many snails, keeping the populations to a manageable size
