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Hard times for the hard-shelled

August 9, 2011
By HILARY SMITH , Adirondack Daily Enterprise

Invasive plants such as Eurasian watermilfoil get a lot of attention from shoreowners, municipalities and media. How could these nuisances not get a second look when they form dense mats at the water's surface, degrade fishing access and clog recreational routes? Forest pests like the emerald ash borer also are attention grabbers. They attack trees, causing noticeable tree death and decline in the wood products industry. What about the organisms not so readily seen?

I recently participated in a training session at the Lake George Association that focused on identification of non-plant aquatic invaders. The session was prompted by the need to become familiar with aquatic invasive animals on the horizon, like round goby and spiny waterflea, and others that have already made inroads to the lake, such as zebra mussels and Asian clam. If we don't know how to recognize these species, how will we know where they are?

Asian clam made headlines when it was detected a year ago in Lake George. A group of organizations from across the Basin and beyond formed the Lake George Asian Clam Rapid Response Task Force to quickly determine containment strategies for this new invader. With potential impacts that include algae blooms, reduced water quality and shell-littered beaches, a well integrated surveillance, education and management campaign was designed to control the clam's spread.

Article Photos

Zebra mussels have a D-shaped shell with black and white banding and average 1 inch in size.
Photo — Amy Benson, U.S. Geological Survey,

There are few silver linings in the fight against invasive species, but the hunt for other locations of the Asian clam in the lake led to an unintended benefit - a greater awareness of the lake's native mollusks. Native mollusks are an important part of the aquatic ecosystem yet one that is largely unknown by most people besides aquatic biologists.

Two groups of freshwater mollusks are common in Adirondack waters: univalves, or snails, which have one shell, and bivalves, such as clams and mussels, which have two shells attached by a hinge. Both native and non-native mollusks are in the region, but not all non-native species are harmful and invasive.

Invasive bivalves include the Asian clam and zebra mussel. Asian clams are yellow to dark brown in color and about 1-and-one-half-inch in size (the size of a dime to a quarter) with distinct ridges that you can feel with your finger. They are usually found in shallow, sandy areas. Zebra mussels have a D-shaped shell with distinct black and white banding and are the size of your thumbnail. Zebra mussels thrive in waters with dissolved calcium levels at least 20 parts per million and pH over 7 and attach to hard substrates (including plants stems, logs and rocks) to depths up to 30 feet. Native bivalve look-alikes include several varieties of freshwater mussels, all larger than a dime and with smooth shells, and a tiny clam the size of a peppercorn.

Though widespread throughout the country, Asian clams are known in the Adirondack region only in Lake George. The distribution of zebra mussels is also limited here: Introduced to Lake Champlain in 1993 and to Lake George in 1999, they are now widespread in Champlain but remain in only a few locations in Lake George. Recent reports of zebra mussel sightings in Lake Harris in Newcomb and Fourth Lake in the Fulton Chain luckily turned out to be false alarms: They were single shelled snails instead of double shelled zebra mussels.

Several snails are on the watch list, too. The Chinese mystery snail is a large snail growing to 2 inches with a hard operculum or "trap door" that seals the opening of the snail's shell. Native snails neither grow this large nor have this visible seal. Though Chinese mystery snails already may be widespread in the region, detection and reporting is limited. The banded mystery snail is another non-native snail that may be widespread but unlike other non-native introductions, this species may not be problematic. It grows to 1 inch, has distinct dark and light banding and is known throughout Lake George and other waters too. The tiniest of the invasive snails is the New Zealand mudsnail. Only one-eighth inch in size, it can reach densities of one-half million per square meter. Unfortunately it is likely only visible once large densities are reached. It invades rivers and streams and is not yet reported in the region.

One introduced organism can quickly become thousands. Monitoring increases the understanding of species distribution and informs prevention and management. If we start looking, what will we find? The next time you're in the water, take a look and see! If you detect a suspect snail or mussel, check to see whether it has one or two shells and then note the size, shape and color - all important identifiers to help distinguish between native and non-native species.


Eye on Invasives is a biweekly column that spotlights a top invader when it is easiest to identify. Hilary Smith directs the Adirondack Park Invasive Plant Program, a partnership program housed at the Adirondack Chapter of The Nature Conservancy in Keene Valley. Find out more about this award-winning program online at



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