Crow's Nest: Asian clams in French Creek

Here's another case of not noticing something that has been all around us. We take kids to French Creek at camp for stream study and play. And on hot days I chill out in the creek after work: better than air conditioning. I knew there are freshwater mussels in French Creek—but I had never seen this species of clam before the camp kids found them this year.

They're buried in the sand of the slow-moving parts of the creek, not in the riffles and rocks we turn over to look for macroinvertebrates and crayfish.

The Asian clam (Corbicula sp.) is also very common. This non-indigenous species is probably the most common clam in many local waterways, according to Natural Lands Trust's New Jersey Regional Director of Stewardship Steve Eisenhauer. Steve reports that they can be found at a density of 20 - 40 per square foot in Union Lake in Millville, New Jersey, where he leads local children's environmental programs.

They appear very inanimate when plucked from the stream. But in a pan of water a foot emerges from the shell and helps them move. And their siphons also emerge; they are filter feeders of algae.

Steve says that the effect of this introduced species on native populations is "complicated"—which sums it up perfectly. It is likely that, as with any species introduction, there are winners and losers. A species so successful at colonizing a new habitat is likely to impact the available resources for other species. North America has a great richness of freshwater mussel species that has undergone a decline due to many causes: water quality, changes in adjacent land use from forest to farm to yards and pavement, and introduced species.

As with the better-known invasive zebra mussel, the Asian clam can alter the clarity of the water, allowing sunlight to penetrate deeper, which changes the community of plants, which changes the community of other creatures that live in them and feed on them.