Saturday, July 28, 2012

The Weird World of Mollusks

What: I don't think I'll ever be able to fully empathize with a mollusk, well at least not with a bivalve. But I do find them terribly fascinating, which isn't a bad start. And like most things, the more I learn and observe them the more I appreciate their quiet existence. While walking the shoreline at Delta Park, I was caught by the odd swirling groves etched into the sand. I had been spending some time trying to figure out how different water currents would manifest in the topography of the underlying sand - in some patches there were long straight furrows like a recently plowed field, while others had overlapping u-shaped mounds consistent in size for a given patch (see end of video for what I mean). I couldn't help but be distracted by the seemingly chaotic lines that the clams had carved across the ripples.

Ecological notes: While walking barefoot through the shallows, we didn't have to worry much about zebra mussels at Delta Park because the substrate is largely sand. Mussels are a subset of clams (see etymology notes below) that have byssal threads that serve to anchor their shells to a solid substrate - no solid substrates, no mussels. Only in a few places, like where we found heaps of downed trees partly submerged in the water, did we find zebra mussels. We did however find an abundance of freshwater clams. I unburied one, then left it with my camera set to take a picture every minute and a half. The time lapse was shot over about an hour and a half. In the first few seconds you can see the animal expel a mess of waste, which appears to be attached by a string.

There behavior still largely mystifies me. With their large rubbery foot the clams slowly reposition themselves along the shoreline - perhaps following changes in wind direction, water depth, or some other factor. Some of their trails were long slow arcs, others overlapping and confused circles. The groves left behind in the wake of their movement created small eddies in which algae settles. Fresher trails appeared as gray fissures bordered with crusty plates of sand fragmented. I checked some of the same trails after about 12 hours and they looked largely in tact, so I'm guessing it takes some stronger currents to breakdown the trails (and that they move really really slowly).
Smaller bivalves burrowed in sediments. Water level is super low right now
Their trails in the sand (hard to see unless you click on image to see a larger version).

Algae settled in old tracks

Where: Delta Park in Colchester (mouth of the Winooski River).

Etymology notes: The term clam is a muddy one to say the least. A clam could be a bivalve (a mollusk with a pair of hinged, clasping shells) that burrows into sediment - as opposed to a mussel, like a zebra mussel, which attaches itself to a substrate like pilings, other bivalves, whales, or just about anything else with a solid surface.  Or a clam could be an umbrella term for all bivalve mollusks (a square is always a rectangle, but a rectangle is not always a square like a mussel is always a clam but a clam not always a mussel). When I called ECHO to ask about help identifying freshwater clams, I was corrected: "freshwater mussels," which further confused the matter since I was referring to bivalves that burrow in the sand. Maybe there's a vernacular understanding of "mussels" as any of those freshwater filter feeders.

Other notes: Not sure about the edibility of the mussels, but they seemed an enticing source of food. Mussels themselves are not toxic, but they are filter feeders so you're essentially eating whatever microscopic stuff is floating down the Winooski or hanging out along the edges of Lake Champlain). Plus I'm not sure what species they are. Vermont's Wildlife Action Plan says that of the 18 species of mussels in Vermont, 13 are listed as threatened, of special concern, endangered, or already extinct (a rate on par with national figures). I used a field guide from Connecticut to try and ID these guys, with little luck. There's another publication UVM has that I plan on checking out to try and narrow it down: Fichtel & Smith's The Freshwater mussels of Vermont. Plus, depending on who you talk to, some species can live 75-200 years. I'd feel bad eating something that old.

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