Wednesday, November 19, 2008

growing clams

Here is some additional information about Sarah D's clam monitoring project, that the invertebrate zoology class helped out with last month. The main part of her project involves measuring recruitment rates for soft-shelled clams (Mya arenaria, also known as "steamers" on local menus) as well as understanding what kinds of factors may enhance recruitment. A definition is probably needed here: when a larval clam, which has spent a couple weeks swimming and feeding in the plankton, encounters a suitable site, it will metamorphose from a larva into a juvenile clam, burrow into the sediment and take up residence, thus recruiting into the local population. Clearly, the larval supply will affect recruitment rate, but larval choice also plays a role, and marked preferences for a variety of factors including substrate texture, flow regime, presence of conspecifics, presence of prey, and absence of competitors or predators, have been demonstrated among a wide range of larvae of marine invertebrates. They may be tiny, but those larvae can exercise some sophisticated decision-making. For one of Sarah's treatments, she added adult clams to her site to measure the effects of adult conspecifics on recruitment rate. These adults were not enhancing the local supply of larvae; any offspring they produced would be widely dispersed during their weeks-long larval life in the plankton. But the adults do provide settling larvae with the information that the site can support clams from settlement to adulthood, and might be a good choice. As long as Sarah and her project supervisor, Chris Petersen, were moving clams (they added hundreds of clams to several large treatment plots), they decided to also measure growth rates of the clams. They marked the outer margin of the shell with permanent marker in the spring, when they added the clams to their site, with the help of students from MDI High School. Then, this month, they measured the marked clams that they found while they were monitoring clam density and recruitment rate. I was surprised that the marks persisted for 6 months, even though the method was suggested by Brian Beal, from U Maine Machias, who definitely knows his way around clam flat research. The marking technique works, and growth rates were quite variable, even among clams in the same plot, with some showing obvious, substantial shell growth, and others not changing size at all since spring. There are clearly plenty of future projects there waiting on the clam flat for some curious and energetic students. Climbing into a pair of waders and splooshing around on the mudflat offers its own unique rewards: fingers numbed from sorting through samples in November, the aroma of anaerobic sediment (think rotten eggs), gooey mud covering everything from tools to clothes to data sheets. What's not to love?

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