A Maimed Muscle Scar?
In a bivalve, the half-shells, or valves, are kept together in part by one (or two, depending on the family) adductor muscles. When threatened, the bivalve will clamp its shell shut by quickly contracting the muscles. The areas of muscle attachment on the inside surfaces of an empty shell remain as muscle “scars.” Late last week Collection Associate Jessica Godwin called my attention to one of the scars on a valve of a freshwater mussel known as Washboard, or Megalonaias nervosa (Rafinesque, 1820), collected in the Meramec River, in Missouri. The anterior adductor muscle scar of that valve shows a branching, “fractal-like” pattern that is not common in most bivalves, where the scar is generally smooth, featureless. I wanted to know more about it, and asked for help from colleagues via Unio, an e-mail discussion group on freshwater bivalves. The consensus was that, although Washboards do show a distinctive anterior adductor muscle scar, usually rough or scalloped (which helps with muscle attachment), the enhanced, exquisite branch-like pattern observed on the scar could be related to enhanced shell dissolution, either naturally, following a fair amount of time in the water, or artificially, via chemical polishing after retrieval by a collector. (Thanks to Kevin Cummings of the Illinois Natural History Survey for the identification and members of the Unio list for their help.) The valve is deposited at the National Shell Museum collection under number BMSM 44523.