Ultrastructure of some pelecypod adductor muscles

The adductor muscles of the pelecypods Placopecten magellanicus, Crassostrea virginica, Arctica islandica, and Astarte undata reveal a series of differences, from striated to obliquely striated, to smooth types. A transverse periodicity of about 5.0 nm was seen in thick filaments of all sectioned and homogenized preparations examined, and in paramyosin crystals from opaque muscle. Occasionally a periodicity of 15.0 nm also was observed. A diagonal periodicity was seen in thick filaments in sectioned and homogenized preparations of those muscles exhibiting catch. It appeared to be a surface feature composed of electrondense patches arranged helically around the filament, occurring only in some filaments. As the diagonal periodicity did not seem to be associated with well-developed cross-linkages, another mechanism, such as electrostatic attraction between the thick filaments, may be responsible for catch.