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By quantitative sodium dodecyl sulfate-polyacrylamide gel electrophoresis, paramyosin:myosin heavy chain molecular ratios were calculated for three molluscan muscles:Aequipecten striated adductor, Mercenaria opaque adductor, and Mytilus anterior byssus retractor; and four arthropodan muscles:Limulus telson, Homarus slow claw. Balanus scutal depressor, and Lethocerus air tube retractor. These ratios correlate positively with both thick filament dimensions and maximum active tension development in these tissues. The role of paramyosin in these muscles is discussed with respect to the following characteristics: force development, "catch," and extreme reversible changes in length.

By sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunodiffusion, we identified paramyosin in two smooth invertebrate "catch" muscles (Mytilus anterior byssus retractor and Mercenaria opaque adductor) and five invertebrate striated muscles (Limulus telson levator, Homarus claw muscle, Balanus scutal depressor, Lethocerus air tube retractor, and Aequipecten striated adductor). We show that (a) the paramyosins in all of these muscles have the same chain weights and (b) they are immunologically similar. We stained all of these muscles with specific antibody to Limulus paramyosin using the indirect fluorescent antibody technique. Paramyosin was localized to the A bands of the glycerinated striated muscles, and diffus fluorescence was seen throughout the glycerinated fibers of the smooth catch muscles. The presence of paramyosin in Homarus claw muscle, Balanus scutal depressor, and Lethocerus air tube retractor is shown here for the first time. Of the muscles in this study, Limulus telson levator is the only one for which the antiparamyosin staining pattern has been previously reported.