Answer
Paramyosin may replace myosin on thick fibers in such a way that either maintains contraction as Ca+ levels fall or are not susceptible to tropomyosin blockage.
Work Step by Step
You could test this hypothesis by altering, in a controlled environment, the Ca+ and tropomyosin concentrations around muscle fibers to see how they are affected. In shellfish that have paramyosin, it does indeed alter the thick fibers in this mechanism.