Lack of correspondence between histochemical and structural fiber typing in antennal muscles of the rock lobster Palinurus vulgaris.

1. Sarcomere lengths and fine structure were examined in three histochemical fiber types of antennal muscles of the rock lobster. 2. Sarcomere lengths are distributed over a continuum of values from 6.5 to 19 microns. 3. Although a correlation between ATPase activity and sarcomere length is demonstrated, fibers with high ATPase activity do not have the sarcomere length typical of fast contracting fibers. 4. These fibers deviated from the typical fast structure in having long sarcomeres (greater than 6.5 microns) and in having some unusual ultrastructural characteristics (absence of the H-band, presence of Z-tubules, high thin to thick ratio, 5:1) associated with other more classical features. 5. This finding demonstrates that sarcomere length measurements do not always accurately predict the physiological performance of a single muscle fiber. 6. The fiber type composition of two antagonistic antennal muscles is compared and the functional significance of the results is discussed with respect to their role in behavior.