THE CLOSER MUSCLE IN THE DIMORPHIC CLAWS OF MALE FIDDLER CRABS

¹ Department of Zoology, Scarborough Campus, University of Toronto, 1265 Military Trail, Scarborough, Ontario M1C 1A4, Canada
² Department of Biology, University of Virginia, Charlottesville, Virginia 22901

The paired claws in male fiddler crabs are asymmetric consisting of a major and a minor type. The homologous closer muscle in these claws are innervated by two excitatory axons which, because they elicit tonic closing of the dactyl with repetitive stimulation and have neuromuscular synapses which facilitate, are regarded as slow type axons. The fiber composition of the paired closer muscles is also similar, being of the slow type. These fibers show typically long sarcomere (10 µm) and A-band (6 µm) lengths, light staining for myofibrillar ATPase denoting low specific activity for this enzyme, but more intense and circumscribed staining for NADH-diaphorase denoting a robust oxidative capacity. However, the protein composition of the paired muscles varies not only in its myosin light chain and paramyosin but also in the expression of isoforms for myosin heavy chain and several regulatory proteins including troponin I, tropinin T, and tropomyosin. Apart from any subtle functional differences imparted by these protein isoforms, the paired closer muscles are homogenously slow in their innervation and fiber type composition.