Independent Development of Bilaterally Homologous Closer Muscles in Lobster Claws

¹ Life Sciences Division, Scarborough Campus, University of Toronto, 1265 Military Trail, Scarborough, Ontario, M1C 1A4, Canada

The fiber composition of the closer muscle in the paired claws of the lobster Homarus americanus was determined in juvenile 8th and 9th stage animals following various experimental manipulations of the substrate and the claws in the early juvenile 4th and 5th stages. One of the paired muscles developed 90% fast fibers and 10% slow fibers, typical of a cutter claw. The other muscle varied in its fiber composition with fast fibers ranging from 10-90%. This range was categorized into three muscle types; a cutter with 70-90% fast fibers, a crusher with 10-30% fast, and an intermediate with 40-60% fast. The paired homologous claw closer muscles therefore develop such that one is preprogrammed as a cutter while the other is more plastic and develops along a gradient ranging from a cutter to a crusher.