The result of evolutionary limb loss on the complement of motor neurons in a crayfish limb nerve revealed by serial homology

Abstract

Many macruran decapod crustaceans show sexual dimorphism of abdominal appendages adapted for use as secondary reproductive organs. Not only does the Australian crayfish, Cherax destructor, show no external, abdominal dimorphism, but both males and females have lost the pleopods of the first abdominal segment entirely. The first nerves of the abdominal ganglia of crayfish and lobsters carry the axons of the pleopod motor neurons. We used intracellular cobalt infusion into the first nerves of the first and second abdominal ganglia to reveal the motor neuron complement of these ganglia in males and females. The first nerves of the second abdominal ganglia of both males and females have approximately 37 motor neurons associated with them. The homologous nerves in the first abdominal segment, where there are no pleopods, have only 8 or 9 motor neurons associated with them. The evolutionary implications of this difference are discussed.