The motor innervation and the musculature of the antennule of the Australian mud crab, Scylla serrata (Forskäl) (Portunoidea, Brachyura)

Abstract

The motor innervation and musculature of the antennule of the Australian mud crab Scylla serrata (Forskäl) (Portunoidea, Brachyura) is described. The musculature of the medial and distal segments of the antennule can be divided into three muscle groups: 30, 31 and 32, which are innervated by 11 motor axons. Muscle group 30 serves to raise the distal segment and is composed of two slow muscles, M30V and M30D. These muscles are innervated by three motor axons, A30V, A30D and A30V–D, which form a specific antennular nerve (nerve 2). Muscle groups 31 and 32 are innervated by 8 motor axons, which also form a specific nerve (nerve 1) in the proximal portions of the medial segment of the antennule. Muscle group 31 serves to depress the distal segment and is composed of two slow muscles, M31S V and M31S m , and one fast muscle, M3IF. Both slow muscles are innervated by branches of two motor axons, A31S 1 and A31S 2 . The fast muscle M31F is innervated by two motor axons, A31F and A31–32F. Muscle group 32 serves to depress the outer flagellum and is composed of one slow muscle and one fast muscle. The slow muscle, M32S, is innervated by two motor axons, A32S 1 and A32S 2 , while the fast muscle, M32F, is innervated by two motor axons, A32F and A31–32F which also innervates M31F. A single motor axon, A32F–S, innervates both M32F and M32S. The anomuran and brachyuran antennules are compared in relation to the homology between specific muscles, the functional adaptations of the antennular muscles, and the total number of motor axons present. The results suggest that, in contrast to the situation in the leg and abdominal neuromuscular systems, the peripheral connections and/or the total number of antennular motoneurons show considerable phylo­genetic plasticity.