The evolutionary transition to sideways-walking gaits in brachyurans was accompanied by a reduction in the number of motor neurons innervating proximal leg musculature.

The forwards-walking portly crab, Libinia emarginata is an ancient brachyuran. Its phylogenetic position and behavioral repertoire make it an excellent candidate to reveal the adaptations, which were required for brachyuran crabs to complete their transition to sideways-walking from their forwards-walking ancestors. Previously we showed that in common with other forwards-walking (but distantly related) crustaceans, L. emarginata relies more heavily on its more numerous proximal musculature to propel itself forward than its sideways-walking closer relatives. We investigated if the proximal musculature of L. emarginata is innervated by a greater number of motor neurons than that of sideways-walking brachyurans. We found the distal musculature of spider crabs is innervated by a highly conserved number of motor neurons. However, innervation of its proximal musculature is more numerous than in closely-related (sideways-walking) species, resembling in number and morphology those described for forwards-walking crustaceans. We propose that transition from forward- to sideways-walking in crustaceans involved a decreased role for the proximal leg in favor of the more distal merus-carpus joint.

Degree of neuromuscular facilitation is correlated with contribution to walking in leg muscles of two species of crab.

Despite decades of work on the neuromuscular physiology of crustacean leg muscles, little is known about how physiological differences between these muscles relate to their behavioral usage. We studied a sideways walking shore crab, Carcinus maenas, and a forward walking spider crab, Libinia emarginata, as part of our work to understand the neural control of locomotion. The two species differed significantly in facilitation at neuromuscular junctions for every muscle studied. Further, these differences are correlated exactly with the walking use of the muscles. The forward walking spider crab showed more facilitation in muscles which operate joints having larger ranges of motion in forward walking. Likewise, greater facilitation was seen in muscles more active during sideways walking in the predominantly sideways walking shore crab. These differences even occur between muscles innervated by the same motor neuron, and become more evident with higher stimulus frequency. The increased presynaptic facilitation might allow selective recruitment of fibers innervated by the same motor neuron and aid in temporal filtering.

Contrasting tactics in motor control by vertebrates and arthropods.

Vertebrates and arthropods share the common problem of controlling a rigid, articulated skeleton using neurally-controlled, striated muscle. Since this condition has arisen independently in the two groups, there is no reason to assume, a priori, that the control mechanisms used by the two groups will be the same. Indeed, there appear to be fundamental differences in the tactics used by the two groups. Insects and crustaceans use small numbers of heterogeneous motoneurons, while vertebrates (mammals especially) use many, more homogeneous, motor axons. In particular, arthropods make extensive use of peripheral neuromodulation to alter the properties of both neuromuscular junctions and muscle fibers. There has been little consideration of the functional consequences of these differences. I suggest that, faced with a size constraint on the number of motor units available, arthropods use peripheral modulation of muscle properties to achieve the flexibility and dynamic range that vertebrates achieve through recruitment of motor units.