Neuroanatomical clues to peripheral locomotor control in small crustaceans (Artemia salina)

Brine shrimp (Artemia salina) were prepared for light and electron microscopy at several stages. Immersion-fixed, rapid Golgi impregnations demonstrated two distinct neuronal types in thoracic appendages of mature, freely swimming Artemia. Isolated motor neurons had large cell somas and thick, radiating dendrites at the body wall-limb junction. A long, elaborate axon extended into the limb. Groups of a second type of neuron with smaller somas and very thin, radiating processes occurred in the distal limb near presumably tactile bristles. Thick axons from motor neurons were traced to terminals associated with limb muscle. Both muscle and axon were best seen with Nomarski optics. Motor axons possessed elongate, irregularly shaped boutons en passant and morphologically variable boutons terminaux; the latter included huge endings with knobbed projectiles arising from thick collaterals, or smaller, round boutons from thin collaterals. In addition, a thick unidentified axon coursed longitudinally within the central body wall, sending short collaterals peripherally. The elaborate peripheral neurons described in this Golgi study may be anatomical correlates for the extraordinary coordination of mature brine shrimp. Because Artemia movements resemble those of leech and decapods, which have been studied extensively electrophysiologically, the possibility of similarly elaborate peripheral structures supplementing central control of locomotion in those invertebrates should be considered.