RESUMO
Hippid crabs are adapted for life in the surf zone of exposed sandy beaches, and their tailfan differs from the tailfans of other crustaceans with respect to morphology and motor control and in having nonspiking stretch receptors (NSR). To investigate how these crabs' mechanosensory systems are adapted to this turbulent environment, I used axonal back-filling and intracellular recording with dye-filled microelectrodes to describe afferent projections from the telson and morphologies and physiological responses of intersegmental and local neurons in abdominal ganglion 6 (A6) in Emerita analoga, then compared them with descriptions of corresponding neurons in A6 of crayfish. The density of afferent terminals and the proportion of projection neuron somata is lower in anterior A6 of E. analoga than in crayfish, perhaps correlated with a reduction in hydrodynamic setae. Many interneurons responded to afferent nerve stimulation and displayed activity correlated with levels of A6 motor output. NSR stretch depolarizes unilateral local neurons and terminals of axons entering A6 from the connective and hyperpolarizes bilateral local and projection neurons. The timing and duration of this inhibition would suppress mechanosensory input from the telson during uropod beating (homologue of crayfish's nongiant tail-flipping). Suppression of reafference during uropod beating may have been pivotal for evolution of hippids' ability to move rapidly across the water-sand interface in the slosh zone of sandy beaches. Homologies between A6 neurons in E. analoga and crayfish, suggested by morphological and physiological similarities, indicate that the NSRs connect to a neuronal network regulating exteroceptive input that was inherited from their tail-flipping ancestors.