Projection and local interneurons in the sixth abdominal ganglion of the sand crab Emerita analoga (Hippidae)

Abstract

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.

Neuronal Network Models of Phase Separation Between Limb CPGs of Digging Sand Crabs

“…Our model assumptions are consistent with recently elucidated neurophysiological details of the motor system in E. analoga (Paul 2004). For example, there are bilaterally paired neurons in the uropod ganglion in E. analoga with properties similar to those in ganglia controlling rhythmic limb movement in crayfish (Hooper and DiCaprio 2004;Murchison et al 1993;Mulloney 1985a,b, 1986).…”

“…For example, there are bilaterally paired neurons in the uropod ganglion in E. analoga with properties similar to those in ganglia controlling rhythmic limb movement in crayfish (Hooper and DiCaprio 2004;Murchison et al 1993;Mulloney 1985a,b, 1986). There are also intersegmental projection neurons long enough to mediate the coupling between the uropod and fourth leg CPGs and potential coordinating interneurons similar to those in crayfish (Paul and Mulloney 1986;Paul 2004;Tschulum 2001).…”

Habitat-Related Divergence among Tailfan Sensory Systems in Reptantian Decapod Crustaceans

“…6 A, C) [Letourneau, 1976;Paul et al, 1985]. The only gross anatomical evolutionary innovation has been the addition of a small sensory branch from motor nerve 6 (N6) in mole crabs that was previously designated N6sensory (N6s) based on the absence of somata in backfills [Paul et al, 1985;Paul, 2004]. We have confirmed that the N6s are sensory and determined that they innervate the bilaterally symmetric tufts of pappose setae on the anterior-dorsal face of the telson ( fig.…”

“…quadrispina , and strongly skewed to more in the posterior cluster in E . analoga [Sigvardt et al, 1982;Paul, 2004;Bock and Paul, 2005;Bock, 2006]). These differences appear unrelated to either phylogeny or neurobehavioral differences, which was unexpected given the documented conservation of neural architecture across phylogeny in other arthropods [Buschbeck and Strausfeld, 1997;Buschbeck, 2000].…”

On the Origin of Grasshopper Oviposition Behavior: Structural Homology in Pregenital and Genital Motor Systems

“…mollusks that do not swim or flightless insects, are studied to see whether 'swim' or 'flight' neurons are present, suggesting whether the behavioral circuits have been lost or whether the absence of behavior is the prim-itive condition of a common ancestor [Arbas, 1983;Arbas et al, 1991;Rast and Braunig, 2001;Whiting et al, 2003;Katz and Newcomb, 2006]. Comparative neuroethological studies, at the level of identified neurons, have shown that similar neural circuits may underlie diverse behaviors, homologous neurons may play different behavioral roles in different animals, and that behavior can be lost in evolution [Arbas, 1983;Paul, 2004;Berkowitz, 2008;Katz, 2010]. Lost behavior can also be recovered, as seen in examples of stick insects where wings have been lost and regained [Whiting et al, 2003].…”