Replacement of an inherited stretch receptor by a newly evolved stretch receptor in hippid sand crabs

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.

Neural mechanisms underlying the evolvability of behaviour

The complexity of nervous systems alters the evolvability of behaviour. Complex nervous systems are phylogenetically constrained; nevertheless particular species-specific behaviours have repeatedly evolved, suggesting a predisposition towards those behaviours. Independently evolved behaviours in animals that share a common neural architecture are generally produced by homologous neural structures, homologous neural pathways and even in the case of some invertebrates, homologous identified neurons. Such parallel evolution has been documented in the chromatic sensitivity of visual systems, motor behaviours and complex social behaviours such as pair-bonding. The appearance of homoplasious behaviours produced by homologous neural substrates suggests that there might be features of these nervous systems that favoured the repeated evolution of particular behaviours. Neuromodulation may be one such feature because it allows anatomically defined neural circuitry to be re-purposed. The developmental, genetic and physiological mechanisms that contribute to nervous system complexity may also bias the evolution of behaviour, thereby affecting the evolvability of species-specific behaviour.

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

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.

The evolution of neuronal circuits underlying species-specific behavior

The nervous system is evolutionarily conservative compared to the peripheral appendages that it controls. However, species-specific behaviors may have arisen from very small changes in neuronal circuits. In particular, changes in neuromodulatory systems may allow multifunctional circuits to produce different sets of behaviors in closely related species. Recently, it was demonstrated that even species differences in complex social behavior may be attributed to a change in the promoter region of a single gene regulating a neuromodulatory action.

Receptor potentials and electrical properties of nonspiking stretch-receptive neurons in the sand crab Emerita analoga (Anomura, Hippidae)

Receptor potentials and electrical properties of nonspiking stretch-receptive neurons in the sand crab Emerita analoga (Anomura, Hippidae). Four nonspiking, monopolar neurons with central somata and large peripheral dendrites constitute the sole innervation of the telson-uropod elastic strand stretch receptor in Emerita analoga. We characterized their responses to stretch and current injection, using two-electrode current clamp, in intact cells and in two types of isolated peripheral dendritic segments, one that included and one that excluded the dendritic termini (mechanosensory membrane). The membrane potentials of intact cells at rest (mean +/- SD: -57 +/- 4. 4 mV, n = 30), recorded in peripheral or neuropil processes, are similar to the membrane potentials of isolated dendritic segments and always less negative than membrane potentials of motoneurons and interneurons recorded in the same preparations. Ion substitution experiments indicate that the membrane potential is influenced strongly by Na+ conductance, probably localized in the mechanotransducing terminals within the elastic strand. The form of the receptor potential in response to ramp-hold-release stretch remains the same as stretch amplitude is varied and is not dependent on initial membrane potential (-70 to -30 mV) or recording site: initial depolarization (slope follows ramp of applied stretch), terminated by rapid, partial repolarization to a plateau (delayed depolarization) that is intermediate between the peak depolarization and the initial potential and sustained for the duration of the stretch. Responses to depolarizing current pulses are similar to stretch-evoked receptor potentials, except for small amplitude stimuli: an initial peak occurs only in response to stretch and probably reflects elastic recoil of the extracellular matrix surrounding the dendritic terminals. The rapid, partial repolarization depends on holding potential and is abolished by 4-aminopyridine (4-AP; 10 mM), implicating a fast-activating, fast-inactivating K+ conductance; TEA (60 mM) abolishes the remaining slow repolarization to the plateau. In intact cells, but not dendritic segments, regenerative depolarizations can arise in response to stretch or depolarizing current pulses; they are reduced by CdCl2 (10 microM) in the saline containing TEA and 4-AP and probably reflect current spread from Ca2+ influx at presynaptic terminals in the ganglion. We found no evidence for other voltage-activated conductances. Unlike morphologically similar "nonspiking" thoracic receptors of other species, E. analoga's nonspiking neurons are electrically compact and do not boost the analogue afferent signal by voltage-activated inward currents. The most prominent (only?) voltage-activated extra-ganglionic conductances are for potassium; by reducing the slope of the stretch-plateau depolarization curve, they extend each neuron's functional range to the full range of sensitivity of the receptor.