1.07.5.1 New Niche Means Altered Sensory Environment
Hippid sand crabs spend their lives alternating between two environments, above and in the sand, on beaches exposed to wave action (Figure 3; Dugan et al., 2000), in which tailfan mechanosensory setae would not be expected to provide much meaningful information. The unpredictable turbulence in the swash zone alone would limit the kinds of information hydrodynamic sensors on the tailfan could provide, in contrast to the importance of this modality in crayfish (Paul, 2004). In addition, hippid uropods beat continuously both for swimming and treading water when the animals are up in the swash zone and while digging into sand, when they are coordinated with the second through fourth thoracic legs (Section 1.07.2.3). Not surprisingly, differences between the mechanosensory integrating network of neurons in the sixth abdominal ganglion of hippids and crayfish are evident. They suggest that alterations in integration of sensory input from the tailfan were important components in the neural evolution that made hippids' new modes of locomotion fully adaptive for their active life on wave-swept beaches. As I treated this subject at length elsewhere (Paul, 2004), I will not discuss it further here.
1.07.5.2 Roles of Hippid Sand Crabs Nonspiking Proprioceptor
Two functions have been ascribed to the NSRs and a third appears likely. They provide clues about these new neurons' connections with ancestral decapod sensorimotor networks serving the tailfan. These clues suggest several potential explanations for the selective advantage of the use of graded potentials in neurons that send signals from the periphery to the central nervous system.
1.07.5.2.1 Suppress reafference during uropod beating The integration of the phylogenetically new NSR input with the sensorimotor network for the tailfan is such that exteroceptive input from the tailfan is depressed whenever the uropods are beating, i.e., continues through the RS and PS phase. The long-lasting nature of the inhibition mediated by the NSR may stem from the combination of the sustained (graded) input and long-lasting inhibitory synapses downstream (Paul, 2004). The ability of hippids to move seamlessly across the water-sand interface, for which continuous uropod beating is required (see above), may have been contingent upon the evolution of proprioceptors that could suppress tailfan mechanosensory input whenever the uropods are beating.
1.07.5.2.2 Switch between motor patterns for swimming and treading water Whenever hippids are above the sand, they are either swimming, i.e., moving with respect to the substrate, or treading water, i.e., keeping themselves suspended vertically (Figure 3) - except when being swept by surging currents! In the electromyogram pattern for swimming (Figure 9c), the phase of the power stroke with respect to RS period is relatively constant (around 0.55), whereas in the electromyogram pattern for treading water, PS muscle bursts occur at relatively constant latency with respect to return strokes. Switching between motor patterns often occurs in the course of one swim sequence. The treading water motor pattern is dependent on intact NSRs, since bilateral ablation of the receptor strands deletes this motor pattern from these animals' behavioral repertoire without diminishing their ability to swim (Paul, 1976).
1.07.5.2.3 Coordinate fourth legs and tail? The
NSR input reinforces power strokes during high-frequency uropod beating (Paul, 1976), which occurs during digging as well as in spurts during swimming. When digging, the fourth legs cycle at the same frequency as the uropods (Figure 4), but never in unison with them. The power strokes of the left and right fourth legs are evenly spaced between the uropod return strokes, which means that the phase relationship between the fourth legs shifts from bilateral synchrony to one-third of a cycle out of phase as uropod frequency declines during a dig (Figure 4). The behavioral significance of this complex motor pattern was explained above (Section 1.07.2.1.2). In other species, including crayfish, leech, and lamprey, sensory feedback is incorporated into the interseg-mental coupling signals between CPGs located in different parts of the body (Friesen and Chang, 2001), and the NSRs may do the same (they influence the activity of several candidate neurons for this coordination; Paul, 2004). Evolutionarily speaking, the interest of this is that it would mean the superposition of the new afferent signals onto an inherited set of coordinating neurons between dissimilar CPGs, paired for the fourth legs and unpaired (most likely) for the uropods -assuming the uropod and nongiant tailflipping circuitries are homologues (see Section 1.07.2.2).
Paul and Bruner (1999) discuss the possibility that the NSRs are ontogenetically related to motoneurons (see also Bush, 1976; Wilson and Paul, 1990).
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