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Figure 3 Sand crabs at home in aquariums and their natural habitat. a, Mole sand crabs (Hippidae; shown here Emerita analoga) filter-feed by extending their long, plumose second antennas above the sand (top left). The other three panels show an individual emerging from sand (bottom left), swimming (top right), and treading water, i.e., beating the uropods to keep itself suspended, but not moving relative to the substrate. To stay positioned at the appropriate level for feeding during the downwash from breaking waves, which shifts with the tide, hippid crabs emerge from the sand periodically to swim with the current (up or down the beach, depending on the direction of the tide), and rapidly dig in again before being carried too far up or down the beach. b, Hippid behaviors are superbly adapted for an active life in the turbulent physical environment of intertidal sandy beaches (Dugan et al., 2000), which makes them easy to collect by hand when they are close to the sand's surface (left; see (a)); shore birds are major predators of hippid crabs (right). c, A spiny sand crab, Blepharipoda occidentalis (Albuneidae) moving across two E. analoga buried just beneath the surface with their first antennas protruding (white arrows). Carapace length of E. analoga = 2.0-2.5 cm and of B. occidentalis ~7cm.

Figure 3 Sand crabs at home in aquariums and their natural habitat. a, Mole sand crabs (Hippidae; shown here Emerita analoga) filter-feed by extending their long, plumose second antennas above the sand (top left). The other three panels show an individual emerging from sand (bottom left), swimming (top right), and treading water, i.e., beating the uropods to keep itself suspended, but not moving relative to the substrate. To stay positioned at the appropriate level for feeding during the downwash from breaking waves, which shifts with the tide, hippid crabs emerge from the sand periodically to swim with the current (up or down the beach, depending on the direction of the tide), and rapidly dig in again before being carried too far up or down the beach. b, Hippid behaviors are superbly adapted for an active life in the turbulent physical environment of intertidal sandy beaches (Dugan et al., 2000), which makes them easy to collect by hand when they are close to the sand's surface (left; see (a)); shore birds are major predators of hippid crabs (right). c, A spiny sand crab, Blepharipoda occidentalis (Albuneidae) moving across two E. analoga buried just beneath the surface with their first antennas protruding (white arrows). Carapace length of E. analoga = 2.0-2.5 cm and of B. occidentalis ~7cm.

of Albuneidae and Hippidae (Faulkes and Paul, 1997b). The members of these families live in dissimilar niches and require different locomotor capabilities. Albuneids are subtidal and intertidal scavengers that remain buried beneath the surface and are weak swimmers. Hippids, in contrast, are intertidal filter feeders that must maintain themselves in the swash zone as the tide shifts, and to do so emerge periodically from the sand to swim up or down the beach before digging in again to feed (see Figure 3a). Their prowess in both digging through sand and swimming or treading water (Section 1.07.5.2.2) is the sine qua non of their existence. The abilities of hippids to disengage leg (digging) movements during swimming and to couple rhythmic movements of the digging legs with the uropods during digging presumably evolved conjointly with their peculiar mode of swimming with the uropods. Furthermore, the phase separation between their fourth legs and uropods, which cycle together, is complex and apparently critical for their ability to maintain their position on surf-swept sandy beaches (Figure 3b; Paul et al., 2002). The power strokes of the left and right fourth legs and uropods remain evenly spaced as frequency changes because the phase relationship between the fourth legs shifts from bilateral synchrony to one-third out of phase as overall frequency drops during the course of a dig (Figure 4b; Faulkes and Paul, 1997a). This pattern ensures continuity in the collective force generated by the fourth legs and uropods. As their force vectors are slanted with respect to the long axis of the body (see tilt of fourth leg and uropod tip trajectories in Figure 4), this coordination maintains the rear-down slant of the body while the powerful digging legs 2 and 3 provide the propulsive force for submerging the animal beneath the sand's surface. The coupling (and uncoupling) of the hind digging legs with the uropods accounts for hippid sand crabs' seamless transitions between two such different media as water and sand. The different coupling patterns between limb segments displayed in sand crabs (Figure 4) exemplifies well the inherent functional and evolutionary flexibility of modularity in the organization of neuromotor networks (Paul et al., 2002) and the developmental mechanisms that give rise to them (Section 1.07.5).

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