The soldier crab Mictyris brevidactylus Stimpson inhabits sandy flats of Southeast Asia. The crabs that we studied fed on deposited matter in the surface sand in two ways. On well-drained sand in the upper part of their habitat, they usually made a single tunnel roofed with sand to conceal themselves while they fed. After feeding, they returned to an ascending shaft from which they had earlier emerged. Then, they descended via the plugged ascending shaft and transformed the shape of the shaft into an ovoid air chamber at a sufficient depth. During the daytime low tide, on the waterlogged, fluid sand near the shoreline, large individuals, mostly males, fed in droves on the surface while moving along the shoreline as it moved with the tide. After feeding for about 2 h, they made a small air chamber slightly above the shoreline by using corkscrew-style digging, and descended into the semi-fluid sand. During the night-time low tide, the crabs fed separately for about 4 h on both the fluid sand near the shoreline and the well-drained firm sand in the upper habitat, and retreated into the sand, although they took longer to retreat into the well-drained sand than into the semi-fluid sand. That is, the crabs feeding on the surface at the daytime low tide returned to the subsurface-feeding area and made a small air chamber during the night-time low tide. Later, at low tide, the small air chamber was elongated to allow subsurface feeding. We consider that the movement to the upper habitat at night-time and the feeding near the shoreline in the daytime help the crabs evade predators that rely on sight, such as birds.
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