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Biology/Natural History: This common species hides in burrows in the sand during the day, then comes out to feed on decaying seaweed at night. They often may move down near the water line to feed but return to burrows the higher beach afterward. They will drown if kept submerged in seawater. Males may fight over burrows near dawn in summer (less in winter), and have a plug of sand at the entrance. The burrows are up to 30 cm deepJuveniles have orange second antennae, while the antennae of adults are reddish. Juveniles also have a dark dorsal "butterfly" pattern and a dark mid-dorsal line.
Before molting, which normally occurs in the burrow, this species becomes opaque white dorsally, and this color spreads rapidly over the rest of the body. The animal twitches violently and splits the exoskeleton transversely between the first and second thoracic segments and longitudinally down both sides above the legs. The animal then lifts the dorsal plates and backs out. It can't jump normally for 24 hours after a molt.
The animals mate in their burrows from June to November in central CA. Females carry dark blue eggs which can be seen through their body wall while they are developing. After she releases 10-100 eggs and the male fertilizes them she carries the eggs in her marsupium (ventral side of her thorax) until they hatch.
Predators include many shorebirds, staphylinid beetles, and racoons. Adults often have parasitic mites such as Gammaridacarus brevisternalis on the ventral surface.