Scientific name Emerita analoga Rank Species | Higher classification Mole crabs | |
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Similar Mole crabs, Hippidae, Crustacean, Decapoda, Anomura | ||
Emerita analoga, the Pacific sand crab or Pacific mole crab, is a species of small, sand-burrowing decapod crustacean found living in the sand along the temperate western coasts of America. It is found on exposed sandy beaches in the swash region of the intertidal zone.
Contents
Sand crab genus emerita analoga in tidepool
Description
The Pacific sand crab is a small crab growing up to 35 mm (1.4 in) long and 25 mm (1.0 in) wide. The female is nearly twice as large as the male and may be identified by the orange egg mass often carried under the telson. The adult is sand-coloured and well camouflaged, and has no claws or spines. It has five pairs of legs and three pairs of pleopods. The crab moults periodically, so its exoskeleton may be found washed up on the beach.
The sand crab is well adapted to life in the sand, which presents an unstable substrate, and its shape is an elongated-dome shape designed for fast burrowing. The eyes are on long stalks and the antennules are also elongated so as to project above the surface of the sand. These form a tube which channels water downwards through the gills. The much longer antennae are retractable. When water is overhead, they also project above the sand surface to collect food particles. The legs and uropods have hairy margins to assist in digging and for use in collecting food and transferring it to the mouth.
Distribution
The sand crab occurs in North America from Alaska to Baja California and in South America, where it is found from Salaverry, Peru, southwards to around Cape Horn and into southern Argentina. The species is common on the beaches of California, but greater changes in population levels occur further north, probably as a result of variations in the coastal currents which in some years passively disperse the planktonic larval stages northwards. In Oregon, the populations seem not to be self-sustaining and recruitment is largely from larvae originating in California.
On any particular beach, the distribution of sand crabs from one part to another can vary greatly for reasons not properly understood. Females tend to be found far down the beach, whereas males and immature crabs are found at higher levels.
Biology
The sand crab always moves backwards when burrowing or crawling. It can also swim (backwards) and tread water using its back legs. It is a suspension feeder. It burrows backwards into the sand and faces the sea. As each wave retreats, it extends its antennae and catches floating organisms. It then retracts the antennae and scrapes the particles into its mouth. It can do this several times per wave. When it becomes uncovered by water, it coils its antennae and burrows backwards deeper into the sand. The diet is plankton, mostly consisting of dinoflagellates.
The sand crab mates in spring and summer. The female lays batches of up to 45,000 eggs each month and carries them about beneath her abdomen tucked under her telson. The eggs hatch in about four weeks. The larvae have five planktonic zoeal stages and a final megalopal stage. The zoeal stages last up to 130 days. The megalopae settle out onto sandy beaches where they moult and develop into juveniles which mature into adults within a few weeks. The long planktonic stage means that the larvae can become widely dispersed and colonise new areas. The adults reproduce in both their first and second summers and most die in the autumn of their second year.
Ecology
The sand crab lives under the surface of the sand, moving up and down the beach according to the state of the tide. As each wave advances and retreats, the crab comes to the surface and extends its antennae to feed. This makes it vulnerable to predatory birds such as the sanderling. These birds actively patrol the part of the beach washed by incoming waves, probing the softened sand with their bills. The sand crab has adopted the strategy of retreating under the sand surface as each wave goes out. By this means it may be out of reach of the bird's beak. To maximise the bird's chance of feeding on sand crabs, it needs to scurry around in the edge of the surf.
Other birds that eat sand crabs include sandpipers, willets, godwits, surf scoters, blackbellied plovers, and curlews. The sea otter also consumes them. The crabs are hosts to the intermediate stages of various parasitic worms. These are passed on to the predators when the crabs are eaten by them and the number of worms ingested has been known to kill the predator.
The barred surfperch (Amphistichus argenteus), found off the coast of California, consumes a large number of sand crabs. Surf fishermen use the crabs for bait and commercial bait fisheries extract them from the beach. The sand crabs with soft shells that have just moulted are kept for bait, while the hard-shelled crabs are thrown back into the sea.
The sand crab has been evaluated as an indicator species for monitoring the level of domoic acid-synthesizing diatoms (Pseudo-nitzschia spp.) which sometimes cause toxic blooms off the coast of California.