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Purple shore crabs (Hemigrapsus nudus) are tiny crustaceans, only about 2 inches long when fully grown, who live on the western seaboard of North America. They are quite adaptable, found from the frigid shores of Alaska all the way to the sunny beaches of Baja California. They are often mistaken for lined shore crabs (Pachygrapsus crassipes) who inhabit the same beaches.
Purple shore crabs live in shallow intertidal waters -- they're rarely found in water deeper than 3 feet. They prefer rocky coastlines where hiding places are plentiful, because they do not burrow in sand like many other crabs. They have the ability to regulate the amount of salt in their systems, a trait called osmoregulation. This enables them to live rather far into brackish estuaries and salt marshes, where the the level of salt in the water changes with the tides.
As their name suggests, their carapaces, or upper shells, are usually deep purple, although they can be olive green or reddish brown. The legs match the color of the carapace and lack the setae, or hairlike bristles, that other species have, giving these crabs their other common name, naked shore crabs. The claws or chelipeds are the same color as the carapace, with raised, darker purple spots and white tips. Size is an accurate method of judging ages of these creatures. They grow very slowly, reaching only a half-inch by the end of their first year. By the end of the second year, most have reached one and a quarter inches. It takes a full three years for them to reach adult size of 2 inches.
Like all crabs, purple shore crabs start their lives as eggs that are glued to the underside of their mother's body. After hatching, the tiny larvae or zoea float to the surface of the water and live as plankton. These zoea look more like insects than crabs. After several molts, the larvae become megalopae and begin to look more like crabs. When they become heavy enough, they sink to the bottom and seek shelter. After several more molts, megalopae become juvenile crabs. They look exactly like adults except smaller. By 7 months of age, most male purple shore crabs have grown large enough to be sexually mature; females won't get there until the end of their first year.
The mating ritual of purple shore crabs involves a bizarre dance during which males and females rise up and embrace, belly to belly. The males clasp females firmly with their legs and claws, and fall over backwards. The pairs stay this way, with the males on their backs and the females on top of them, until the males finish depositing sperm. A single female can produce and carry as many as 36,000 fertilized eggs from a single mating. One species of barnacle affects male crabs. Once one of these rhizocephala embeds under the male's carapace, it can alter the crab's body chemistry. The crab's growth slows down, molts delay and the crab's hormones make the crab no longer a male but a reproductively complete female, capable of producing eggs. Studies have shown that as many as one-quarter of male purple shore crabs may be infected with rhizocephala.
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