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Exploring the beautiful nature of California


California Nature:  Rock Crabs


Crustaceans are of great direct and indirect importance to humans. The larger crustaceans (shrimps, lobsters, and crabs) are used as food throughout the world, and are therefore important to human economies. Small planktonic crustaceans, such as copepods, water fleas, and krill, are a major link in the marine food chain between the photosynthetic phytoplankton and larger carnivores such as fish and whales. Still others are crucial in recycling nutrients trapped in the bodies of dead organisms.

Crustaceans are the arthropods that dominate marine habitats, but they are also found in large numbers in freshwater and a few groups have made their way successfully onto on land. When found on land crustaceans are either found in moist protected habitats like under logs or in leaf litter in cool forests, or they are encysted (enclosed in a tough protective capsule, nearly dried out, and dormant).
Crabs play a vital role in California nature California's coastal waters are home to a multitude of invertebrates (species lacking a bony skeleton). A small fraction of these, including a variety of crab species, are actually targeted by California's recreational and commercial fisheries.

Rock crabs are fished along the entire California coast. The catch is made up of three species: the yellow rock crab, the brown rock crab, and the red rock crab.

Yellow rock crabs range from Humboldt Bay into southern Baja California, brown rock crabs from northern Washington to central Baja California, and red rock crabs from Kodiak Island to Central Baja California. All three species inhabit waters from the low intertidal zone down to depths of 300 feet or more.

Although these species may occur together throughout much of their range, yellow rock crabs are most abundant in southern California, brown rock crabs in central California and red rock crabs in northern California. Yellow rock crabs prefer open sand or soft bottom habitat, while brown and red rock crabs prefer rocky or reef-type substrate.
Rock crabs, like other crustaceans, grow in a step-wise fashion with each molt of the external shell. Yellow and brown rock crabs molt 10 to 12 times before reaching sexual maturity at about three inches carapace width.

Crabs of this size may molt twice a year, while crabs as large as six inches carapace width or more may molt once a year or less. Growth-per-molt, as a percentage of size, decreases as the crab increases in size and age.

Males of all three species attain sizes 10 to 15 percent larger than females. Yellow rock crabs grow to exceed seven inches in carapace width, brown rock crabs 6.5 inches, and red rock crabs eight inches. While the longevity of rock crabs is not well known, many crabs may reach five or six years of age.
There is a wide variety of beautiful crabs off the coast of California Mating takes place after the females molt and are still in the soft-shell condition. In southern California, rock crab mating is most common in the spring, but occurs throughout the year.

About three months after mating, the eggs are laid, then fertilized from a sperm packet left by the male during mating. The developing eggs are carried in a mass under the abdomen of the female. Depending on size and species, nearly four million eggs may be carried by a female rock crab. After six to eight weeks, the eggs hatch into planktonic larvae, which undergo seven developmental molts before settling to the bottom as juveniles.

Rock crabs do not appear to migrate or to undertake large-scale movements. Tagged adults have moved several miles, but no pattern was apparent. Some local movements also may occur in relation to mating or molting. Egg-bearing yellow rock crabs are known to congregate in rock-sand interface habitats.

Rock crabs are both predators and scavengers, feeding on a variety of other invertebrates. Strong crushing claws allow them to prey on heavy-shelled animals such as snails, clams, abalone, barnacles, and oysters. The olfactory sense of crabs is well developed and allows them to detect and locate food at a distance.

Rock crabs, especially juveniles, are preyed upon by a variety of other marine organisms. Fishes such as cabezon, barred sand bass and several species of rockfish are known to feed on rock crabs. Important invertebrate predators include the octopus and certain sea stars.

As rock crabs grow larger, they become less susceptible to predators except during the soft-shell post-molt period; however, the sea otter is one animal that is an effective predator on large rock crabs.
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