Crab

Brachyura

APPEARANCE

Crabs are decapod (‘ten-footed’) crustaceans. They have thick external skeletons and a single pair of claws (known as chelae) which give them their characteristic appearance.

Did you know that many crabs also have a tail? It is very short, and usually entirely hidden under their thorax so it easily goes unnoticed. Crabs have no backbone – they are invertebrates. Instead, their exoskeleton protects their organs. Crabs vary in size; the smallest crabs are only a few millimetres wide, while the largest – the Japanese spider crab – has a leg span of four metres! There are over 35 species of crab in BC alone.

RANGE AND HABITAT

Most crabs live in the ocean, although there are some freshwater crabs. Some crabs live entirely below water, in the ‘subtidal zone’, while other more resilient crabs live in the intertidal zone, where they must endure changes in temperature and salinity (the saltiness of water).

Crabs often prefer to be hidden from predators under rocks or other vegetation. Many crabs burrow into the sand to hide.

LIFE CYCLE 

Before they mate, female crabs shed their shell. Prior to this ‘molting’ process, the female releases pheromones which attract male crabs. The female crab rides on top of the shell of the male crab as she molts, and only after she molts does she accept the male’s sperm. Once the female has grown a new hard shell, she detaches herself from the male and allows her eggs to be fertilized by the sperm. For several weeks she carries the fertilized eggs in a sac on her abdomen, until the larvae hatch. Many crabs only undergo this reproductive process once in their lifetime!

The newly hatched larvae look like shrimp and must pass through several developmental stages before they resemble crabs. The larvae simply drift through the water with the ocean currents; it is only the megalops – the name for crabs once they develop claws and legs – that can swim in the water. Megalops swim near the ocean surface and eventually settle onto the ocean floor, where they continue to develop into juvenile crabs.

As crabs grow, they shed their smaller shells by moulting. When crabs moult, they absorb sea water to grow and split their old shell, and then grow and harden a new shell over the course of a few weeks.

ANIMAL USES

Crabs are decomposers! They consume decaying biological (either plant or animal) matter. Crabs also eat some live animals, such as clams, other crustaceans and small fish. Crabs are also an important food source for animals higher in the food chain like fish, birds and sea mammals.

TRADITIONAL USES BY INDIGENOUS PEOPLES

People have been consuming crabs as long as humans have lived near the seashore. For some First Nations people in British Columbia, crab is a fundamental component of their diet and also an important part of their culture.

OTHER USES

Many cultures globally consider crabs a delicacy, and crab fishing is an important source of income for many people.

STATUS

Most species of crabs are not endangered, but they are negatively impacted by ocean pollution. Pollution often sinks to the bottom of the ocean where crabs usually live. This is common in Victoria, BC because of the high levels of industrial pollution in Victoria’s harbours.

COSEWIC: Not at Risk

CDC: Yellow

PHOTO: Seattle Aquarium