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 B.C. alone.
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 B.C. alone.
Range & Habitat
Most crabs live in the ocean, although there are some freshwater crabs. Some crabs live entirely below water, in the ‘subtidal zone’. 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.
Crabs often prefer to be hidden from predators under rocks or other vegetation. Many crabs burrow into the sand to hide.
Reciprocal Relationships
Crabs play an important role as decomposers in our ecosystem! They consume decaying biological (either plant or animal) matter. These nutrients are then released for terrestrial and aquatic plants to absorb and fuel growth. Crabs also eat some live animals, such as clams, other crustaceans and small fish. Crabs are an important food source for animals higher in the food chain like fish, birds, humans and sea mammals.
Dungeness Crab harvesting is protected as an Aboriginal right in British Columbia due to their significance in diet and potlatch ceremonies for many Coastal First Nations. However, in B.C. the invasive green crab has been outcompeting our native crabs for food and habitat. Now people are actively working to protect and steward crab populations throughout coastal B.C.
Dungeness Crab harvesting is protected as an Aboriginal right in British Columbia due to their significance in diet and potlatch ceremonies for many Coastal First Nations. However, in B.C. the invasive green crab has been outcompeting our native crabs for food and habitat. Now people are actively working to protect and steward crab populations throughout coastal B.C.
Lifecycles
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 molting. When crabs molt, 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.
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 molting. When crabs molt, 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.
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, B.C. because of the high levels of industrial pollution in Victoria’s harbours.
COSEWIC: Not at Risk (variable based on species)
CDC: Yellow (variable based on species)
COSEWIC: Not at Risk (variable based on species)
CDC: Yellow (variable based on species)
Photo Credit
Seattle Aquarium
More Information
If you belong to a First Nation with a story or piece of information not represented here and you would like to share more about this species, please email us at education@sierraclub.bc.ca.
