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Knowing Nature . . .

Curious Crabs

May 8, 2007
Jocie Ingram

When a beach rock is tipped over, small crabs scuttle for cover under neighbouring rocks. This simple act is something we discover as children, and rediscover again as parents and grandparents. These creatures, with their sideways "crab walk", armoured shells, pincers and stalked eyes, fascinate and delight us.

There are a great variety of crabs in our area. Some prefer rocky places, while others like to hide in the seaweed or bury themselves in sand. There are big crabs and small crabs, some with long spidery legs, and others with short stocky ones. They come in many colours and may be red, brown, green, yellowish, or have intricate tortoise-shell patterns. Some crabs are very hairy, and some are smooth. They are often masters of camouflage, and may piggyback bits of seaweed on their backs to escape notice.

Shore Crab photo © Dave Ingram

Despite this rich diversity, there are a few basic things that all crabs have in common.  Crabs are crustaceans, a group that includes creatures such as barnacles, shrimp and lobster.

Crabs have a rigid external skeleton, called an exoskeleton, made of chitin impregnated with calcium salts. As a young crab grows, it has to moult the old skeleton and grow a new one. A soft new skeleton develops inside of the old one, and when the time is right the crab squirms out, leaving the old skeleton completely intact. Beachcombers often mistake these moults for dead crabs.

Ten jointed legs radiate from the crab's main body. There are four thinner legs on each side, and two larger legs tipped with pincers. Crabs are able to lose leg segments and regenerate them. When under stress, or grabbed by a predator, a crab will drop a limb. Soon, a new bud will form, and the missing segment will grow anew.

Crabs have separate male and female individuals. In some species, males may have larger pincers than females. Generally, males may be told apart from females by flipping them onto their backs. Males have a line that forms a rough inverted V (like a lighthouse), while females have an inverted U-shaped line (like a beehive).

After mating, female crabs carry grape-like clusters of eggs, encased in a flap tucked against the abdomen. The eggs hatch into tiny larvae that are free-floating in the sea. These larvae look very different from adult crabs. After going through several larval stages and moults, the larvae gradually transform into an adult crabs.

Along sheltered rocky beaches of our coast, the most common crabs to be found are the green shore crabs (Hemigrapsus oregonensis). These small, hairy-legged crabs are scavengers, eating particles of waste. The body is 3-5 cm across, and colouration varies from green to yellow, olive grey or mottled. Another familiar crab, slightly bigger, is the purple shore crab (Hemigrapsus nudus), which has hairless legs. Though colouration may vary, these crabs are typically reddish purple with purple polka dots on the pincers.

Hermit Crab photo © Dave Ingram

Peering into a tide pool with anemones, limpets, barnacles and periwinkles, hermit crabs may be observed. These curious crabs take up residence in empty snail shells, to protect their soft and curled lower abdomen. A small shell that appears to be walking is likely home to a hermit. When a hermit grows too large for its shell, it must find a new home. These feisty crabs often battle over desirable shells.

Crabs known as spider crabs may lurk amongst the seaweed. The kelp crab (Pugettia producta) likes to cling pieces of kelp or eelgrass. With long thin legs and spines on its back, this is an aggressive crab. Handling should be avoided. The decorator crab (Oregonia gracilis) garlands itself with bits of seaweed and other materials with a sticky glue-like substance from its mouth. Some refer to this crab as "a walking garden".

The biggest crabs to be found in our area are the red rock crab (Cancer productus) and the dungeness or edible crab (Cancer magister). The red rock crab is a deep reddish brown, and has black-tipped pincers. It may be found in rocky areas, or eel grass beds on sand flats. Though edible, this crab is not large or plentiful enough to be harvested.

The larger Dungeness crab measures up to 20 cm across the back, and frequents sandy or muddy bays. A light brownish-orange, this crab has claws that are proportionally smaller than its body. Dungeness crabs like to hide in the sand, burying themselves so that only eyes and antennae protrude. These crabs are harvested commercially by setting crab traps on the sandy ocean floor. Though many crabs are scavengers, some are also predators, and the Dungeness likes to feed on small clams.

Exploring intertidal life is a fun way to spend a spring day, and watching crabs and their antics is a great way to reconnect with the child within.