What are crabs?
Crabs are crustaceans, part of the Arthropod phylum of invertebrates. Arthropods have jointed appendages, no backbone and a hard external skeleton, or exoskeleton, made of a substance called chitin. Crustaceans also have a large shell called a carapace covering the main part of their body, four pairs of walking legs, two pincers, stalked eyes and two pairs of antennae. Other crustaceans include lobsters, shrimp, copepods, barnacles and crayfish. Most crabs live in the ocean, although some are also found in freshwater.
Many species of crabs are commercially important as a human food source; others are both predators and prey in marine food webs. For example, river and sea otters eat crabs, while crabs feed on clams, other crustaceans, small fish, and marine algae such as sea lettuce.
Several types of crab are common in the marine areas around Victoria:
- Dungeness crab (Cancer magister) is the species most commonly harvested commercially in this area. They are brown or tan while alive. The male can grow to over 20 cm wide, while the female rarely exceeds 15 cm.
- Graceful crab (Cancer gracilis) looks similar to the Dungeness crab, but is much smaller, has smooth pincers (unlike the Dungeness’s spiny pincers) and a white perimeter around its shell.
- Red rock crab (Cancer productus) is a deep red colour with black-tipped pincers. Although generally too small for commercial harvest, it is sometimes taken by recreational fishers.
- Kelp crab (Pugettia producta) is also sometimes called a spider crab due to its very long legs. Its greenish-brown carapace is shaped like a shield and may be up to 10 cm across. It lives among kelp and eelgrass, as well as on pilings and rocky shorelines.
- Decorator crab (Oregonia gracilis) also has long legs but is usually less than 3 cm across. This species has the unique habit of attaching bits of live seaweeds, sponges and assorted debris onto their carapace for camouflage, by secreting a “glue” with their mouth.
- Green shore crabs (Hemigrapsus oregonensis) and purple shore crabs (H. nudus) are the most common types of crabs found under rocks in the intertidal zone. They are rarely larger than 5 cm across.
- Hermit crabs (Pagurus spp) are unique in that they do not grow shells of their own; they take over unoccupied snail shells, and must find new larger ones as they grow. These crabs are commonly seen in tide pools.
Where do crabs live?
Crabs live in a variety of marine habitats, from estuaries to rocky shorelines. Some are found only in the subtidal zone, where they are constantly submerged, while others can withstand the rigours of widely fluctuating temperature, salinity and moisture of the intertidal zone. Crabs often hide in rock crevices, under vegetation or rocks, and by burrowing into soft sand and mud. Turn over almost any medium-sized boulder in the intertidal zone, and you will find a multitude of crabs; just remember to carefully put it back again.
How do crabs live & reproduce?
Crabs feed on a variety of prey (e.g. clams, snails, marine worms and fish), both alive and dead; some, such as shore crabs, also eat vegetation like sea lettuce. Most crabs can move surprisingly quickly, to pursue prey or flee from danger.
As crabs grow, they must shed their old shells in a process called moulting. Just before the moult, their tissues absorb extra water, causing their body to swell and split open the carapace. The crab then backs out of its old shell. It takes a number of weeks for the new shell to harden; during this time the crab is more vulnerable to predators so it takes shelter in protected areas such as eelgrass beds. Dungeness crabs go through 10 or 11 moults before they reach sexual maturity at about two years of age.
Male and female crabs moult at different times of the year, because only just after moulting is the female’s shell soft enough for the male to penetrate it. During mating, the male clasps the female so that their abdomens are pressed together. The male may carry the smaller female around for a number of days, under his belly, to ensure that only he mates with her. The eggs are not fertilized with sperm until they are extruded by the female, some months after mating. Eggs remain attached to the female’s abdomen until they hatch.
Newly hatched crab larvae look more like shrimp, and drift with the ocean currents. After passing through several larval stages, they take on more of a crab-like form, with claws and legs, but can still swim; at this point they are called megalops. Megalops swim to the surface of the water to feed at night, and during the day they descend to about 20 metres. Eventually, the megalops settle to the sea bottom and become juvenile crabs.
Why are crabs important?
People have probably eaten crabs for as long as they have lived near the seashore. They are known as a delicacy throughout the world, and both the sport and commercial crab fishery contribute to the local economy. For some First Nations in B.C., crab harvesting is an important subsistence and cultural activity.
Crabs are one of the main decomposers in the marine ecosystem. In other words, they help to “clean up” the sea bottom by harvesting decomposing plant and animal matter. Many fish (e.g. halibut and dogfish), birds (e.g. great blue herons) and sea mammals (e.g. sea otters) rely on crabs for a food source.
What threatens crabs?
Pollution is one of the main threats to crabs. Many types of pollutants tend to sink to the bottom and bind to sediments. Since crabs are in contact with the bottom environment, they may be exposed to high levels of substances such as dioxins, furans, heavy metals, PCBs and pesticides. Some of these substances are present in the harbours around Victoria due to historic industrial pollution, while others are ongoing problems such as contaminants in runoff that enters harbours through storm drains.
Another pollution concern stems from nutrient pollution such as sewage and fertilizers. Nitrates in these substances can cause abnormally large algae blooms; as the algae are broken down by bacteria, oxygen in the water is used up. Fish and crabs may then die from a lack of oxygen.
Excess sediment in the water may interfere with the crabs’ ability to breathe and feed. This can arise from construction activities, logging and erosion in the watershed. Erosion is compounded by the large area of impervious surfaces in this urban region. Instead of gradually soaking into the earth, rainwater that falls on pavement and roads is washed into storm drains and creeks, and can lead to sudden high flows that damage streams and deposit sediment in the marine areas.
Commercial and recreational fisheries must also be carefully managed, to ensure that enough crabs are left to reproduce and maintain stocks.
How can I help protect crabs?
For information on protecting Crabs, please visit our How Can I Help?
Additional Links & Information ©Top Image courtesy of Minette Layne, bottom image courtesy of Jody Watson