Cancer irroratusAtlantic rock crab

Geographic Range

Cancer irroratus inhabit the Atlantic ocean, and are extensively distributed along North America's east coast. Their range stretches as far north as Labrador, Canada and reaches southward to South Carolina, U.S. (Gendron,2001).


Cancer irroratus live in the benthic zone of the ocean (Ristvey & Rebach 1999). They are able to adapt to extreme variations in depth. The depths that they exist at range from deep waters at 2,600ft. to very shallow waters and occasionly well inland from the low tide line (NJ Scuba, 1994-96). They live on a variety of substrate types including rocky and loose material. Smaller crabs with the cephalorthorax width of about 50 mm tend to inhabit muddy or sandy bottoms, while other crabs seem to prefer the rocky bottoms (Gendron, 2001).

Physical Description

The rock crab's size varies considerably between males and females. A fully mature male can reach a cephalothorax width (CW) between 50mm-140mm (Gendron,2001), and range in weight from 49-202g (Miller & Addison, 1994). CW is measured as the distance between the two most lateral notches on the carapace (Miller & Addison, 1995). It is uncommon for females to surpass 100mm CW (Gendron,2001). The shape of a rock crab's shell is wide and oval. It is a yellowish color and frequently shades of red and purplish brown are visible. The different array of colors gives this creature a unique and colorful carapace. Another interesting characteristic of the rock crab is its denitition, or teeth. The rock crab has either nine broad and smooth or rough jagged teeth, depending on its diet (Gosner,1978).

  • Range mass
    49 to 202 g
    1.73 to 7.12 oz
  • Average mass
    108.50 g
    3.82 oz


Cancer irroratus reproduction occurs in the autumn. Sexual maturity in females is attained once they reach the CW size of 60 mm. Males have a slightly larger size of CW 70 mm at sexual maturity. Before mating commences, both male and female rock crabs moult their shell. This event for males happens during the winter months to ensure that their shells are completely firm before the fall. Females moult their shells during the fall so that they remain soft during mating. It takes rock crab shells between 2-3 months to fully harden. After fertilization females lay their eggs and store them under their stomach for almost a year. Depending upon the female's size the number of eggs produced can range from 125,000 to 500,000. After this period of time the eggs hatch and between the months of June to September the larva stay in the water column. By the time autumn returns the larva that were hatched metamorphose into extremely small crabs called megalops. For the first part of their life these young crabs are extremely vulnerable to predators and water turbulance. To increase their chance for survival they remain in shallow water and at the bottom of the ocean floor for protection (Gendron,2001).


Cancer irroratus does not remain in one spot for too long. As a scavenger it is extremely mobile. Each season it moves between varying depths. In the autumn, which is their season of reproduction, most rock crabs remain at shallow depths (Gendron, 2001). They tend to bury themselves in rocky areas and in tide pools. Their main method of defense is to pinch if provoked (Gosner, 1978). Their most important predator is the lobster. Lobsters usually consume megalops (tiny rock crabs that have just metamorphsed) or crabs that are still in their larva stage. Once rock crabs reach their full size they seem to be safe from lobsters (Gendron, 2001). Many times rock crabs enter lobster pots and steal bait and are captured themselves as a result (NJ Scuba, 1994-96).

Food Habits

Rock crabs are opportunistic feeders. Since they inhabit the benthic zone of the ocean from shallow waters to depths extending to 2,600 ft., their food source is limited to species that can survive in this zone as well. They eat an assortment of items including algae, polychaetes, mussels, gastropods, and various crustaceans including hermit crabs. An interesting thing regarding Cancer irroratus food habits, is their responsiveness and affinity to food types they were already accustomed to rather than to ones with which they were unfamiliar. After being continuously fed one of two species they had the capability of noticing the difference in odors exhibited by both species, and expressed a preference for the most familiar one. This showed that rock crabs are able to detect and respond to prey odors (Ristvey & Rebach, 1999).

Economic Importance for Humans: Positive

The rock crab industry provides economic benefits. This fishery in the Atlantic Ocean is generally new. Lobsters have been the most popular fishery in this region over the years. The rock crab fishery first began in the 1970's but took several years to grow into an industry. As more markets developed for the seafood industry interest began to rise about the rock crab. Regulations were created by the Department of Fisheries and Oceans (DFO) to ensure the safety and survival of the species. These restrictions included- mandatory licenses in order to fish for rock crabs, a quota for the number of rock crabs fisherman could catch at one time, a minimum size of rock crabs fisherman were allowed to keep, set at CW 102 mm, and a ban on catching females to help promote a minimal amount of reproductive potential. This industry shows great promise for the future as more areas are being discovered and explored for rock crab exploitation (Gendron, 2001).

Economic Importance for Humans: Negative

Cancer irroratus were considered pests by fisherman catching lobsters because they were known to enter the lobster traps and steal the bait. However, once the rock crab industry began to develop they were considered a profitable item themselves (NJ Scuba, 1994-96). The rock crab fishery can have negative effects on the lobster industry. Lobsters are known to depend upon smaller rock crabs as a food source. As the number of fisherman exploiting rock crabs increases, the number of smaller rock crabs (including megalops) begin to decrease in number. This could cause a negative effect on the lobster industry by disrupting the lobster food supply (Gendron, 2001).

Conservation Status

As the rock crab industry continues to grow, it is having an undertermined effect on the conservation of the species. However steps have been taken to help prevent the overfishing of rock crabs. Organizations, such as the DFO, are monitoring and making efforts to maintain a steady population and reproductive potential by placing guidelines and restrictions on fisherman (Gendron, 2001).

Other Comments

Other studies that have been performed on Cancer irroratus include studies of osmoregulation and salinity tolerance and the trapping interactions of rock crabs and lobsters. A minimal amount of information has been compiled on the osmoregulation of larval and post-larval crustaceans. In their early stages of development rock crabs are hyper-osmoconformers in order to adjust to their surroundings. As they become adults they adjust between being osmoconformers in higher saline solutions to being hyper-regulators in less concentrated media. Larger adult rock crabs were more effective regulators than smaller adult rock crabs. Cancer irroratus salinity tolerance changes throughout its life cycle. When first born the larva has a moderate tolerance towards salinity. Its tolerance decreases as it begins to develop and drops to a minimum when it metamorphoses into a megalop. However, once the rock crab reaches maturity its salinity tolerance increases again (Charmantier & Charmantier-Daures, 1991). It was discovered, that crustacean catch rates are effected if there are other animals in the vicinity of the trap. When large rock crabs were present near the trap it decreased the capture of smaller rock crabs. In comparison, when lobsters were placed around the traps there was also a reduction in catch rates of both the small and large rock crabs. These results showed that there was a significant effect on crustacean catch rates when rock crabs experienced interactions with other threatening organisms, (Miller & Addison, 1994).


Krista Page (author), Western Maryland College, Randall L. Morrison (editor), Western Maryland College.


Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

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Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

native range

the area in which the animal is naturally found, the region in which it is endemic.


Charmantier, G., M. Charmantier-Daures. February 1991. Ontogeny of Osmoregulation and Salinity Tolerance in Cancer irroratus; Elements of Comparison with C. borealis (Crustacea, Decapoda). Biol. Bull., 180: 125.

Gendron, L. May 2001. "Rock Crab Of The Inshore Waters Of Quebec: DFO, Atlantic Fisheries, Stock Status Report C4-02" (On-line). Accessed April 24, 2001 at

Gosner, K. 1978. Peterson Field Guides Atlantic Seashore. 215 Park Avenue South, New York, New York 10003: Houghton Mifflin Company.

Miller, R., J. Addison. 1994. Trapping interactions of crabs and American lobster in laboratory tanks. Can. J. Fish. Aquat. Sci., 52: 315-318.

NJ Scuba A New Jersey Marine Life Slide Show, 1994-96. "Northeast Marine Life Slide Show: Atlantic Rock Crab" (On-line). Accessed April 12, 2001 at

Rebach, S., A. Ristvey. December 1999. Enhancement of the Response of Rock Crabs, Cancer irroratus, to Prey Odors following Feeding Experience. Biol. Bull., 197: 361-366.