Chionoecetes opilio is a predominantly epifaunal crustacean found at shelf depths in the northwest Atlantic Ocean and north Pacific Ocean. It is a well-known commercial species of Chionoecetes, often caught with traps or by trawling. Male C. opilio with a total length above 95 millimetres (3.7 in) long are the most commonly trapped, especially around Canada and Newfoundland. This crab genus is found all across both the Atlantic and Pacific oceans. There are seven species in the genus Chionoecetes, all of which bear the name "snow crab." Chionoecetes opilio is also related to Chionoecetes tanneri, commonly known as the tanner crab, and other crab species found in the cold, northern oceans.

Anatomy

Snow crabs have equally long and wide carapaces, or protective shell-coverings, over their bodies. Their tubercles, or the bodily projections on their shell, are moderately enclosed in calcium deposits, and they boast hooked setae, which are rigid, yet springy, hair-like organs on their claws. Snow crabs have a horizontal rostrum at the front of the carapace; the rostrum is basically just an extension of the hard, shell covering of the carapace and it boasts two flat horns separated by a gap. They have triangular spines and well-defined gastric and branchial regions internally.^[2] Snow crabs also have little granules along the border of their bodies, except their intestinal region. Concerning their walking legs, their first three are compressed; their chelipeds, or pincers, are usually smaller, shorter, or equal to their walking legs.^[3] Snow crab are iridescent and range in color from brown to light red on top and from yellow to white on the bottom,^[1] and are bright white on the sides of their feet.^[4]

Distribution and habitat

Snow crab are very abundant in the Atlantic Ocean region. More specifically, they are found in the Western Atlantic area near countries such as Greenland, Newfoundland, and in the Gulf of Maine.^[2] This crab species is also found across the North Pacific area, in areas ranging from Alaska to northern Siberia, and through the Bering Strait to the Aleutian Islands, Japan, and Korea.^[5]

Snow crab are often found in the ocean's benthic shelf and upper slope, in the sandy and muddy bottoms, and in depths as shallow as 20 metres (66 ft) and as deep as 1,200 m (3,900 ft). The most snow crab can be found at 70–280 m (230–920 ft) in the Atlantic waters.^[3] There are interesting differences in where male and female snow crab are found in the ocean depths: Small adult and senescent adult males occur mainly at intermediate depths over much of the year, while large and hardy adult males are found mostly at depths greater than 80 m (260 ft). Adult females are gregarious and congregate at depths of 60–120 m (200–390 ft).

Diet

Chionoecetes opilio eat other invertebrates that reside in the benthic shelf, such as crustaceans, bivalves, brittle stars, polychaetes, and even phytobenthos and foraminiferans. Snow crabs are also scavenger eaters, and aside from preying on other benthic shelf invertebrates, they also prey on annelid worms and mollusks. Males typically prove to be better predators than mature females, and prey type depends upon predator size, with the smallest crabs feeding mainly on amphipods and ophiuroids, while the largest crabs feed mainly on annelids, crustacean decapods and fish.^[6] Cannibalism is also practised at times among snow crabs, most frequently by intermediate-sized females.

Size and population structure

The snow crab grows slowly and is structured according to its size, with at least 11 recognised stages of growth for male crabs. The male crabs are usually twice the size of the female crabs. Male snow crabs can grow to be up to 150 mm (5.9 in), while females can grow up to 90 mm (3.5 in). Male carapaces are usually around 70 mm (2.8 in) in width and length, with the female carapace usually at about 55 mm (2.2 in) in width and length.^[4]

Off the coast of Newfoundland, two amphipod species – Ischyrocerus commensalis and Gammaropsis inaequistylis – have been found to live on the carapace of the snow crab.^[7]

Breeding patterns

Snow crab have a very high reproductive potential: each year, every single female carries eggs. Females are fertilised internally and can carry up to 150,000 eggs under their abdomens after mating. Females usually lay their eggs in very deep areas of the ocean, such as in deposits of phytodetritus. Males are also capable of mating at both immature and mature stages of their lives.^[8]

Adult snow crabs usually live between five and six years; before their deaths, they usually moult, mate a final time, and then die. New snow crab offspring hatch along with the late spring phytoplankton boom, so that they have an ample food source to take advantage of upon hatching. When they hatch, the snow crabs are in the zoeal stage, meaning that they are developing larva that can swim on their own. Then, they morph into the megalopa stage and settle to the ocean floor among the phytodetritus; the megalopa stage is the advanced larval stage that the crab develops into before becoming a true adult.

Commercial importance

This species of crab was commonly caught by trappers in the 1980s, but trapping of this type of crab has decreased since then. Much of the trapping has been in Canada for commercial use, such as for edible purposes.^[9]

Taxonomy

The species was first described by Otto Fabricius in 1780, under the name Cancer phalangium,^[10] a name which was invalid due to Johan Christian Fabricius having used it previously for the species now known as Inachus phalangium.^[11] The first valid scientific name was provided by Otto Fabricius in 1788, when he redescribed the species as Cancer opilio. The type locality is Greenland.^[11]

As the genus Cancer was divided up, the species C. opilio was transferred to a new genus, Chionoecetes by Henrik Nikolai Krøyer in 1838. Chionoecetes opilio was the only species in the genus at first, and is therefore the type species.

Mary J. Rathbun described a subspecies, C. opilio elongatus, in 1924. This is now generally recognised as a full species, Chionoecetes elongatus.^[12]

References

Further reading

• A. J. Paul, ed. (2000) (PDF). Bibliography of Research on Snow Crab (Chionoecetes opilio). University of Alaska Sea Grant College Program. ISBN 978-1-56612-063-0. http://seagrant.uaf.edu/lib/aksg/0001/AK-SG-00-01.pdf.
• "Species Fact Sheet: Chionoecetes opilio (O. Fabricius, 1788)". Fisheries and Aquaculture Department. FAO. http://www.fao.org/fishery/species/2644/en.