Chionoecetes opilio

Chionoecetes opilio
Blue snow crab (typically more reddish in color)
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Subphylum:	Crustacea
Class:	Malacostraca
Order:	Decapoda
Infraorder:	Brachyura
Family:	Oregoniidae
Genus:	Chionoecetes
Species:	C. opilio
Binomial name
Chionoecetes opilio (O. Fabricius, 1788)
Synonyms
Cancer phalangium O. Fabricius, 1780 non J. C. Fabricius, 1775: preoccupied Cancer opilio O. Fabricius, 1788 Chionoecetes behringianus Stimpson, 1857 Chionoecetes chilensis Streets, 1870 Peloplastus pallasi Gerstaecker, 1856

Chionoecetes opilio, also known as snow crab, is a predominantly epifaunal crustacean native to 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. Seven species are in the genus Chionoecetes, all of which bear the name "snow crab". C. 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. 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. Snow crab are iridescent and range in color from brown to light red on top and from yellow to white on the bottom, and are bright white on the sides of their feet.

Distribution and habitat

Snow crabs are native to the Northwest Atlantic and the North Pacific. In the Northwest Atlantic, they are found in the areas near Greenland, Newfoundland, in the Gulf of St. Lawrence, and on the Scotian Shelf. In the North Pacific, this crab is found in areas ranging from Alaska to northern Siberia, and through the Bering Strait to the Aleutian Islands, Japan, and Korea.

In 1996, they were recorded in the Barents Sea for the first time. They are considered an invasive species there, but how they arrived there is unclear. Another commercially important species, introduced deliberately to the same region, the red king crab, already has established itself in Barents Sea. Similarly, snow crabs likely will have an adverse effect on the native species of the Barents Sea.

Snow crabs are found in the ocean's shelf and upper slope, on sandy and muddy bottoms. They are found at depths from 13 to 2,187 m (43–7,175 ft), but average is about 110 m (360 ft). In Atlantic waters, most snow crabs are found at depths of 70–280 m (230–920 ft). Where male and female snow crab are found in the ocean depths may vary: 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). Snow crabs mainly reside in very cold waters, between −1 and 5 °C (30–41 °F), but can be found at temperatures up to 10 °C (50 °F).

Diet

C. opilio crabs eat other invertebrates that reside in the benthic shelf, such as crustaceans, bivalves, brittle stars, polychaetes, and even phytobenthos and foraminiferans. Snow crabs also are scavengers, and aside from preying on other benthic shelf invertebrates, they 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. Cannibalism is practised at times among snow crabs, most frequently by intermediate-sized females.

Size and population structure

Two snow crabs with the larger male perched on top of the female, Bonne Bay, Newfoundland, Canada

The snow crab grows slowly and is structured according to its size. There are at least 11 recognised stages of growth for male crabs. Usually, the male crabs are almost twice the size of the female crabs. Male snow crabs can grow up to 16.5 cm (6.5 in) in carapace width, while females can grow up to 9.5 cm (3.7 in). Male carapaces are usually approximately 7 cm (2.8 in) in width and length, with the female carapace usually close to 5.5 cm (2.2 in) in width and length. Males caught in commercial fisheries generally weigh 0.5–1.35 kg (1.1–3.0 lb) and females generally about 0.5 kg (1.1 lb).

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.

Breeding patterns

Snow crabs have a very high reproductive potential: each year, every 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 also are capable of mating at both immature and mature stages of their lives.

Adult snow crabs usually live to five or six years. Before their deaths, they usually moult, mate a final time, and die. New snow crab offspring hatch along with the late spring phytoplankton boom, so 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 metamorphose into the megalopa stage and settle to the ocean floor among the phytodetritus; the megalopa stage is the advanced larval stage into which the crab develops before becoming an adult.

Commercial importance

This species of crab was commonly caught by trappers in the 1980s, but trapping has decreased since then. Much of the trapping has been in Canada for commercial use. The first commercial fishing for the species in the Bering Sea (where not native) began in 2013, and the stock of this region likely will reach levels similar to eastern Canada in the future.

Taxonomy

The species was first described by Otto Fabricius in 1780, under the name Cancer phalangium, a name that was invalid due to Johan Christian Fabricius having used it previously for the species now known as Inachus phalangium. 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.

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, so it is 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.

Gallery

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  Steamed snow crab
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  Steamed snow crab legs
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  Snow crab fried rice on crab shell

Further reading

• A. J. Paul, ed. (2000). Bibliography of Research on Snow Crab (Chionoecetes opilio) (PDF). University of Alaska Sea Grant College Program. ISBN 978-1-56612-063-0.
• "Species Fact Sheet: Chionoecetes opilio (O. Fabricius, 1788)". Fisheries and Aquaculture Department. FAO.
• Top and Bottom Views of the Opilio, Snow Crab, Chionoecetes opilio - Dana Point Fish Company