Underwater World

Snow Crab

Northwestern Altantic

At a glance | Vital stats | Curiosities | Closeup

At a glance

Snow crabs live on muddy bottoms in cold waters. They are sexually dimorphic, meaning males and females have two different forms, with obvious size differences between them. As they grow these crabs periodically shed their shell (a process called the molt) including the carapace-the large portion on the crab's back. Molting is an important sign of a snow crab's growth as it matures and gets ready to reproduce.

Vital stats

Scientific name:
Chionoecetes opilio
Taxonomy:
Crustacea, Decapoda, Majoidea, Oregoniidae
Region:
Northwestern Atlantic (from Greenland to the Gulf of Maine), the northern Pacific including the Bering Sea and Sea of Japan, the Chuckchi, Beaufort and Barents Seas in the Arctic Ocean

Curiosities

To protect this species and its reproduction, commercial harvesters can only catch male snow crabs. Catching-or 'landing'-female crabs is not allowed.

Closeup

About the snow crab

Male and female snow crabs have one obvious physical difference at maturity: size. The male is much larger, with a carapace up to 16.5 centimetres wide, a leg span of 90 centimetres, and a weight of about 1.35 kilograms. By comparison, the female's carapace does not often grow beyond 9.5 centimetres; its leg span is about 38 centimetres and it weighs just under half a kilogram.

Apart from size, snow crabs also differ in shape: males have a triangular abdomen and relatively large claws, while the female's abdomen is circular and their claws are rather small.

Snow crabs feed by preying or scavenging on polychaetes (marine worms), crustaceans (shrimp, crab and smaller crustaceans such as amphipods), bivalves and gastropods (both are kinds of mollusks), fish, brittle stars, sea urchins, hydrozoans, large zooplankton. Snow crabs will also eat their smaller congeners.

Lifecycle and reproduction

After spring hatching, snow crabs go through a larval stage in which they float with the plankton in the sea, then eventually settle to the bottom. Benthic juveniles (those living on the sea bottom) grow by shedding their shell (moulting) each year, usually sometime in late winter or early spring.

Male and female snow crabs develop differently, though each undergoes three stages of development.

For male snow crabs, the stages are:

  • juvenile-when its reproductive organs do not function;
  • adolescent-when its reproductive organs do function but its claws are not yet enlarged; and
  • adult-when its claws are enlarged (are 'differentiated').

Juvenile and adolescent males may moult every year until they develop enlarged adult claws, a feature that improves their mating ability by allowing them to fight off other male crabs. The last moult of the male snow crab can occur anytime between four and eleven years of age-after about 8 to 13 moults.

For females, the three stages are:

  • immature-when they have a narrow abdomen and no detectable ovaries;
  • prepubescent-when their ovaries begin to develop; and
  • adult-when their abdomen gets broader and they have the ability to reproduce.

Most females become sexually mature at their last (terminal) moult when their carapace width reaches 40 to 75 millimetres. This occurs between the ages of four and six years, or between 8 and 10 moults.

Up to three weeks before mating begins, a male will start holding a female preparing for terminal moult and assist her through moulting. The male becomes her protector, fending off other males and predators. The competition for females may be fierce and males often become injured in the process.

Females may be quite selective when choosing a mating partner. Some have died resisting mating attempts from unwanted males.

In spring, mating pairs migrate to shallow waters. Female snow crabs carrying their first clutch of eggs are called 'primiparous'. Females are multiparous when they carry egg clutches for a second or third time.

Primiparous females mate after their terminal moult, sometime from February to mid-March. The male fertilizes the female's eggs before they are released. The eggs are deposited on the female's pleopods, under her abdomen, and carried there until the larvae hatch-one to two years later! Water temperature determines how long the eggs are carried.

A female can produce from 12,000 to 160,000 eggs depending on her size. Sperm that accumulates during the mating process is stored in a sack called the spermatheca. After the larvae are released the female may mate again or use the stored sperm to fertilize future clutches of eggs. The larvae are released from April to late May and are found throughout the water column (between the surface and the bottom). Depending on temperature, this planktonic larval development lasts three to five months. The larvae will go through various stages before settling on the sea bottom.

The underwater world of the snow crab

Predators and Diseases

Predators of the snow crab include the halibut, skates (primarily thorny skate), cod, seal and the American plaice. Smaller crabs and newly molted soft-shell crabs are especially vulnerable to predators.

Bitter crab disease (BCD), caused by a parasitic alga, commonly infects crabs in the Bering Sea, and off Newfoundland. Infected crabs have a bitter taste, their leg meat becomes milky white in color and their mortality is thought to be high. The rate of mortality is higher in August-September and decreases towards the mid-winter. Other diseases such as carapace diseases caused by fungus and bacteria are also known but are less detrimental compared to BCD.

Commercial Fishing

Commercial fishing of the snow crab began in the mid-1960s in Canada. The main fishing grounds range from Quebec to western Newfoundland-along the northern shore of the St. Lawrence River, around the Gaspé Peninsula to the Magdalen Islands, around Cape Breton Island toward southwestern Nova Scotia, and from southeastern Newfoundland midway up to Labrador. This is a male-only fishery with the minimum size limit set at 9.5 cm in carapace width. The fishery is also managed by fishing season, number of traps, and quota. The total landings in Atlantic Canada were at 90,000 tonnes in 2007.

Crab fishing vessels vary in size depending on whether the fishery is inshore, mid-shore or offshore. Snow crabs are trapped using various baits: most often herring, mackerel or squid. Once captured, the crabs are kept alive on ice in the hold of the fishing boat. Some vessels have saltwater circulation systems that maintain the quality of the crabs until processing, which usually happens within hours of their delivery to the processing plant. Eastern Canada's snow crab fisheries are at maximum capacity.

Climate impact

Snow crabs prefer a narrow temperature range of cold water less than 3 to 4°C. The effects of temperature depend on the stage of the snow crab's lifecycle. If the oceans warm, snow crabs may lose their primary habitat.

Revised: October 2009