Chionoecetes opilio

Geographic Range

Alaska snow crabs are found primarily in the northern Pacific Ocean, in the Bering, Beaufort, Chukchi, and Japan Seas, as well as the Sea of Okhotsk and off the Aleutian Islands. They may also be found along the eastern coast of Russia, along the coast of northern Siberia, on the shelf of North Primorsky Krai, and in Peter the Great Bay. They are also found on the Scotian Shelf,in the northwestern Atlantic Ocean, from northern Labrador, Newfoundland, and Greenland to the Gulf of Maine, including the southern Gulf of St. Lawrence.

• Biogeographic Regions
• nearctic nearctic
  • native native
• palearctic palearctic
  • native native

• Other Geographic Terms
• holarctic holarctic

Habitat

Alaska snow crabs are benthic animals that are found at a wide range of depths, from 13 to 2187 m, with the majority of individuals found no deeper than 110 m. Temperature and substrate surfaced seem to be the most important factors in habitat determination; these crabs prefer to live on muddy bottoms, although smaller individuals prefer gravely substrate. They generally live at temperatures ranging from -1 to 5°C, but may be found at temperatures as high as 10°C.

• Habitat Regions
• polar polar
• saltwater or marine saltwater or marine

• Aquatic Biomes
• benthic benthic

Physical Description

Alaska snow crabs have a relatively round carapace with a short rostrum; when fully grown, the carapace width ranges from 40-160 mm. Males are typically much larger than females. Females have a maximum carapace width of only about 80-95 mm, and leg spans averaging 38 cm, while males may have a carapace width of up to 165 mm and leg spans of approximately 90 cm. On average, commercially caught males weigh 0.5-1.35 kg, while females weigh only 0.5 kg. Sexes can also be distinguished by claw size (males with larger claws) and abdominal flap shape: females have more rounded flaps while male flaps are more triangular. The carapace and legs are typically reddish in color and covered with small bumps or spikes; however, the carapace of crabs that are about to molt are dark greenish in color. The legs are whitish in color along the sides and bottoms, and these crabs are yellowish-white underneath. Alaska snow crabs are bilaterally symmetrical, with 5 pairs of limbs. The back 4 pairs of limbs are used for crawling along the sea floor, while the front pair are chelae (claws), used for catching and manipulating food. The first 3 pairs of walking legs are much longer than the chelipeds (claw-bearing appendages). The eyes are green or blueish-green.

• Other Physical Features
• ectothermic ectothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• male larger
• sexes shaped differently

Development

Fertilized eggs are carried on a female's pleopods until they hatch (dependent largely on water temperature, typically from April through late May), as much as 1-3 years after fertilization. Larvae are shrimp-like and free-swimming. After three molts (typically taking 3-5 months to complete), larvae become become known as megalops. During this stage, intermediate between larvae and adult, megalops settle to the sea floor and feed on detritus for about 30 days. After this, they molt again, becoming juveniles. Males and females go through three slightly different stages of development after the megalops phase. Males begin as juveniles, mature to adolescents (at which time their reproductive organs become functional), and finally reach adulthood, in which their larger claws are apparent. Females begin development immature, then progress to a prepubescent stage in which ovaries begin to develop, and finally become reproductively mature adults. Both sexes molt 1-3 times per year until sexual maturity and completing a terminal molt.

• Development - Life Cycle
• metamorphosis metamorphosis

Reproduction

Alaska snow crabs are polygynandrous. In the presence of excess females, males mate with multiple partners. Females are multiparous (capable of laying eggs multiple times). In the absence of additional males, a female may store sperm in her spermatheca and use it to fertilize her eggs at a later time. As early as three weeks before mating, a male will hold a female, preparing to aid her through her terminal molt, also feeding and caring for her during this time. He will defend his position fiercely from other males. Females are very selective when choosing mates, fighting off unwanted suitors. After the female has molted, the male will transfer sperm to her spermatheca, where fertilization occurs. When the eggs begin to hatch (up to 1-3 years later, dependending on water temperatures), a male will assist in larval release by holding and waving the female around. Larvae are generally released in the spring, when there are high levels of phytoplankton in the water. Once released, the larvae are completely independent.

• Mating System
• polygynandrous (promiscuous) polygynandrous (promiscuous)

Male Alaska snow crabs reach reproductive maturity at 4-11 years (8-13 molts) while females take only 4-6 years (8-10 molts) to reach sexual maturity. These crabs mate in the spring after migrating to shallower water; there is evidence that, at least in a lab setting, males are fertile year-round. Females carrying their first clutch of eggs during a given breeding season are known as primiparous. Multiparous females are those carrying a second or third egg clutch, either fertilized by a different male or by sperm stored from a previous copulation. After fertilization, females carry a clutch of 12,000-160,000 eggs for 1-2 years, until they are ready to hatch and water conditions are appropriate. 100% of females carry eggs each year.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• fertilization fertilization
  • external external
• oviparous oviparous
• sperm-storing sperm-storing
• delayed fertilization delayed fertilization

Parental investment occurs pre-fertilization (males) and pre-hatching (females). Prior to fertilization, a male holds on to a female for up to three weeks, during which he protects her from predators, feeds her, and then helps her to molt before releasing his sperm; after this, there is no further male investment. After fertilization, a female carries fertilized eggs for 1-3 years before releasing larvae, during which time she provides protection to the eggs but does not provide any nutrients. After larvae are released, they are completely independent from both parents.

• Parental Investment
• pre-fertilization
  • provisioning
  • protecting
    • male
• pre-hatching/birth
  • protecting
    • female

Lifespan/Longevity

The estimated maximum age for females is 12-13 years (approximately 5 years after the terminal molt), while the maximum age for males is estimated to be 13-19 years (4-5 years after the terminal molt).

Behavior

Alaska snow crabs are motile, using different types of locomotion at different stages in their life cycle. In the zoeal (early) larval stage, they use their appendages to swim. However, once zoeal larvae morph into megalops larvae, they settle on the sea floor. Juveniles and adults use their legs to crawl along the sea floor. These crabs migrate to shallower waters to breed and also are known to migrate to deeper waters as they age, in order to accommodate their changing preferences for prey, water temperature, and substrate types. Outside of breeding, males and females are solitary and segregated, with males found on muddy bottoms in deeper waters and females found on gravelly or rocky bottoms in shallower waters.

• Key Behaviors
• diurnal diurnal
• nocturnal nocturnal
• motile motile
• migratory migratory
• solitary solitary

Home Range

These crabs are not known to occupy a particular home range or defend territories, outside of males defending the immediate proximity of a female when preparing to mate.

Communication and Perception

It is thought that pheromones may play a role in mate attraction for this species. In general, crabs perceive their environments visually using compound eyes on stalks which can be retracted into sockets on the carapace for protection. Setae on their walking legs are used to perceive tactile cues, such as differences in substrate.

• Communication Channels
• visual visual
• tactile tactile
• chemical chemical

• Other Communication Modes
• pheromones pheromones

• Perception Channels
• visual visual
• tactile tactile
• chemical chemical

Food Habits

Adult Alaska snow crabs feed on sessile or slow moving benthic invertebrates including shrimps, brittle stars, polychaete worms, gammarid amphipods, bivalves, hydroids, sea stars, squids, and gastropods. They may also feed on algae, sponges, and bryozoans. In addition to feeding on live organisms, Alaskan snow crabs are scavengers, feeding on detritus and dead organisms such as fish and other crabs. Larval stages consume plankton.

• Primary Diet
• carnivore carnivore
  • eats non-insect arthropods
  • molluscivore molluscivore
  • vermivore
  • eats other marine invertebrates
  • scavenger scavenger
• omnivore omnivore
• planktivore planktivore
• detritivore detritivore

• Animal Foods
• carrion carrion
• mollusks
• aquatic or marine worms
• aquatic crustaceans
• echinoderms
• other marine invertebrates
• zooplankton zooplankton

• Plant Foods
• algae
• phytoplankton phytoplankton

• Other Foods
• detritus detritus

Predation

Alaska snow crabs may burrow into soft substrate to avoid predation. Their reddish brown coloration also helps to camouflage them against the ocean floor, and their sharp claws can deter potential predators. Larval stages are prey to fishes and, as they grow, juveniles and adults become prey to marine mammals and invertebrates as well.

• Anti-predator Adaptations
• cryptic cryptic

Ecosystem Roles

Alaska snow crabs are scavengers to some extent, cleaning organic material from the ocean floor. They also serve as prey for larger organisms. They are known hosts to two species of amphipods: Gammaropsis inaequistyli and Ischyrocerus commensalis . These amphipods live on a crab's carapace, with no negative effect to their host. Alaska snow crabs may also host other polychaete, bryozoan, and hydrozoan epizoites, as well as endoparasites, particularly a dinoflagellate that causes Bitter Crab Disease.

• Ecosystem Impact
• biodegradation biodegradation

Economic Importance for Humans: Positive

Alaska snow crabs are primarily important to humans as a source of food due to their flesh's sweet taste and high protein but low fat content. This species became more important to fisherman in the early 1980s when the king crab market crashed and fisherman needed another source of income. Alaska snow crabs were fished heavily until 1999, when they were declared overfished and more regulations were imposed. Since then, populations have been managed effectively, with fishing quotas reaching 89.9 million pounds in 2011/2012. This species has become better known and increasingly economically important with the popularity of the television series "Deadliest Catch" on the Discovery Channel, which documents the struggles of crab fishing on the Bering Sea. The series began airing in April 2005.

• Positive Impacts
• food food

Economic Importance for Humans: Negative

There are no known adverse effects of this species on humans.

Conservation Status

Alaska snow crabs are abundant and populations are not currently threatened. Regulations put in place in the late 1990s and early 2000s are still in place, keeping fishing sustainable. Only adult males can be caught during the winter season, when mating and molting does not occur. A cap, set annually and based on the previous year's population, is also placed on the total number of crabs that can be harvested each season.

Additional Links

Encyclopedia of Life

Contributors

Justin Siegel (author), University of Michigan-Ann Arbor, Alison Gould (editor), University of Michigan-Ann Arbor, Jeremy Wright (editor), University of Michigan-Ann Arbor.

References

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