Biology
Mature male and female snow crab. The male, pictured on top, grows to a larger size than the female, pictured on bottom.
"Pencil clawed" male on top has not yet reached terminal moult. Larger clawed male on bottom has reached terminal moult.
Moulting process.
Life History
The snow crab (Chionoecetes opilio, Brachyura, Majidae, O. Fabricius) is a subarctic species resident along the east coast of North America from northern Labrador to the Gulf of Maine. In the Scotian Shelf Ecosystem (SSE), commercially fished snow crab are generally observed between depths of 60 to 280 m and between temperatures of -1 to 6°C.
Mean annual bottom temperatures on the Scotian Shelf for 2010 (left). Depth (ln; m ) of the Scotian Shelf(right).
Predicted probabilities of viable habitat for fishable snow crab (takes into account temperature, depth and other environmental factors)
Near 7°C, metabolic costs are thought to match metabolic gains (Foyle et al. 1989). Snow crab are generally observed on soft mud bottoms although small-bodied and moulting crabs are also found on more complex (boulder, cobble) substrates (Sainte-Marie and Hazel 1992; Comeau et al. 1998).
Early Life Snow crab eggs are brooded by their mothers for up to 2 years, depending upon ambient temperatures, food availability and the maturity status of the mother (up to 27 months in primiparous females – first breeding event; and up to 24 months in multiparous females – second or possibly third breeding events; Sainte-Marie 1993). More rapid development of eggs (from 12 to 18 months) has been observed in other systems (Elner and Beninger 1995; Webb et al., 2007). A primiparous female of approximately 57.4 mm CW would produce between 35,000 to 46,000 eggs which are extruded between February and April (in the Baie Sainte-Marguerite; Sainte-Marie 1993). The actual range of fecundity is however quite large, especially as multiparous females are thought to be more fecund with more than 100,000 eggs being produced by each female. Eggs are hatched from April to June when the larvae become pelagic, feeding upon the plankton for 3 to 5 months (zoea stages 1 and 2 and then the megalopea stage). The larvae settle to the bottom in autumn to winter (September to October in the Gulf of St. Lawrence area). In the SSE, pelagic stages seem to have highest abundance in October and so may begin settling as late as January. Very little is known of survival rates at these early life stages.
Once settled to the bottom (benthic phase), snow crab grow rapidly, moulting approximately twice a year (Sainte-Marie et al. 1995; Comeau et al. 1998). The first inter-moult stage (instar 1) is approximately 3 mm CW. After the 5th instar (15 mm CW) the frequency of moults decline, moulting occurring once a year in the spring until they reach a terminal maturity moult. Growth is allometric with weight increasing approximately 250% with each moult.
Growth curves determined from Scotian Shelf male snow crab.
The left diagram shows the significant size gain from each moult for a male snow crab. The carapace width is about 50% larger with each moult and the weight approximately doubles(right diagram).
Returning immature (small / pencil claw) male crab to the water substantially increases the yield per crab to the snow crab fleet.
This can have great positive benefit to the stock in future years both in biomass and reproductive potential. Terminal moult has been observed to occur between the 9th to the 13th instar in males and the 9th to 10th instar in females. Just prior to the terminal moult, male crab may skip a moult in one year to moult in the next (Conan et al. 1992). Male snow crab generally reach legal size (≥ 95 mm CW) by the 12th instar; however, a variable fraction of instar 11 snow crab are also within legal size. Male instar 12 snow crab represent an age of approximately 9 years since settlement to the bottom and 11 years since egg extrusion. Thereafter, the life expectancy of a male is approximately 5 to 6 years. Up to 10 months are required for the shell to harden to carapace conditions 1 and early carapace condition 2 and up to 1 year for meat yields to be commercially viable. After hardening of the carapace (carapace conditions 3 to 4) the male is able to mate. Near the end of the lifespan of a snow crab (carapace condition 5), the shell decalcifies and softens, often with heavy epibiont growth. In some warm-water environments (e.g., continental slope areas), epibiont growth occurs at an accelerated rate creating some uncertainty in the classification of carapace condition 5 crab.
The growth stanzas of male snow crab. Each instar is determined from carapace width bounds obtained from modal analysis and categorized to carapace condition (CC) and maturity from visual inspection and/or maturity equations. Snow crab are resident in each growth stanza for 1 year, with the exception of CC2 to CC4 which are known from mark-recapture studies to last from 2 to 5 years.
Females reproducing for the first time (primiparous females) generally begin their moult to maturity at an average size of 60 mm CW and mate while their carapace is still soft (early spring: prior to the fishing season in Eastern Nova Scotia, and during the fishing season in Crab Fishing Area 4X). A second mating period later in the year (May to June) has also been observed for multiparous females (Hooper 1986). Complex behavioural patterns have also been observed: the male helps the primiparous female moult, protects her from other males and predators and even feeds her (indirectly; Hooper 1986). Pair formation (a mating embrace where the male holds the female) may occur up to 3 weeks prior to the mating event (Hooper 1986). Upon larval release, males have been seen to wave the females about to help disperse the larvae (i.e., prior to a multiparous mating). Females are selective in their mate choice, as is often the case in sexually dimorphic species, and have been seen to die in the process of resisting mating attempts from unsolicited males (Watson 1972; Hooper 1986). Males compete heavily for females and often injure themselves (losing appendages) while contesting over a female. Larger males with larger chela are generally more successful in mating and protecting females from harm.
References
Comeau, M., Conan, G. Y., Maynou, F., Robichaud, G., Therriault, J. C. and Starr, M. 1998. Growth, spatial distribution, and abundance of benthic stages of the snow crab, Chionoecetes opilio, in Bonne Bay, Newfoundland, Canada. Canadian Journal of Fisheries and Aquatic Sciences, 55:262–279.
Conan, G. Y., Comeau, M. and Robichaud, G. 1992. Life history and fishery management of majid crabs: the case study of the bonne bay (newfoundland) Chionoectes opilio population. International Council for the Exploration of the Seas, C.M.1992/K:21–21.
Elner, R. W. and Beninger, P. 1995. Multiple reproductive strategies in snow crab, Chionoecetes opilio: physiological pathways and behavioural plasticity. Journal of Experimental Marine Biology and Ecology, 193:93–112.
Foyle, T., O’Dor, R. and Elner, R. 1989. Energetically defining the thermal limits of the snow crab. Journal of Experimental Biology, 145:371–393.
Hooper, R. 1986. A spring breeding migration of the snow crab, Chionoectes opilio (O. Fabr.), into shallow water in Newfoundland.
Crustaceana, 50:257–264.
Sainte-Marie, B. 1993. Reproductive cycle and fecundity of primiparous and multiparous female snow crab, Chionoecetes opilio, in the Northwest Gulf of Saint Lawrence. Canadian Journal of Fisheries and Aquatic Sciences, 50:2147–2156.
Sainte-Marie, B., Raymond, S. and Brethes, J.-C. 1995. Growth and maturation of the benthic stages of male snow crab, Chionoecetes opilio (Brachyura: Majidae). Canadian Journal of Fisheries and Aquatic Sciences, 52:903–924.
Sainte-Marie, B. and Hazel, F. 1992. Moulting and mating of snow crabs, Chionoecetes opilio, in shallow waters of the northwest gulf of st. lawrence. Canadian Journal of Fisheries and Aquatic Sciences, 49:1282–1293.
Watson, J. 1972. Mating behaviour of the spider crab, Chionoecetes opilio. Journal of the Fisheries Research Board of Canada, 29:447–449.
Webb, J. B., Eckert G. L., Shirley, T. C., and Tamone, S. L., 2007. Changes in embryonic development and hatching in Chionecetes opilio (Snow Crab) with variation in incubation temperature. Biological Bulletin, 213:67-75. Jae