Moulting & growth
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  Moulting & growth is divided into a section on Cancer, considered here, and sections on OTHER GENERA and HANDEDNESS presented elsewhere.
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Research study 1

graph showing growth of Dungeness crabs Cancer magister in Boundary Bay, British Columbiaphotograph of Dungeness crab Cancer magister courtesy Iain McGaw, U Nevada
Studies on growth and moulting in Dungeness crabs Cancer magister in Boundary Bay, British Columbia show that individuals of both sexes moult about 12 times before reaching sexual maturity at 4-5yr of age.  Both sexes moult continuously throughout their life, as there is no terminal moult in the species, and moult timing is tied to reproductive cycle.  Relative size-increase with each moult slows down considerably with age.  Thus, at 1cm carapace width an individual will increase in size by about 40%; however, by 10cm carapace width, the increase is only 9 and 15% for males and females, respectively.  The legal size limit for harvesting in British Columbia is 16.5cm carapace width, reached after about 16 moults.  MacKay 1942 Bull Fish Res Bd Can 62: 1. Photo courtesy Iain McGaw, U Nevada.

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Research study 2

graph showing growth of Dungeness crabs Cancer magister in Haida Gwaii, British Columbia
In a later study on growth of Cancer magister in Haida Gwaii, British Columbia the legal size limit for harvesting is reached after 14 moults at an age of 4yr. Both sexes are reproductively mature at 2yr of age.  Butler 1961 J Fish Res Bd Can 18: 873.

NOTE  lit. “islands of the people” Haida; formerly known as the Queen Charlotte Islands

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Research study 3

photograph of limbs of Dungeness crab showing autotomy schedule in a study of limb regenerationhistograms of % increase in size of shore crabs Hemigrapsus oregonensis and Pachygrapsus crassipes in Bodeg Bay, CaliforniaCrabs that lose limbs replace them within a moult or two.  It has long been known that limb loss and replacement influences intermoult duration in decapods (leading to either longer or shorter intermoult periods depending upon species), but the effect on size increase at the next moult is not as well known.  Studies at the Bodega Marine Laboratory, California on shore crabs Hemigrapsus oregonensis and Pachygrapsus crassipes show that limb regeneration reduces expected size at subsequent moult and that the effect of limb loss is additive.  The authors induce crabs to autotomise either 2, 4, or 8 limbs by squeezing the merus portion of each limb. 

Results show, first, that there is no regeneration if moulting occurs within 15d of autotomy. In regenerating crabs, post-moult body size is smaller, and the effect appears to be additive with greater limb loss (see histograms; no statistical tests are applied to the data).  As to why there is a decrease in post-moult size, the authors offer 2 explanations.  The first, and simpler one, is that with the additional fluid required to expand the new limb buds, there is less available for carapace expansion.  However, in an experiment (using 8-limb “autotomees”) in which the 8 new limb buds are removed before they can expand (and use up fluid), size increase is still the same as in specimens in which the 8 limb buds are allowed to expand.  The second and perhaps more convincing explanation offered by the authors is that energy allocation is programmed early in the preceding pre-moult period and is adjusted commensurate with the degree of expected limb regeneration.  Kuris & Mager 1975 J Exp Zool 193: 353.

NOTE  an 8-limb removal leaves the crab with only the pair of 4th walking legs.  Under lab conditions where the treatment crabs are isolated from other crabs, mortality from these treatments is surprisingly low (<5% in both treatment and control groups).  The authors note that loss of 8 limbs would be unusual in nature.  In their collections of H. oregonensis, totalling 10,000, only a single specimen is found with 6 limbs missing, and none is found with 7-8 limbs missing

NOTE  this suggests that the moult cycle is already programmed and is not changed by loss of limbs.  In this case, there is no reduction in post-moult size

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Research study 4

graph showing growth of Dungeness crabs Cancer magister in Grays Harbor, Washingtonphotograph of Cancer magister cast-off moult looking in from the rearA 14-mo study of distribution and growth of Dungeness crabs Cancer magister in Grays Harbor, Washington shows an ontogenetic change in habitat preference.  Eelgrass/mud is a good habitat for the first few instar stages (0+ age class), but later the crabs move to subtidal channels (1+ years of age) or migrate seawards (2+ years).  Growth during the first summer is relatively quick, with a 280-fold increase in dry mass occurring during between May-Oct (see accompanying graph). Subsequent winter growth is slower.  The graph shows growth of 4 age classes, with another one (in early 1981) just commencing.  The truncated record for the 3+ age class owes to an absence of large crabs, thought by the authors to be associated with movement of mature individuals out of the Harbor to spawn.  Stevens & Armstrong 1984 Fish Bull 82: 469.


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Research study 5

graph showing growth of Dungeness crabs Cancer magister in San Juan Island, WashingtonA comprehensive comparative study on post-metamorphic growth in 4 sympatric species of Cancer crabs in San Juan Island, Washington provides detailed information on moulting frequency, size at each moult, and onset of reprodutive maturity.  Note in the graph on the Left the stepwise pattern of growth in Cancer magister caused by periodic moulting. Sexual maturity in C. magister occurs in mid-summer of the second year of life at a size of about 110mm carapace width.  

The chart below Right shows that the total number of instars ranges from 7-13 depending upon species and sex.  Note that 8 out of 11 instars in female C. oregonensis are potentially fertile, as compared with only 3 instars for females of the other species.  Although the species have different general habitat requirements (C. magister lives in sand/mud areas, C. gracilis in muddy areas bordering eelgrass beds, and C. oregonensis
and C. productus in spatially complex rocky substrata), the distributions of 3 species overlap to some extent in the high intertidal regions during nighttime foraging. 

schematics of carapace width vs. moults in males and females of several west-coast crab species

The authors describe 3 different mating “systems” among the 4 species: 1) resource defense in C. oregonensis, 2) female defense in C. gracilis and C. productus), and 3) “explosive” breeding assemblages in C. magister.  Sexual dimorphism in claw sizes tends to support this idea, as male claws are larger than female claws in the first 3 species, but not in C. magister.  In the first mating “system”, resource defense, male C. oregonensis defend small but desirable refuge areas, which presumably attract females.  This species has the greatest sexual dimorphism in claw size among the 4 species, and the large size of the male’s claws is perhaps useful in territorial defense.  In the female defense strategy, females of C. gracilis tend to cluster for extended periods, and the most successful males are those that hover about and make their presence known to the females.  The mechanism of clustering is not known, as males are never seen to herd the females, but likely involves a female-attraction pheromone.  In the 3rd strategy, explosive breeding assemblages, both sexes of C. magister converge for a limited-duration mating period.  The paper is filled with useful comparative information on the 4 Cancer species and has a rich literature-cited section.  The brief summary presented here does not do the paper justice.  Orensanz & Gallucci 1988 J Crust Biol 8: 187.


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Research study 6

map showing location of study sites for collections of Dungeness crabs Cancer magister to assess growth rates
Other studies show, not surprisingly, that growth patterns of crabs vary considerablly with locality.  A review of a number of growth studies on Cancer magister conducted at locations from Alaska to southern California show that individuals of both sexes moult once per year but, after reaching a size of  about 50-60mm carapace width in both sexes, moult (growth) increments become highly variable. After reaching a size of 100mm CW in females and 150mm CW in males, moult increments begin to decrease. Finally, at about 160mm CW in females and 190mm CW in males, moults are virtually absent.  Wainwright & Armstrong 1993 J Crust Biol 13: 36.

NOTE the data points in the graphs correspond with the letter designations for the location of the studies on the map, but identification of these in the graphs is virtually impossible because of their small size and extent of overlap. Careful and almost microscopic scrutiny, however, shows that there is, indeed, a considerable amount of geographic variability in growth rates in west-coast C. magister

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Research study 7

histogram of claw-breakage frequencies in 6 species of Cancer crabsMoulting in crustaceans is required for body growth, but is also a way for the biting surfaces of claws, mandibles, and so on to be rejuvenated, and for damage to the claws incurred by crushing to be repaired.  A survey of field populations of 6 species of Cancer1 crabs in Barkley Sound, British Columbia reveals that about 6% of all crabs have some sort of breakage damage in addition to wear (see graph on Left). The data show little difference between sexes for any of the 3 categories2 of claw-wear shown.  Breakage is correlated with degree of wear, so claws later in the intermoult period are the ones most likely to fail. 

histogram showing safety factors in claws of male and female Cancer crabs of 6 different speciesLaboratory tests3 of crushing strengths and breakage resistance in the claws at Bamfield Marine Sciences Centre, British Columbia reveal high variability in degree of safety4 factor built into the claws.  For example, Cancer oregonensis has an unusually high safety factor (7), C. magister has an unusually low factor (3), and the other 4 species are also on the low side (3-4). High safety factors tend to be associated with species that inhabit rocky substrata and that presumably consume a higher proportion of hard-shelled prey, but are also associated with relative claw size and with the degree of sexual dimorphism in claw size.  With safety factors of even the lowest magnitude recorded, healthy crabs that bite hard-shelled prey with maximal force should not break their claws. However, the field data show that claws do fail in field populations, so breakage must result from loads (perhaps attack by predator or intraspecific battles for resources) or conditions (fatigue, accumulation of microcracks) other than those experienced in the laboratory. The authors note that agonistic interactions between males are unlikely to be the cause of claw breakage because there are no significant differences in safety factors between males and females.  The authors are uncertain which of habitat, diet, or sexual selection are primarily responsible for the differences in safety factors observed among the species. Taylor et al. 2000 Biol J Linn Soc 70: 37.

NOTE1 species assayed include antennarius, branneri, gracilis, magister, oregonensis, and productus

schematic drawing of strain gauge used to assess crab-claw crushing forcesNOTE2 degree of tooth wear is rated as none (no visible wear), intermediate wear (> half the volume of teeth present), and extreme wear (< half the volume of teeth present)

NOTE3 crushing forces are measured with the tips of the chela inserted into steel rings attached to a strain gauge, with the start position of the claw at 60% gape (see diagram on Right).  Breaking forces are obtained by suspending an empty container to the lower ring of the device with the claw clamped firmly, and adding measured amounts of sand to the container until the claw breaks

NOTE4 this is the load over and above that normally experienced by the crab that causes fracture - calculated as break force/crush force.  The authors stress that safety factors are not static.  They vary among individuals, and change with body size, time, and environment

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