Descriptions and durations of premolt setal stages in female Dungeness crabs, Cancer magister

Abstract

For crustaceans with a well-defined annual molting season, such as adult female Dungeness crabs (Cancer magister Dana), setal molt staging can, in principle, be used to predict molting destiny of individual crabs. Size-specific predictions of molting probability could, in turn, be useful for estimating mortalities due to molting. Female Dungeness crabs were collected January–March 1997 at depths of 10–30 m off the California coast, USA (41°N). Intermolt stage A1–C4 and premolt stages D0, D1′, D1′′, D1′′′ and ≥D2 were described based on features of the branchial epipod. Laboratory experiments suggested that stage D1′ was the earliest stage beyond which eventual molting was inevitable so that molting destiny could be determined. Estimated mean number of days from the beginning of stage D1′ to molting was 85. Estimated individual stage durations, based on laboratory experiments, were 24, 51, −4, 18, and 20 days, respectively, for stages D0, D1′, D1′′, D1′′′, and >D2. The estimated −4 days for stage D1′′ suggests that this stage must be very brief and raises a question of its utility in designation of molt stages. Molt staging of three field samples, 300–600 crabs in each, collected prior to the 1997 annual molting season, indicated an increase in the size and frequency of crabs staged D1′ or later as the time to molt approached, although crabs of 150 mm carapace width and larger showed few signs of molt preparation. Because the duration of the molting season (approximately 120 days) for adult female C. magister in northern California exceeds the estimated maximum duration of reliable prediction of molting destiny (85 days), it does not appear that molt staging can be used to predict molting destiny in this population. However, the procedures that we have used in this paper for application to female C. magister might be used with success for other crustaceans if the duration of stages D1′ to molting exceeds the duration of the molting season, and furthermore may be used for describing temporal molting trends.