Direction, ontogeny, and mechanism of the temperature-size rule operating in a large marine crab, Chionoecetes opilio

The classic temperature-size rule (TSR) states that ectotherms mature smaller in warmer than in colder conditions; the reverse TSR is the opposite response. We combined field observations with laboratory experiments and published information to synthesise the response of snow crab (Chionoecetes opilio), a marine brachyuran with determinate growth, to temperature. Size at onset of physiological maturation/maturity and after terminal moult (TM) were positively related to temperature, thus indicating the reverse TSR. Moult increment varied little with temperature, but crabs were larger at instar in colder than in warmer water due to an initial difference in settlement size that propagated to higher instars, suggesting classic TSR prior to settlement. The pattern of increasing TM size with temperature was caused by crabs moulting more times before TM in warmer than in colder water. Intermoult period (IP) declined exponentially with temperature, and lower instars were more temperature sensitive than higher instars. Temperature effects on IP were strong enough to explain changes in size and instar number at TM under a possible time-invariant maturation schedule. Skip moulting was observed in the smallest crabs reared in the laboratory and resulted in high mortality. The reverse TSR in snow crab seems to be adaptive to coping with ectotherm predation.