Moulting in captive terminally mature spider crabs, Maja brachydactyla Balss, 1922 (Decapoda: Brachyura: Majidae), exposed to different environmental conditions

Abstract

The Atlantic spider crab Maja brachydactyla Balss, 1922 exhibits determinate growth, with a terminal moult marking the onset of sexual maturity and the cessation of somatic growth. To evaluate larval production, 219 mature crabs were maintained in captivity for two years under different sex-ratio involving male presence or absence and constant and delayed photoperiod, and for one year under different salinity regimes combined with natural or constant photoperiod. Contrary to expectations, 13 mature individuals moulted after the presumed terminal moult in all treatments. Post-terminal moulting produced substantial growth, particularly in males, and may help explain the marked variation in adult body size reported for this species. Evidence of post-terminal moulting in mature crabs, both herein and in previous reports, suggests three hypotheses. First, degeneration of the Y-organs, which produce moulting hormones and reportedly regress markedly after the terminal moult, may be reversible, restoring moulting capacity under conditions unsuitable for reproduction. Second, seasonal photoperiod may be a stronger environmental cue for reproductive success than salinity. Third, crabs may alter their life-history strategy when conditions deviate from those favouring reproduction, redirecting energy towards additional growth while postponing reproduction. As ocean warming is expected to shift species distributions poleward and expose populations to novel photoperiodic and salinity regimes, the capacity to resume growth after maturity may represent an adaptive response with important implications for population dynamics and fisheries management.