TIMING AND SEASONALITY OF THE TERMINAL MOLT AND MATING MIGRATION IN THE SPIDER CRAB, MAJA BRACHYDACTYLA: EVIDENCE OF ALTERNATIVE MATING STRATEGIES

Timing and synchronization at individual and population levels of the processes related to the terminal molt, gonad maturation, accumulation of energy reserves and migration in the spider crab Maja brachydactyla are analyzed. Also, the intra and intersexual variability is established. Two hypotheses are tested to explain the temporal and population variability: (1) physiological hypothesis: males and females begin migration when they reach the appropriate physiological stage (the optimum level of energy reserves) and (2) mating opportunities hypothesis: the timing of the migration maximize mating opportunities and mate quality. Our results show that males carry out the terminal molt before females, the former having a peak in July and the latter in August. The onset of gonad maturity in females occurs between two and three months after they have reached morphometric maturity (starting in October), coinciding with the period prior to and during the mating migration to deep waters. In an analysis of the spermathecae of primiparous females, it was found that practically no mating activity occurred in shallow waters, whereas the first copulations took place in the migration corridor. However, probably most of the mating activity occurs in deep mating grounds. Males reached gonad maturity prior to morphometric maturity. No differences were observed in the physiological status (muscle, gonad and hepatopancreas relative mass) between migrating and nonmigrating crabs, which is a clear indication that the physiological hypothesis does not hold true for this species. A higher percentage of postmolt crabs were caught in the migration corridor than in shallow waters. The physiological condition improved over time in males and females in the specimens caught in the shallow area as well as in the migration corridor. Therefore, the crabs that start migrating first did so in poorer physiological condition. Moreover, the early migrators had a significantly lower mean size than the late migrators. Our results suggest that variability within populations and between sexes are related to the different reproductive strategies and not to a physiological limitation. Thus, the poor-quality males (with a reduced competitive ability) would migrate at the beginning of the season to maximize the mating opportunities that would not be feasible if they had to compete with the late migrators, which are larger in size and in better physiological condition. Sperm competition is not clear i