Morphometric and gonad maturity in the spider crab 'Maja brahcydactyla'. Acomparison of methods for the estimation of the size at maturity in species with determinate growth

摘要

Ontogenetic changes in the relative growth of males and females of the spider crabMaja brachydactyla, a species with terminal moult and determinate growth, wereanalyzed and related to their reproductive (maturity) status. Sampling was carried out inthe Ría de A Coruña (NW Spain) from December 1997 to November 1999. Acombination of principal component analysis of morphometric variables, nonhierarchicalK-means cluster and discriminant analysis allowed to differentiate twomorphometric groups of males defined as juvenile and adult phases and developstatistical methods to assign maturity status to individual crabs. A significant change inallometry of cheliped size was detected in juvenile males with a breakpoint at 96.2 mmcarapace length (CL), separating the inmature and adolescent phases. Histologicalanalysis of males showed that more than 60% of morphometrically juveniles and 100%of morphometrically adults presented spermatophores in the gonad. Size at gonadmaturity in males (estimated as CL50) was 96.2 mm CL. Size at morphometric maturitywas estimated comparing two methodologies: the size at 50% maturity (CL50) and themedian size of adult cohorts. For the whole sampling period, two adult-year classes,CL50 was estimated to be 136.5 mm for males and 130.3 mm for females, while mediansize of adult cohorts for the same period was estimated to be 139.9 mm for males and148.7 mm for females. In all cases the size at maturity obtained from the median of thecohort was greater than the one resulting from the CL50 method with differences of over20 mm, showing the bias that methods based in CL50 could cause in animals withdeterminate growth. Due the spatial segregation of adults and juveniles and the availability of morphological and morphometrical methods to estimate the maturitystatus of any crab, we propose a combination of spatial closures and direct protection ofjuvenile habitat as management strategies alternative to minimum landing sizes