Recursos Marinos y Pesquerías

Morphometric and gonad maturity in Maja brachydactila [Working paper]

11/02/05 ,

Corgos A & J Freire (2005). Morphometric and gonad maturity in the spider crab Maja brahcydactyla. A comparison of methods for the estimation of the size at maturity in species with
determinate growth. Working paper (v.1, Feb 2005). [pdf]

Ontogenetic changes in the relative
growth of males and females of the spider crab Maja brachydactyla, a species with terminal moult and
determinate growth, were analyzed and related to their reproductive (maturity)
status. Sampling was carried out in the Ría de A Coruña (NW Spain) from
December 1997 to November 1999. A combination of principal component analysis
of morphometric variables, non-hierarchical K-means cluster and discriminant
analysis allowed to differentiate two morphometric groups of males defined as
juvenile and adult phases and develop statistical methods to assign maturity
status to individual crabs. A significant change in allometry of cheliped size
was detected in juvenile males with a breakpoint at 96.2 mm carapace length
(CL), separating the inmature and adolescent phases. Histological analysis of
males showed that more than 60% of morphometrically juveniles and 100% of
morphometrically adults presented spermatophores in the gonad. Size at gonad
maturity in males (estimated as CL₅₀) was 96.2 mm CL. Size at morphometric
maturity was estimated comparing two methodologies: the size at 50%
maturity (CL₅₀) and the median
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 median size of adult cohorts for the same
period was estimated to be 139.9 mm for males and 148.7 mm for females. In
all cases the size at maturity obtained from the median of the cohort was
greater than the one resulting from the CL₅₀method with differences
of over 20 mm, showing the bias that methods based in CL₅₀ could
cause in animals with determinate growth. Due the spatial segregation of adults
and juveniles and the availability of morphological and morphometrical methods
to estimate the maturity status of any crab, we propose a combination of
spatial closures and direct protection of juvenile habitat as management
strategies alternative to minimum landing sizes.