Morphometry and sexual maturity in the spider crab Maja squinado (Decapoda: Majidae) in Galicia, Spain

Abstract

Ontogenetic changes in the relative growth of males and females of the spider crab Maja squinado were analyzed and related to their reproductive (maturity) status in order to define criteria to assign individuals to each growth phase. The sampling was carried out in two different areas of southern Galician waters, northwestern Spain, in shallow water (<10 m) with mixed rocky-sandy bottoms (juvenile and postpubertal adult areas) and deep water (>20 m) with soft bottoms (adult habitat). A Principal Component Analysis (PCA) of morphometric variables and a nonhierarchic K-means cluster of PCA scores differentiated two morphometric groups defined as juvenile and adult phases. A significant change in allometry of cheliped size was detected in juvenile males with a break point at a 109-mm carapace length (CL). This point may indicate a change in the relative growth of juveniles, separating the immature and adolescent phases. Histological analysis of a subsample of males showed that sperm were present in most adolescent crabs, but not in immature crabs. Bivariate morphometric linear discriminant functions allow for the identification of juvenile and adult males (classified previously by K-means cluster) with over 99% correct classification using CL and the length or height of the right cheliped. Carapace length at the onset of sexual maturity was estimated to be 132.7 mm (50% of adult males), although in a broad range, 112-165 mm, the size of juveniles and adults overlap. The life history of male spider crabs shows 3 phases differentiated by the relative growth rate of chelipeds separated by two 2 critical molts: prepubertal molt (immature-adolescent) determines a slight increase in allometry, and pubertal or terminal molt (adolescent-adult) determines an increase in the relative size and allometry of chelipeds and the onset of functional maturity. Females showed only 2 phases separated by the terminal molt. Growth of chelipeds in females showed no changes in allometry and was similar to juvenile males. Juvenile females presented a smaller relative width and a higher allometry of the abdomen with regard to adult females. Unlike juvenile females with a flat abdomen, adult females with a domed abdomen have well-developed pleopods, gonads, and seminal receptacles. Carapace length at the onset of sexual maturity was estimated to be 130.4 mm (50% of adult females). The range of overlap of the size of juveniles and adults (114-143 mm) was smaller than in males.