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 ( 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. The spider crab Maja squinado (Herbst) inhabits the Northeast Atlantic from the British Isles to Guinea and the Mediterranean Sea (de Kergariou, 1984; Le Foll, 1993). It is subject to an intense fishery, especially in the waters along the coast of the English Channel and Galicia, northwestern Spain. As for other species of the Majidae, the postlarval life history of Maja squinado consists of two main phases, growth and reproduction, which are separated by a terminal molt (Vernet-Cornubert, 1958; Hartnoll, 1963, 1978; Conan and Comeau, 1986; Comeau and Conan, 1992; Meyer, 1993; Gonzalez-Gurriaran et al., 1995; Freire et al., in press). The first morphometric study done on this species (Teissier, 1935) distinguished three main developmental phases in males, separated by two critical molts. In males, the most obvious morphological change throughout growth is the development of the chelipeds. This led morphometric studies to be based primarily on changes in relative growth rates, or allometric levels, of different variables measuring the size of the chelipeds. Females undergo changes in morphology and in the allometry of the width of the abdomen. Later, these changes in external morphology were used in growth and maturity studies of other species of the Majidae (Vernet-Cornubert, 1958; Hartnoll, 1978; Conan and Comeau, 1986). In studies carried out on other majids, the different phases of the life history and critical molts were based on morphological, morphometric, physiological (gonad maturation), and behavioral criteria (Fig. 1). In males, the first postlarval phase, known as immature or juvenile, gives rise to a second phase, known as prepubertal, juvenile, or adolescent, after a prepubertal, pubertal, or juvenile molt.