Growth at moult and moulting seasonality of the spider crab, Maja squinado (Herbst) (Decapoda: Majidae) in experimental conditions: implications for juvenile life history

Abstract

Prepubertal growth (moult size increment and duration of the intermoult period) and the seasonality of moulting in the spider crab Maja squinado from the southern coast of Galicia (NW Spain) were analyzed under experimental conditions (laboratory and extensive culture). Two annual moult periods were observed; the first corresponded to the prepubertal moults that take place in spring (reaching their peak at the end of March and in April), and the second to the pubertal (terminal) moult in summer during which time the animals reach morphometric maturity. The peak for the terminal moults within the population took place in June in the experiment under extensive culture conditions and in July and August in the laboratory and in the field, where the maximum annual temperatures were lower and occurred later (17–19 °C in August and September) than in the former experiment (21–22 °C in July). The modal number of annual moults per crab was 2, and ranged between 1 and 3. The number of annual moults decreased with the size of the animal at the beginning of the moulting season, and the animals delayed ecdysis in accordance with body size (there was a significant positive correlation between moulting date and body size within each of the two periods). There were no significant differences between males and females in moulting seasonality. In the laboratory, the duration of the intermoult period prior to the pubertal moult was estimated to be 104 days, and for the prepubertal moult it was 85 days. For specimens between 60 and 130 mm carapace length (CL), the moult size increment was estimated to be an average of 29 mm CL, which constitutes an average of 32% of the pre-ecdysial size. There were no significant differences in the moult increment between sexes or between the laboratory and extensive culture experiments. There were however significant differences between the pubertal and non-puberty moults (27 and 36%, respectively). This was due to the larger sizes of the prepubertal animals, and to the negative correlation between body size and the percentage moult increment (the absolute moult size increment was positively correlated with CL). The samplings carried out in juvenile habitats in shallow areas in the outer part of the Ría de Arousa (Galicia, NW Spain) in summer and winter confirm these experimental results. The body size and intermoult and maturity stages of the animals caught indicate that most of the juveniles (mean CL ≈ 110 mm) carried out the terminal moult in summer and had moult increments similar to those in the experimental conditions (mean body size of postmoult mature animals was aproximately 140 mm). In winter, however, the population was mainly composed of juveniles in intermoult.