A technique for identifying the early-premolt stage in the male snow crab Chionoecetes opilio (Brachyura: Majidae) in Baie des Chaleurs, southern Gulf of St. Lawrence

Abstract

New criteria for determining the early-premolt stage (D0) of the snow crab Chionoecetes opilio are described, based on histological observations. Identifying the early-premolt stage in snow crabs on the basis of observations of setagenesis poses difficulties for inexperienced observers. At the start of the premolt phase, numerous actively secreting cells beneath the epithelium and Leydig cells in the connective tissue can be observed using histological techniques. A semitransparent substance observed between the membranous layer and epithelium by histological means was further subjected to transmission electron microscope observations, which suggested that this was the molting fluid whose presence represents the beginning of the process of deposition of the epicuticle. At the time when the molting fluid is secreted in snow crabs, no morphological characteristics of premolt stage D1 are discernible, therefore these features can be used to identify D0. Using this technique, the molt stages of adolescent male crabs in the southern Gulf of St. Lawrence were examined, revealing the existence of two different types of molters that enter into the molting process in different times of the year. The majority of adolescent males go through the premolt stages in August-December and start to molt in March-April of the following year; another group of adolescent males with more epibionts on the carapace in the spring (adolescents that did not molt in the previous spring) are in the premolt stages in May-July and probably start to molt in December-January. The present study reveals that the molting patterns of snow crabs are far more flexible than the previous literature had suggested. The implications for long-term stock management are quite important, and further study on the structure and dynamics of molters is needed.