Sea lice, in common with other crustacean groups studied, undergo periodic moults allowing changes in size and morphology between stages. Periodic moulting is necessary to all arthropods during growth phases given the constrictions of a hard exoskeleton. In other crustacean groups, and particularly in decapods, the moult cycle has been subdivided by many authors into a number of stages which rely for their recognition upon morphological criteria assessed by physical attributes (e.g. hardness of the cuticle) and appearance as noted by naked eye, light microscopic or electron microscopic observation (inter alia Drach, 1939; Drach & Tchernigovtzeff, 1967; Skinner 1962, 1985). According to the stages recognised by Skinner (1962), following shedding of the old cuticle during ecdysis (Stage E) comes metecdysis or postmoult (Stages A-C3) during which the new endocuticle (inner procuticle) is completed, anecdysis or intermoult (Stage C4) a resting phase which follows completion of the cuticle and which may last for a considerable period in some species and finally, proecdysis or premoult during which time the old cuticle separates from the epidermis (apolysis) and is resorbed and finally shed as the new cuticle is constructed beneath it (stages D0-D4). Subsequent authors who have tried to apply the proposed criteria to smaller species such as Daphnia magna Straus (Halcrow, 1976) have used electron microscope studies to establish similar stages since larger scale observations are not easily attainable.
Observations of moulting in copepods are rare. Only Dexter (1981) has made any attempt to tie moulting in a copepod (Calanus marshallae Frost) to those recognised by other authors and even in this case, only three stages – premoult, postmoult and intermoult were designated (according to observation of setogenesis using light microscopy). Observation of moulting in parasitic copepods is similarly sparse, with naked eye observations of moulting in mobile stages of Lepeophtheirus pectoralis (Müller) being provided by Anstensrud (1990) and scanning and transmission electron microscope studies of cuticle folding in moulting females by Smith & Whitfield (1988) for Lernaeocera branchialis (L.).
The purpose of the work presented here is to describe the moult cycle in the chalimus phase of L. salmonis in order to provide a baseline for studies seeking to examine changes in cuticle structure / moulting which may arise as a result of exposure to exogenous compounds or from use of vaccines. An important facet of the present study was to tie observations, where possible, to the stages classically recognised in other crustacean groups.