An overview is presented on the interactive parameters that influence fish susceptibility to infection. In particular, the importance of genetically determined resistance, stress, immunocompetency, and nutrition are discussed in relation to their influence on susceptibility of salmonids to infection with sea lice. It is suggested that these factors should be taken into account, together with other factors that determine infection intensity, such as the source and number of infective stages of sea lice, when devising programs for the management of wild and aquacultured salmonids.1998 International Council for the Exploration of the Sea

Caligus elongatus, known as a sea louse, is a very important pathogen of sea-farmed Atlantic salmon (Salmo salar). The life cycle of C. elongatus was examined under laboratory conditions. The observations were made at 10 °C, with ambient photoperiod. Small Arctic char (Salvelinus alpinus) kept in full-salinity seawater were used as experimental host fish. They were infected individually by offspring of several C. elongatus females. The hosts were killed at consecutive time periods after infection and developmental stages of the parasite were recovered. The life cycle consists of 8 stages: 2 nauplii, 1 copepodid, 4 chalimi, and adult. A separate preadult stage does not occur. The copepodid is infective and all subsequent stages live on fish. The time at which a particular stage is attained by 50% of individuals in a cohort (T50) was as follows: chalimus I, 7.1 days post infection (dpi); chalimus II, 11.4 dpi; chalimus III, 14.0 dpi; chalimus IV, 17.6 dpi; adult, 24.7 dpi. The generation time was 43.3 days. The females can lay eggs at least twice. The males die after copulation (38–54 days after hatching). Overwintering females show signs of diapause.

Atlantic salmon (Salmo salar), naturally infected with Caligus elongatus in seapen cages in Brandy Cove, St. Andrews, N.B., were examined histologically and ultrastructurally for signs of pathology associated with the attached chalimus larvae. Naturally -infected fish and fish that had never been exposed to sea lice infections were examined serologically for altered white blood cell counts and for any antibody response to the adult parasite. The chalimus larva feeds on the epidermal cells of the scale to which it is attached and denudes the epidermis down to the basement membrane. Ruptured and pycnotic cells line the hole created by the larva, and there is some evidence of hyperplasia around stage IV chalimus larvae. Little other damage or host reaction is evident. There is no seral antibody response to the patent infection as assayed by electroimmuno transfer blot, and white blood cell counts from infected and control groups of fish are statistically the same.

The mature spermatozoa from Bothrimonus sturionis (Pseudophyllidea), Pseudanthobothrium hanseni (Tetraphyllidea), and Monoecocestus americanus (Cyclophyllidea) were examined using transmission electron microscopy. Transverse sections of the sperm of B. sturionis indicate that the number of sperm axonemes varies from one to eight, with approximately one-third of the sperm containing two axonemes. Likewise, the number of peripheral microtubules lying just within the external plasma membrane varies from 12 to 20. The nucleus is electron lucent and fibrous in appearance. The spermatozoa of B. sturionis show great variation in the material examined and the majority of them are believed to be aberrant. The spermatozoon of P. hanseni contains a single axoneme with the nucleus wrapped in a crescent around it in the anterior region of the sperm. The posterior portion of the spermatozoon is characterized by a helical flange which projects from the main body of the sperm. The spermatozoon of M. americanus is elongate and slender, containing a single axoneme with an electron-dense nucleus coiled around it in the anterior one-third of the sperm. Electron-opaque bodies, which may be glycogen, fill the cytoplasm. The spermatozoa of all three species contain neither an acrosome nor mitochondria. The flagella of all the spermatozoa have a 9 + "1" arrangement of microtubules. The importance of the ultrastructure of spermatozoa in the phylogeny and taxonomy of cestodes is discussed.