Lepeophtheirus salmonis, the salmon louse, occurs in cold temperate waters of the northern hemisphere. L. salmonis is an ectoparasite which occur on all salmonid species; salmon, trout and char. They may be found on other fish species as a stop-gap measure while seeking out their salmonid host.
L. salmonis has a direct life-cycle (i.e. a single host) with eight life stages (Hamre et al. 2013) . The adult female sea louse extrudes a pair of egg-strings and the planktonic nauplii stages hatch directly into the water column. The duration of the egg stage varies from 17.5 days at 5°C to 5.5 days at 15°C.
These planktonic nauplii cannot swim directionally against the water current but drift passively and have the ability to adjust their vertical depth in the water column. They are almost translucent in colour and are about 0.5-0.6mm in length. At 5°C the nauplius 1 stage lasts about 52 hours and about 9 hours at 15°C.
Nuaplius 2 takes 170 hours and 36 hours at these temperatures, respectively. They are responsive to light and salinity. Low salinities appear to have a greater effect on the planktonic stages than on the parasitic stages. Newly hatched larvae do not survive below salinities of 15‰ and poor development to the infective copepodid occurs between 20‰ and 25‰. Nauplii and copepodids are positively phototactic and exhibit a daily vertical migration, rising during the day and sinking at night. The ability to find their host is not light dependent. They have been shown to be responsive to low frequency water accelerations, such as those produced by a swimming fish. Finding their migratory host in the vastness of the ocean is still a mystery for scientists to solve but the species has managed to do this effectively for millennia.
Life cycle of L. salmonis
These free swimming nauplii then moult into the infective copepodid stage and this can take from 2 to 14 days depending on water temperature. The planktonic stages must live off their fat reserves, they cannot feed until they find a host and moult to the parasitic chalimus stage.
Initial attachment for the copepodid typically occurs on the fins of the fish or the scales. The copepodid clasps the host tissue, then undergoes a moult to the first sessile stage in the life cycle. The settlement and survival of copepodids at 10 days post infection is significantly greater at 12°C than at 7°C. The copepodid measures about 0.7-0.8mm. This next stage is called the chalimus, which attaches itself by means of a frontal filament (penetrative thread) which punctures the epidermis of the host.
The chalimus stages moults through two stages which are attached to the fish before becoming a pre-adult or mobile stage and then are able to move around on the surface of the fish and can also swim in the water column.
Duration times are approximately 10 days for copepodid, ~10 days chalimus I, ~15 for chalimus 2, ~10 days for pre-adult 1 female and ~12 days for pre-adult 2 female at 10°C. Males develop faster, spending ~8 days as pre-adult 1 and ~9 days as pre-adult 2 at 10°C. Chalimus stages measure in length from c 1.1mm at stage 1 to c 2.3mm at stage 2.
Adult Lep, pre-adult 1 and pre-adult 2
Two pre-adult stages are followed by the fully mature adult phase. In the pre-adult stages the genital complex is under-developed and the mean length is about 3.6mm. Final moults to adult stages, both male and female, then take place. The female is larger than the male with males measuring 5-6mm and females 8-18mm. Female adult L. salmonis can produce ten to eleven pairs of egg strings over their life cycle. Mean egg numbers per string (fecundity) have been recorded as 152 (+16) with a range from 123 to 183 at 7.2°C by Heuch et al. 2000.
The development to sexual maturity following attachment to the host fish depends on water temperature and the generation time, from egg to mature adult, and ranges from 32 days at 15°C to 106 days at 7.5°C. Egg strings tend to be longer with higher fecundity at lower temperatures but factors affecting egg production are poorly understood.
The sea louse generation time is around 8-9 weeks at 6°C, 6 weeks at 9°C and 4 weeks at 18°C. The lifespan of the adult under natural conditions has not been determined but under laboratory conditions, females have lived for up to 210 days.
Hamre LA, Eichner C, Caipang CMA, Dalvin ST, Bron JE, et al. (2013) The Salmon Louse Lepeophtheirus salmonis (Copepoda: Caligidae) Life Cycle Has Only Two Chalimus Stages. PLoS ONE 8(9): e73539. doi:10.1371/journal.pone.0073539
Heuch P.A., Nordhagen J.R., Schram T.A. (2000) Egg production in the salmon louse [Lepeophtheirus salmonis (Krøyer)] in relation to origin and water temperature. Aquaculture Research 2000;31:805-814. doi: 10.1046/j.1365-2109.2000.00512.x
Schram TA (1993) Supplementary descriptions of the developmental stages of Lepeophtheirus salmonis (Krøyer, 1837) (Copepoda: Caligidae). In: GA BoxshallD. Defaye. Pathogens of wild and farmed fish: sea lice. New York: Ellis Horwood. pp. 30-47