Each year the salmon louse ( Lepeophtheirus salmonis Krøyer, 1838) causes multi-million dollar commercial losses to the salmon farming industry world-wide, and strict lice control regimes have been put in place to reduce the release of salmon louse larvae from aquaculture facilities into the environment. For half a century, the Lepeophtheirus life cycle has been regarded as the only copepod life cycle including 8 post-nauplius instars as confirmed in four different species, including L . salmonis . Here we prove that the accepted life cycle of the salmon louse is wrong. By observations of chalimus larvae molting in incubators and by morphometric cluster analysis, we show that there are only two chalimus instars: chalimus 1 (comprising the former chalimus I and II stages which are not separated by a molt) and chalimus 2 (the former chalimus III and IV stages which are not separated by a molt). Consequently the salmon louse life cycle has only six post-nauplius instars, as in other genera of caligid sea lice and copepods in general. These findings are of fundamental importance in experimental studies as well as for interpretation of salmon louse biology and for control and management of this economically important parasite.

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The development rate of the salmon louse Lepeophtheirus salmonis is greatly influenced by seawater temperature. This study describes how the growth rate of L. salmonis changes with temperature and identifies the extreme high and low temperatures at which development to the adult stage is compromised. Atlantic salmon Salmo salar were infected with copepodids and development was monitored in 8 temperature groups spanning 3 to 24°C until the lice were adults. Development was severely compromised at 3 and 24°C, while the lice developed normally in the temperature range from 6 to 21°C. At 6°C, most female lice had become adults at 72 d post infection (432 degree-days). At 21°C, development was significantly faster and most females were adults after 13 d, at only 271 degree-days. After infection, lice grew through 5 stages before reaching the adult stage, all of which, with a few exceptions, appeared to last approximately equally long. Thus, a simple model describing the mean daily growth rate (stages per day) as a function of temperature was made for each sex. The relationship between mean daily growth rate and temperature was best described by a second-order polynomial. The term relative age is introduced and used to describe the pattern of development in terms of percent of total development time to the adult stage. This was applied to calculate the timing of developmental events as a function of temperature. Photoperiod and development under rising or decreasing temperatures had minor effects on development rate.

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