Seasonal progression of embryo size and lipid reserves in sea lice Lepeophtheirus salmonis collected from salmon farms

Abstract

Sea liceLepeophtheirus salmonisare marine copepods that are the primary parasitic threat to Atlantic salmonSalmo salaraquaculture. Prior to infecting a host,L. salmonisembryos and larvae rely entirely on maternally derived lipid reserves, offering a unique lens for investigation of energetic trade-offs and reproductive investment. In the current study, we combined histology and image processing to assessL. salmonisembryo size, number of lipid droplets per egg, and lipid area across monthly collections (2018-2019) fromS. salarfarms in Maine, USA. Results indicate consistent embryo areas from season to season, peak lipid metrics in May, and minima in lipid quantities from October-December. Therefore, gravid females appear to invest the highest lipid levels in their embryos under biologically favorable conditions, when future larvae may thrive in the plankton and infection typically begins to surge on farms. In contrast, maternal lice likely allocate proportionately more energy into metabolizing their own lipid stores for vertical migration and survival through the winter. A detailed understanding of seasonal lipid reserves is fundamental for the improvement of infection models. These indicators at the earliest developmental stage partially encode recruitment of subsequent planktonic larvae, enabling unique forecasting potential to inform pest management on salmon farms.