Transcriptomic insights into the low-salinity tolerance of the sea louse Caligus elongatus

Caligus elongatus is a marine copepod ectoparasite on a wide variety of fish species. It has also been observed on fish farms cultivating Atlantic salmon and reports shows that this parasite can be a problem for the industry and for the fish's welfare. Freshwater is used as one of the non-medical treatment methods against the salmon louse (Lepeophtheirus salmonis). However, the efficacy of freshwater treatment against C. elongatus is still unknown. This study aims to fill this gap by examining the salinity tolerance limits of both adult and copepodid life stages of C. elongatus. Our findings reveal that detached adult C. elongatus exhibit low tolerance to reduced salinity, with mortality occurring within hours at salinities below 20 ppt. In contrast, copepodid stages demonstrated a slightly higher tolerance, surviving at salinities as low as 15 ppt for one day. Adult lice attached to a host quickly detached from the fish as soon as the salinity was lower than 20 ppt, suggesting that freshwater delousing might be effective in this species. To further understand the genetic basis of acclimation to reduced salinities, we performed RNA-sequencing to assemble the first transcriptome of this species and identify differentially expressed genes. Several genes regulated upon low-salinity transfer were identified. These include genes involved in proline metabolism, energy metabolism, and the transport of various ions and betaine, an osmolyte. The potential roles of these genes in salinity acclimation are discussed within an evolutionary context, providing valuable insights into the survival mechanisms of C. elongatus under low-salinity conditions.