Secretory/Excretory Protein Profiling of <i>Lepeophtheirus salmonis</i> (Gulen Strain) Copepodites Induced by Dopamine: A Comparative Analysis Between Natural and Artificial Sea Water

ABSTRACT Lepeophtheirus salmonis is a crustacean ectoparasite that poses a significant threat to the salmon aquaculture industry, causing substantial economic losses. The copepodite stage is critical for successful host infestation, as failure to attach to the host results in the parasite's death. Once attached, copepodites must evade the host immune system to establish infestation and grow. Secretory/excretory proteins (SEPs) released by copepodites are hypothesized to play a role in immune evasion and host manipulation. In this study, SEPs were collected from 1000 copepodites ( n = 3) incubated in artificial seawater (ASW) and natural seawater (NSW), both with and without dopamine stimulation. Using LC–MS/MS‐based proteomics, we identified 1815 non‐redundant proteins, representing the largest dataset of L. salmonis SEPs reported to date. A higher number of proteins were identified in dopamine stimulated SEPs from ASW compared to NSW. Key protein classes included proteases (metalloproteases, serine proteases, cysteine proteases, aspartic proteases), protease inhibitors, peroxidases, and oxidoreductases. Gene ontology (GO) enrichment analysis revealed key molecular functions, including oxidoreductase, peptidase and catalytic activity, biological processes related to energy metabolism and cellular components such as proteasomes and ribosomes. These findings highlight the metabolic shifts and immune modulation strategies employed by copepodites during host attachment. Our findings demonstrate the utility of ASW for in vitro SEP profiling, with lower background noise compared to NSW and underscore the importance of dopamine stimulation in enhancing SEP release. This study provides valuable insights into the molecular mechanisms underlying L. salmonis parasitism. However, further characterization of these proteins is necessary to identify their specific roles in host immune evasion, infestation success and might offer strategies to mitigate its impact on aquaculture.