Diversity of ligand-gated ion channels in free-living and parasitic copepods (Crustacea)

Ligand-gated ion channels (LGICs) are essential mediators of synaptic transmission and sensory processing in animals, yet their diversity and evolution in copepods remain poorly understood. Copepods are among the most abundant metazoans in marine ecosystems and also occur across a wide range of salinity regimes, including freshwater habitats. They play central roles in food webs, biogeochemical cycling, and host-parasite interactions, with some parasitic taxa having significant impacts on aquaculture. In this study, we conducted a comprehensive analysis of LGICs across 20 copepod species (13 free-living and 7 parasitic) spanning four orders, Calanoida, Cyclopoida, Harpacticoida, and Siphonostomatoida, using transcriptomic and expressed sequence tag (EST) datasets. We identified 6456 putative LGICs belonging to four major families: Cys-loop, ionotropic glutamate receptors (iGluRs), P2X, and ASIC/Deg/ENaC. Phylogenetic and clustering analyses revealed both conserved and lineage-specific patterns, with approximately 20 % of LGICs showing taxonomic restriction to copepods, particularly within the ASIC/Deg/ENaC family. Notably, P2X receptors, previously reported as largely absent in arthropods, were consistently detected across copepod species, suggesting that this family is comparatively well represented in the group. We also identified multiple receptors associated with environmental sensing, including ionotropic receptors (IRs), shared across free-living species, as well as potential pharmacological targets such as GluCl and GABA receptors in parasitic taxa. Our findings highlight the evolutionary complexity of LGICs in copepods and provide a foundation for future research on their roles in environmental adaptation, and chemosensory biology, with potential pharmacological applications in aquaculture.