A Salmon's Defense Peptide Is Accidentally a Homing Signal for Its Worst Parasite

Sea lice copepodids were exposed to four concentrations of salmon cathelicidin-2 (0, 7, 70, 700 ppb). Researchers measured three independent outcomes: neural activity (neurophysiology), swimming behavior (behavioral assays), and gene expression profiles (transcriptomics). The convergence of all three lines of evidence confirmed the peptide's role as a host-associated chemical cue.

Sea lice are the most costly parasite in salmon aquaculture, causing billions in losses annually. Discovering that they use a specific antimicrobial peptide to find hosts opens entirely new approaches to parasite control — potentially blocking the chemical signal or developing decoys. More broadly, it reveals that antimicrobial peptides can have ecological functions beyond pathogen defense, fundamentally expanding our understanding of what these peptides do in nature.

Cathelicidins are one of the major families of antimicrobial peptides across vertebrates — in humans, LL-37 is the sole cathelicidin and has been extensively studied. This discovery that a fish cathelicidin serves as a parasite attraction signal challenges the assumption that antimicrobial peptides are purely defensive. It raises the question of whether human cathelicidins might also have unintended ecological or pathological signaling roles that haven't been recognized yet.

The study was conducted under laboratory conditions that may not fully replicate the complex chemical environment of the ocean. The abstract doesn't report whether other salmon-derived molecules were tested as controls. It's unclear whether Cath-2 is the only chemical cue sea lice use or one of several. The concentrations tested may or may not reflect natural levels released from salmon skin.