Atlantic salmon release an antimicrobial peptide (cathelicidin-2) from their skin to fight infection, but sea lice have evolved to detect this same peptide as a chemical signal to locate their salmon hosts.
Defense peptide = parasite beaconSalmon cathelicidin-2, meant to fight infection, is exploited by sea lice as a chemical cue to locate hosts — confirmed by neurophysiology, behavior, and transcriptomics
What the researchers found
The antimicrobial peptide cathelicidin-2 (Cath-2) released from Atlantic salmon skin serves a completely unexpected second function: it's the chemical signal that sea lice (Lepeophtheirus salmonis) use to find their hosts. When exposed to Cath-2 at concentrations of 7, 70, and 700 ppb, sea lice copepodids showed triggered chemosensory neural activity, altered swimming behavior, and upregulated chemosensory-related genes.
This reveals a remarkable evolutionary twist — a peptide that evolved to defend against microbial infection is being exploited by a parasite as a homing beacon to locate its host.
Why it matters
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.
The numbers in context
Cath-2 tested at 0, 7, 70, and 700 ppb · Neurophysiology, behavior, and transcriptomics all consistent · Chemosensory neural activity triggered · Swimming behavior altered · Chemosensory genes upregulated
How the study worked
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.
Who was studied
Atlantic salmon (Salmo salar) cathelicidin-2 peptide tested on sea lice (Lepeophtheirus salmonis) copepodids
What this study cannot tell us
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.
How to read the evidence
This is a preliminary animal study, but the convergence of three independent lines of evidence (neurophysiology, behavior, transcriptomics) makes the finding robust. Published in Scientific Reports (Nature portfolio). The ecological application (aquaculture parasite control) remains untested.
When this study was published
Published in 2018, this study identified a novel ecological role for antimicrobial peptides. Sea lice remain a major aquaculture challenge, and this finding continues to inform parasite control research.
The bigger picture
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.
Questions still open
- Could blocking cathelicidin-2 release or creating decoy molecules disrupt sea lice host-finding and reduce aquaculture infections?
- Do other antimicrobial peptides serve as unintended homing signals for parasites or pathogens in other species, including humans?
- Is the parasite's ability to detect Cath-2 an evolutionary arms race, or has it stabilized because the salmon can't stop producing the peptide without losing immune defense?
Common questions
How can a defense peptide help a parasite?
Could this discovery help the salmon farming industry?
Read the original research
The Atlantic salmon (Salmo salar) antimicrobial peptide cathelicidin-2 is a molecular host-associated cue for the salmon louse (Lepeophtheirus salmonis).
Scientific reports, 8(1), 13738
Citation
Núñez-Acuña, Gustavo; Gallardo-Escárate, Cristian; Fields, David M; Shema, Steven; Skiftesvik, Anne Berit; Ormazábal, Ignacio; Browman, Howard I. (2018). The Atlantic salmon (Salmo salar) antimicrobial peptide cathelicidin-2 is a molecular host-associated cue for the salmon louse (Lepeophtheirus salmonis).. Scientific reports, 8(1), 13738. https://doi.org/10.1038/s41598-018-31885-6