Author: Frank Nilsen, Professor, Department of Biological Sciences, University of Bergen (Norway)

About the organism

Sea lice is the collective name for a group of ecto-parasitic copepods (Crustacea: Caligidae), mainly found on many different species of marine fish. Salmon louse (Lepeophtheirus salmonis) is the best known of all sea lice species due to its significance for the salmon farming industry in the northern hemisphere. Salmon louse is a natural parasite on salmonids in the marine environment with two subspecies (an Atlantic and a Pacific subspecies) and with two exceptions, all anadromous salmonids are susceptible (but to varying degrees) to this parasite that feeds on skin and blood from the host.  

Sea lice like the salmon louse have a life cycle comprising eight developmental stages (instars) separated by moulting. The life cycle consists of three free-living developmental stages for dispersal and infection and five subsequent parasitic instars including the adult stage. Sea lice have free-living larvae specialised for dispersal. These non-feeding nauplia and copepodids can be transported by the water current over large distances increasing likelihood for encountering a potential host. This feature, combined with long-living reproducing females, has likely evolved to facilitate transmission to a host occurring at low density (which is the case for wild Atlantic salmon in the Norwegian sea). The direct life cycle has facilitated artificial production of lice larvae so that the life cycle can be completed in tanks at wet-lab facilities such as LiceLab (ie the wet-lab at Sea Lice Research Centre (SLRC), hosted by University of Bergen). The pre-adult and adult stages of salmon lice can move freely on their host and this feature opens for off-host manipulation of the parasite, which is not possible for most parasites. This has facilitated the development of RNAi protocols for the salmon louse. Furthermore, this allows the establishment of various bioassays useful for both basic research as well as more applied studies such as assessment and evaluation of novel chemicals for treatment.   

Picture of sea lice
Picture of sea lice (courtesy of Frank Nilsen).

Relevant fields of research

Salmon louse is a large challenge for the salmon farming industry due to the cost related to lice control. Traditionally sea lice have been controlled by anti-sea lice medicine but emerging resistance to most available compounds have made a shift in treatment tools towards mechanical types of de-lousing. This switch has caused significant welfare issues for the farmed fish due to increased handling of the fish. The economic significance of sea lice has been a driver for research which has elevated the level of knowledge significantly during recent decades. However, most aspects of salmon louse biology, including host-parasite interaction, is not well studied like many of the parasites infecting livestock or humans. In the future, basic studies on salmon louse regarding key biological processes like development, reproduction and various physiological processes will be crucial for research and development of new treatment tools. Atlantic salmon are not able to eliminate salmon louse. This means that the lice can control/resist the host immune system in a way that ensures successful lice reproduction. Such immunomodulation is well-known in other parasites and, for some species, the mechanism behind this has been identified. Understanding how this sea lice-controlled host immune response is played out may be crucial for the development of prophylactic measures.  

What are the available tools or techniques to study the marine organism?

The genomes from both the Atlantic and the Pacific sub-species of the salmon louse have been sequenced. A fully annotated genome of the Atlantic salmon louse is available through LiceBase (a database for sea lice genomics established by SLRC) and at Ensembl. In addition, sequence data (ESTs, RNAseq, genomic sequences) are available from other sea lice species and available in public databases like Genbank. 

At LiceBase the genome is linked to functional data such as RNAseq data from various life stages and tissues, an in situ gallery, and a collection of phenotypes for RNAi experiments conducted mainly at SLRC at the University of Bergen. The available resources in LiceBase are a very useful tool/starting point for further studies on the salmon louse. We currently have two well-established protocols for performing gene knockdown by RNAi for L. salmonis. One is for knockdown in the free-living larvae and the other is for knockdown in pre-adult and/or adult lice. In addition, various protocols for in situ hybridisation and immunohistochemistry have been established, allowing accurate localisation of transcripts or proteins.  

Where is the organism available in Europe? 

Sea lice (ie salmon louse and Caligus elongatus) are available at SLRC. At LiceLab, several different strains of L. salmonis and C. elongatus are available, together with experimental facilities and expertise to conduct a wide-range of experiments. For the salmon louse, both fully sensitive strains and strains resistant to one or several of the anti-sea lice medicines are available. Part of the facility is special designed to conduct experiments with sea lice. At Licelab, imaging equipment is available allowing documentation of lice just after removal from the host fish. 

Impact & commercial application

The salmon louse is very common on farmed Atlantic salmon and is found in most farms. Control of salmon lice is a significant cost for the farming industry and in a country like Norway there are limits for how many lice farmed fish can have. This means that efficient treatment tools must be available to reduce lice numbers when necessary. Salmon louse has also become an environmental challenge for the farming industry since lice larvae originating from salmon farms may infect wild salmonids and reduce their fitness. All types of parasites, including the salmon louse, have a large ability to develop tolerance or resistance to various treatments, and particularly medical/chemical treatments where resistance is a widespread problem. Access to lab strains and experimental facilities is very important for the development of new treatment tools to ensure a sustainable development for the salmon farming industry. 

Relevant publications

  • Komisarczuk, AS., Grotmol, S. & Nilsen, F (2017). Ionotropic Receptors signal host recognition in the salmon louse (Lepeophtheirus salmonis, Copepoda). PLoS ONE 12(6): e0178812
  • Overgard, AC, Hamre, LA, Kongshaug, H & Nilsen, F (2017). RNAi-mediated treatment of two vertically transmitted rhabdovirus infecting the salmon louse (Lepeophtheirus salmonis). Scientific Reports 7: 14030
  • Eichner, C, Dondrup, M & Nilsen, F. (2018). RNA sequencing reveals distinct gene expression patterns during the development of parasitic larval stages of the salmon louse (Lepeophtheirus salmonis). Journal of Fish Diseases 41 (6): 1005-1029 
  • Heggland, E, Eichner, C, Stove, SI, Martinez, A, Nilsen, F & Dondrup, M (2019). A scavenger receptor B (CD36)-like protein is a potential mediator of intestinal heme absorption in the hematophagous ectoparasite Lepeophtheirus salmonis. Scientific Reports 9: 4218.
  • Borchel, A, Heggland, E & Nilsen, F (2021). The transcriptomic response of adult salmon lice (Lepeophtheirus salmonis) to reduced salinity. Comparative biochemistry and physiology. Part D, Genomics & proteomics 37: 100778.

Interested in using sea lice for your research? Check out the EMBRC service catalogue to identify available services (at EMBRC Norway).

Sabrina Gaber
Communication Officer

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