The salmon louse is feeding on blood from its host during the later parts of its life-cycle. Blood-feeding is a common strategy among arthropods. Blood is a rich source of nutrients for the parasite, among them proteins, lipids, iron, and other metal-containing molecules. But iron and heme that are present in the blood in large amounts can also be toxic. In a new study, we have investigated how the settlement site on the fish affects the blood-feeding of the salmon louse [1]. In an infection trial, we studied lice larvae attached to the skin and compared them with lice that we put on the gills of the fish. Gill tissue is rich with blood-vessels, and therefore, the parasite gains access to blood much earlier than it normally would. We sequenced and analyzed the transcriptome of the parasite using high-throughput sequencing that creates millions of short sequences and allows us to study how gene expression across the entire genome changes between lice settling on the skin and those on the gills.

Interestingly, we found that chalimus larvae that settle on the gills and prematurely gain access to blood develop much slower than those on the salmon skin. Also, gene expression is remarkably different between these groups of animals. We detected several hundred genes that were differentially expressed at different developmental stages and between settlement sites. Among those are well-known iron handling proteins like ferritins [2], or the heme scavenger receptor LsHSCARB that we have recently discovered [3].

Gill settlement results in a difference in gene expression and affects vital biological pathways. We conclude that premature blood-feeding likely causes the parasite to develop at a slower pace.

[1] Heggland, Erna Irene, Michael Dondrup, Frank Nilsen, and Christiane Eichner. “Host Gill Attachment Causes Blood-Feeding by the Salmon Louse (Lepeophtheirus Salmonis) Chalimus Larvae and Alters Parasite Development and Transcriptome.” Parasites & Vectors 13, no. 1 (2020): 225. https://doi.org/10.1186/s13071-020-04096-0.
[2] Heggland, Erna Irene, Christiane Tröße, Christiane Eichner, and Frank Nilsen. “Heavy and Light Chain Homologs of Ferritin Are Essential for Blood-Feeding and Egg Production of the Ectoparasitic Copepod Lepeophtheirus Salmonis.” Molecular and Biochemical Parasitology 232 (September 1, 2019): 111197. https://doi.org/10.1016/j.molbiopara.2019.111197.
[3] Heggland, Erna Irene, Christiane Eichner, Svein Isungset Støve, Aurora Martinez, Frank Nilsen, and Michael Dondrup. “A Scavenger Receptor B (CD36)- like Protein Is a Potential Mediator of Intestinal Heme Absorption in the Hematophagous Ectoparasite Lepeophtheirus Salmonis.” Scientific Reports 9 (2019): 1823. https://doi.org/10.1038/s41598-019-40590-x.