New structure discovered on salmon lice

A researcher at Norwegian science institute Nofima has, for the first time, described a previously unknown structure at the very front of the salmon louse’s body. Resembling a small spike, it may play a role both in how the louse senses salmon and in its initial attachment to the fish’s skin, writes Nofima in an article on its website.

The structure, named the rostral spike, is visible at the stage when salmon lice need to find a host. The discovery could offer new opportunities to develop tools for controlling salmon lice.

The louse proboscis was discovered during microscopy of copepodites, the free-living stage where the sea lice actively seek out and infect a host. Amritha Johny, a researcher in fish health at Nofima, had decided to study the behavior of the sea lice. This could give her a better basis for investigating how the lice affect the health of the salmon.

Johny used wild sea lice collected from farmed salmon as well as laboratory strains. The egg strings hatched and developed into copepodites in the laboratory at Nofima in Ås. She screened the sea lice with a microscope at Nofima, before the samples were further analyzed with electron microscopy at the NMBU Imaging Centre. She noticed that sea lice spend a lot of time sensing, probing and penetrating the skin, and that this small structure, which resembles a spike, was very active in that process.

“The fact that it was moving made me curious, so I wanted to know what it was,” says Johny about how she became aware of the unknown structure. She identified the louse proboscis in all the lice she had studied under the microscope.

She asked leading researchers, who also didn't know about it.

What does the louse proboscis do?

Through the microscope, Johny found that the structure is flexible and moves in sync with the rest of the body, and that it can be pushed out or pulled back into the body shell. The images also revealed a pore-like opening near where the spike attaches to the body.

Need to know more about the role of the structure

“We still know too little about how sea lice find their host and attach themselves to it. Amritha Johny has discovered a structure in sea lice that has not been described in previous research,” says Nick Robinson, senior researcher at Nofima.

Robinson leads the large international project CrispResist , which researches the mechanisms behind the salmon’s resistance to sea lice.

“If the salmon louse's ability to detect and infect a host depends on specialised structures like this louse's proboscis, it opens up the possibility of disrupting the process even before the louse has time to attach to the host,” says Johny.

The researcher points out that this structure was not found in another louse species, Caligus elongatus, in an initial study. This may mean that the louse proboscis is unique to the salmon louse, Lepeophtheirus salmonis.

This could make it a more precise target for new measures against sea lice. But Johny emphasises that more research is needed before we can know for sure how the lice proboscis works. Among other things, the researchers need to examine the structure more closely, especially what function it has.

An article by Johny describing the research in detail is published in the open access journal Parasitologia.