New sea lice discovery offers clues to how the parasite finds its host

Researchers at Norwegian food research institute Nofima have identified a previously undescribed structure at the front of sea lice, Lepeophtheirus salmonis, which they believe could play a role during the earliest stages of infection.

The structure, described as a rostral spike-like structure in a study published in July in the academic journal Parasitologia, is found on copepodites, the free-swimming infective stage of sea lice that search for a host before attaching to the fish.

Nofima researcher Amritha Johny first noticed the small protrusion while using microscopy to study the behaviour of sea lice during interactions with salmon tissue.

"The fact that it was moving made me curious, so I wanted to know what it was," Johny said.

According to the study, the structure was observed in more than 40 copepodites from different batches and locations that had not yet been exposed to fish tissue. Scanning electron microscopy of a further 10 specimens confirmed its position at the front of the parasite.

The researchers found that the spike appeared flexible and could be seen in both protruded and retracted positions. Electron microscope images also revealed a slit or pore-like feature close to its base.

During laboratory observations, copepodites repeatedly probed salmon scale and fin tissue using structures at the front of their bodies before attachment. The newly identified spike was located in the same general region where the frontal filament later emerges after a salmon louse has attached to its host.

However, the study does not establish what the structure does. Johny proposes that it could be involved in sensing or examining the host surface, or in the initial attachment process, but said further investigation is needed.

"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," said Nofima senior researcher Nick Robinson.

Robinson leads CrispResist, an international research project studying the mechanisms behind salmon resistance to sea lice.

"If the salmon louse's ability to detect and infect a host depends on specialized 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," Johny said.

An initial comparison with another sea louse species, Caligus elongatus, did not identify a comparable structure in the 14 copepodites examined. The researchers said this raises the possibility that the feature could be specific to L. salmonis.

The next step will be to investigate the structure in greater anatomical and functional detail to determine whether it does in fact play a role in host recognition or attachment, Nofima said.

As previously reported by WeAreAquaculture, Nofima's CrispResist project published findings last year suggesting that a rapid immune reaction in salmon may be crucial for resisting infestation by sea lice, having previously established that chemical signals released by the fish are critical for sea lice to locate their salmon hosts.