International project CrispResist has been investigating the chemicals released by Atlantic salmon that serve as attractants to sea lice.
The encounter between a salmon louse and its host begins when the louse is in its free-swimming larval stage, known as a copepodite. However, being tiny organisms in a vast ocean, how do they locate salmon as their host?
“Chemical signaling is believed to play a key role in host-parasite communication, and scientists have confirmed this,” said Nicholas Robinson from Nofima.
Robinson is coordinating CrispResist, which brings together a leading team of scientists from Norway, the UK, the USA, Canada, Sweden and Australia. Their goal is to uncover the mechanisms behind cross-species variation in host resistance to sea lice and apply this knowledge to enhance Atlantic salmon resistance in aquaculture.
A crucial objective is to identify and document genes and mechanisms responsible for the difference in sea lice resistance between salmonid species. It is well-established that certain Pacific salmon species are resistant to sea lice, whereas Atlantic salmon are highly susceptible.
The team working with chemical signaling in salmon, gathered for trial at Austevoll in Norway. Aleksei Krasnov to the far right. Credits: David Fields.
Senior fish health scientist Aleksei Krasnov at Nofima is involved in studies of chemical communication between lice and salmon. Working with the global team, Krasnov identified putative semiochemicals or compounds of biological origin that affect the behavior of animals of the same or other species. The type of semiochemicals called kairomones help lice find salmon by scent.
The discovery involved a wide range of chemical analyses and lice behavior tests. Water that was conditioned with Atlantic salmon, Pacific salmon and other fish species was analyzed. Twenty-one candidate semiochemicals were selected for lice behavior tests.
Additionally, mucus from Atlantic salmon families with high and low resistance to lice was studied to determine whether resistance could be linked to the chemical composition of the mucus. Behavioral tests were carried out in Norway and a few in Sweden using different methods.
The research showed that water conditioned only with salmon stimulated copepodite activity, confirming the presence of kairomones. Interestingly, conditioned water also contained compounds that deterred lice, suggesting that Atlantic salmon can repel lice as well. Moreover, the tests suggested that semiochemicals may be produced in various tissues of Atlantic salmon, especially the skin. Salmon from families susceptible to sea lice were found to produce mucus that had a higher stimulatory effect on lice than salmon from families with high resistance to lice.
“Overall, the findings suggest that host-parasite communication is highly complex and likely involves multiple cues,” said Krasnov who believes that developing molecular tests is the most promising approach for continuing semiochemical research in this field.
The research is a part of the project CrispResist, which is financed by the Norwegian Seafood Research Fund – FHF. The project is a collaboration between 12 partners from science and industry. In particular, Rothamsted Research (UK), University of Gothenburg (Sweden), Bigelow Laboratory of Ocean Science (USA) and Nofima (Norway) have contributed to this part of the project.
