The objective of the present study was to understand how and when the frontal filament (FF) in the salmon louse Lepeophtheirus salmonis is produced by examining the sequence of morphological changes leading to FF production in the copepodid and early chalimus stages. Atlantic salmon (Salmo salar) were heavily infested with newly molted copepodids. Sea lice were sampled prior to infestation and at 1, 2, 3, 4, 5, 6, 7, 8, and 9 days postinfestation. FF morphogenesis from newly molted copepodid to chalimus II, i.e., through 2 molts, was studied using high-resolution light microscopy of serial transverse and sagittal resin sections. Three groups of cells, identified as A, B, and C, are thought to be involved in the production of the secretions (S1 and S2) that form the filament material. The amount and shape of S1 and S2 and their association with B- and C-group cells, respectively, changed with the molt cycle. The following scenario for FF development is proposed: the first secretion to form after the molt for both copepodid and chalimus stages is S1, and it is formed by B-group cells and becomes the basal plate of the external FF. C-group cells produce S2 during mid-intermolt to premolt stage. The S2 becomes the stem of the external FF. In premolt larvae, S1 and S2 were contained within a cuticle-lined invagination that had a form similar to that of the extruded filament. The axial duct present in both copepodid and chalimus originates from the A-group cells and probably carries a secretion used to attach the filament to the host. This study provides strong evidence that L. salmonis produces a new filament with each molt, creating the possibility of using a sea lice control method based on interference with filament production more feasible.

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