The salmon louse (Lepeophtheirus salmonis) is an important parasite of Atlantic salmon (Salmo salar). It is widely spread in aquaculture facilities and leads to economic losses every year. As it has developed resistances against many common treatments, new control methods must be established. Here we characterize a novel gene family of the salmon louse, consisting of two genes, which has not been described in other species before. We analyzed temporal expression patterns of both genes, the localization of mRNA and protein. An RNAi mediated gene knockdown lead to information about the function of the protein. Overall, these two genes are expressed only in sperm ducts of male sea lice. The mucin-like proteins can additionally be found in the wall of spermatophores, which are responsible for sperm transfer to females. Knockdown showed that both genes are essential for successful fertilization of females. Overall, all results indicate that the two analyzed genes are necessary for reproduction in sea lice as they are essential for the formation of a wall surrounding the spermatophores, which is needed for fertilization. Therefore, we name them Mucin-like spermatophore wall protein 1 & 2 (MLSWP1 & MLSWP2). Analysis of sequence data from other copepod species suggests that MLSWPs are present in many copepod species and may also play a similar role in reproduction in those species. This article is protected by copyright. All rights reserved.