Interspecific interactions between parasites and hosts can influence the evolution of behavioural and morphological adaptations of both parasites and their hosts. There is, however, little empirical evidence available regarding the evolution of reproductive traits driven by these interactions. In this paper, we investigated host selection and ovipositor length in nine sympatric marine sculpins that oviposit into tunicates or sponges. Field and genetic studies have revealed host use for eight out of nine species of sculpins investigated here: five species of Pseudoblennius, two species of Furcina and one species of Vellitor. For one species studied (V. minutus), no egg masses could be found. Ovipositor length reflects morphology of host species utilised: six sculpin species had extremely long ovipositors allowing females to attach eggs to the deep atrium of solitary tunicates, whereas the two species that attached their eggs to the small space of atrial siphon of colonial tunicates and the spongocoel of sponges had short ovipositors. Ovipositor length varied between solitary-tunicate spawners and species with longer ovipositors selected larger tunicates. Since the ancestral form is non-parasitic, the ovipositor evolved as an adaptation to utilise sponges and tunicates as hosts. Sculpins found sympatrically may show host specificity to avoid interspecific competition for spawning niches and ovipositors may have evolved depending on the species and size of host invertebrates.