Mating systems describe the number of mates different sexes in a species have in their lifetime. Mating systems are key to sexual selection and the evolution of species, as a mating system sets the rules for future evolution and sexual conflict. Spiders show a wide variety of mating systems and extreme adaptations and behaviours are associated with the evolution of monogynous (males mate once in their life) mating systems, including males breaking their genitalia and sacrificing themselves to be eaten by their mate. Another process that is highly influential on evolution is introgression, the movement of genes from one species to another. This generally occurs through species mating to produce hybrids which then facilitate gene transfer. Despite requiring two different species to interbreed, introgression is often not investigated through the lens of mating behaviours and how these can facilitate or limit introgression. Dolomedes aquaticus and D. minor are fishing spiders endemic to New Zealand. Their mating systems and behaviour are undescribed, but members of this genus in other parts of the world are known for their extreme mating behaviour including obligate monogyny, spontaneous male death and sexual cannibalism. Previous genomic work on D. aquaticus and D. minor revealed a one-way and geographically limited introgression occurring between the two species. That is, genes are being transferred between the two species, but only via D. aquaticus females mating with D. minor males; and only in the extreme south of the two species’ shared range. In this thesis I investigated the mating systems and introgression of D. aquaticus and D. minor, using laboratory experiments, field surveys, and 3D morphometrics. I found D. aquaticus females will generally only mate once, followed by a syndrome of aggression after mating, whilst males will mate multiply. D. minor mating is typified by low probability of mating for males and females, but potential for multiple mating in both sexes, rapid escape behaviour by males following copulation, and genital damage. I also found that the two species are able to meet outside of the Introgression Zone, making it unlikely that habitat plays a key role in the introgression. Instead, my results suggest behaviour plays a significant role in limiting the introgression, with male-choice controlling the geographic element and female-choice the one-way element. Using micro-computed tomography and 3D morphometrics I analysed the potential role of genital shape in this introgression. Whilst the genitalia of the two species were quantifiably different, there was little evidence that genital shape limits the introgression. The results of this thesis show that closely related species can have drastically different mating systems, showing the strong impact sexual selection can have on the behaviour of species. They also highlight the importance of behaviour to promoting and limiting introgression between species. Future work on these species should include investigation of sperm dynamics, the rapid copulation behaviour of D. minor, and testing the effectiveness of genital plugging to reduce sperm competition in D. minor.