The extent of parental investment can significantly influence reproductive success of a species. It necessitates the allocation of precious resources to incubate developing embryos, often prompting adaptations for larger body sizes, and meticulous regulation of immune responses to prevent the rejection of these embryos. Syngnathids are the only known vertebrates to have evolved male pregnancy, making them an invaluable resource for comparative research on the reproductive investments and potential trade-offs exhibited, independent of the female reproductive tract.

In this thesis, I explore the reproductive ecology, gill gene expression, and immune sexual dimorphisms in Stigmatopora macropterygia, a pipefish species endemic to New Zealand. Reproductive ecology methods were used to assess the selection gradient for male reproductive success based on body length and width, examine possible sexual size dimorphism, and analyse population sex ratios using field catch data. The results indicate notable variations in female body width in comparison to males, with no significant distinctions in body length between the sexes. Moreover, neither body length nor width appeared to significantly impact male reproductive success. Sampling data highlighted a greater prevalence of females in the population.

Genetic analysis, incorporating a differential gene expression analysis, highlights substantial similarity in gene expression patterns between male and female gill tissues. The minimal differential expression between sexes was also reflected in the immune-related genes of the gills.

This research provides insight into the complex interplay between morphological evolution and gene regulation in this New Zealand endemic species. It underscores the need to include alternative ecological factors, such as breeding timing, mate preferences, and female size, in shaping male reproductive success. While gene expression remains largely consistent in gill tissues, the identification of exclusive differentially expressed genes in males hints at unique genetic strategies.

My thesis not only contributes to our understanding of the evolutionary patterns and reproductive ecology of S. macropterygia but also pave the way for future investigations into the broader evolutionary patterns within syngnathids.