Freshwater colonization drives divergent reproductive strategies in shrimps of the genus Palaemon (Decapoda: Palaemonidae)

Freshwater colonization is associated with profound shifts in the reproductive traits of invertebrates, and these phenomena are still rarely investigated in an integrated framework across closely related species. In this study, we examined three species of shrimps of the genus Palaemon, each occupying distinct habitat types, to identify how the transition to freshwater habitats shapes reproductive investment strategies. For each species, we quantified multiple reproductive traits, including fecundity, spermatozoa count, embryo and spermatozoa dimensions, reproductive output, per-offspring investment, and investment in weaponry. Our results revealed parallel patterns between sexes: P. northropi, the marine species, exhibited the highest fecundity and spermatozoa production, whereas P. yuna, the freshwater species, displayed the lowest values for both traits. Embryo and gamete sizes varied coordinately, with P. yuna producing larger embryos and spermatozoa, consistent with the abbreviated larval development typical of freshwater species. Per-offspring investment in P. yuna was up to approximately 16 times higher than in the other species, reflecting the substantial energetic costs associated with this developmental strategy. Overall, we show that freshwater colonization imposes adaptive selective pressures that reshape multiple components of reproduction in an integrated way, affecting not only females but also males. These findings demonstrate that coordinated adjustments in the number, size, and energetic cost of gametes represent key evolutionary responses underpinning reproductive success in freshwater environments.