Sperm morphometry and motility in an African CICHLID, Pseudocrenilabrus multicolor victoriae across divergent habitats

Abstract

There is a paucity of studies on the natural variation in sperm morphometry and motility in a single species across dissolved oxygen regimes. This study measured the natural variation in reproductive traits including sperm morphometry and motility of male Pseudocrenilabrus multicolor victoriae across a wide range of habitats in Uganda, Africa. I investigated whether fish displayed differences in testes mass, testes asymmetry, sperm morphometry, and sperm motility. P. multicolor were collected from nine sites characterized by three oxygen regimes: seasonally fluctuating, hypoxic and normoxic. Fish traits were measured and analyzed by site and by oxygen regime. I found that P. multicolor display variation in reproductive traits across habitats, and that males in hypoxic regimes are smaller bodied, have lower testes asymmetry, shorter sperm, and a higher sperm velocity relative to males in normoxic or fluctuating regimes. Males may be able to invest more energy into reproduction in hypoxic sites due to a lack of predators in these sites. Additionally, over the long term, males in hypoxic sites may have locally adapted to chronic conditions, allowing them to invest more energy into testes and sperm to offset the costs of living under hypoxia. In contrast, males from the fluctuating regime were large bodied, had high testes asymmetry, and the longest sperm with the lowest sperm velocity. Males in fluctuating regimes may be experiencing energetic trade-offs between growth and reproduction, due to the less predictable oxygen levels in their habitat. Future studies should assess P. multicolor for trait differences across oxygen regimes in a split-brood laboratory study to control for confounding effects of food availability, predation risk, and mating competition on reproductive traits.