Physical and physiological impacts of ocean warming alter phenotypic selection on sperm morphology

Global warming may threaten fertility, which is a key component of individual fitness and vital for population persistence. For males, fertility relies on the ability of sperm to collide and fuse with eggs; consequently, sperm morphology is predicted to be a prime target of selection owing to its effects on male function.
In aquatic environments, warming will expose gametes of external fertilisers to the physiological effects of higher temperature and the physical effects of lower viscosity. However, the consequences of either effect for fertility, and for selection acting on sperm traits to maintain fertility, are poorly understood.
Here, we test how independent changes in water temperature and viscosity alter male fertility and selection on sperm morphology in an externally-fertilising marine tubeworm. To create five fertilisation environments, we manipulate temperature to reflect current-day conditions (16.5°C), projected near-term warming (21°C), and projected long-term warmi...