Sperm maturation and male tactic-specific differences in ejaculates in a marine fish

The sperm maturation process prepares sperm for the competitive and harsh environment in which fertilization of eggs takes place. However, the role of sperm maturation in sperm competition is rarely considered in species with alternative reproductive tactics (ARTs), where guarder and sneaker male tactics face different levels of sperm competition risk. Using the plainfin midshipman fish (Porichthys notatus), a species with ARTs, we investigated how sperm maturation shapes sperm competitive abilities. We compared sperm performance and morphology before and after final sperm maturation by sampling sperm from the testes and stripped ejaculates, respectively, of guarders and sneakers. In accordance with sperm competition risk theory, ejaculates from sneaker males had three times as much sperm as ejaculates from guarder males, and sneaker males produced faster swimming sperm than guarder males, but this was only the case after final sperm maturation had occurred. Additionally, fully mature sperm found in ejaculates had larger heads and midpieces than sperm found in the testes. These results emphasize the important role played by non-sperm components of an ejaculate in mediating sperm performance and potentially also morphology.