Sperm competition is a powerful selective force that has shaped sexual traits throughout animal evolution. Yet, how fertilization mode (i.e. external versus internal fertilization) influences the scope and potential for sperm competition to act on ejaculates remains unclear. Here, I examine how fertilization mode shapes ejaculatory responses to sperm competition in fishes, a diverse group that constitute the majority of vertebrate biological diversity. Fishes are an ideal group for this examination because they exhibit a wide range of reproductive behaviours and an unparalleled number of transitions in fertilization mode compared to any other vertebrate group. Drawing on data from cartilaginous and bony fishes, I first show that rates of multiple paternity are higher in internally than externally fertilizing fishes, contrary to the prevailing expectation. I then summarize how sperm competition acts on sperm number and quality in internally and externally fertilizing fishes, highlighting where theoretical predictions differ between these groups. Differences in how ejaculates respond to sperm competition between fertilization modes are most apparent when considering sperm size and swimming performance. Clarifying how fertilization mode influences evolutionary responses in ejaculates will inform our understanding of ejaculate evolution across the animal tree of life.

This article is part of the theme issue ‘Fifty years of sperm competition’.

精子竞争是一种强大的选择力量，在整个动物进化过程中塑造了性特征。然而，受精模式（即体外受精与体内受精）如何影响精子竞争对射精起作用的范围和潜力仍不清楚。在这里，我研究了鱼类的受精模式如何影响精子竞争的射精反应，鱼类是构成大多数脊椎动物生物多样性的多样化群体。鱼类是这项检查的理想群体，因为与任何其他脊椎动物群体相比，它们表现出广泛的生殖行为和无与伦比的受精模式转变次数。根据软骨鱼和硬骨鱼的数据，我首先表明，体内受精鱼类的多重亲子鉴定率高于体外受精鱼类，这与普遍的预期相反。然后，我总结了精子竞争如何影响体内和体外受精鱼类的精子数量和质量，强调了这些群体之间理论预测的不同之处。当考虑精子大小和游泳性能时，射精对受精模式之间精子竞争的反应差异最为明显。阐明受精模式如何影响射精的进化反应将有助于我们了解整个动物生命树的射精进化。

本文是“精子竞争五十年”主题的一部分。