Female reproductive biology and the coevolution of ejaculate characteristics in fish

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Abstract
The influence of female reproductive biology in the evolution of male ejaculate characteristics has received little attention. Fish are an ideal group for investigations of this kind because they exhibit two discrete modes of fertilization (internal versus external) and wide variation in both the size and number of gametes produced. Here, using comparative methods, we examine how sperm length, numbers, and logevity are related to ova numbers, size, and the mode of fertilization across fish species. We find that sperm numbers increase with ova numbers among externally fertilizing species, fitting the predictions of theoretical models. However, sperm numbers are apparently not related to ovum size. The sperm of internally fertilizing species are generally longer than those of externally fertilizing relatives, and sperm length is positively correlated with number of ova (but not ovum size) among external fertilizers. There is a significant positive association between sperm longevity and ovum diameter in externally fertilizing freshwater species. Likely explanations for the observed relations between female reproductive biology and ejaculate characteristics in fish are discussed.