Sperm cells exhibit extraordinary phenotypic variation, both among taxa and within individual species, yet our understanding of the adaptive value of sperm trait variation across multiple contexts is incomplete. For species without the opportunity to choose mating partners, such as sessile broadcast spawning invertebrates, fertilisation depends on gamete interactions, which in turn can be strongly influenced by local environmental conditions that alter the concentration of sperm and eggs. However, the way in which such environmental factors impact phenotypic selection on functional gamete traits remains unclear in most systems. Here, we analyse patterns of linear and nonlinear multivariate selection under experimentally altered local sperm densities (densities within the capture zone of eggs) on a range of functionally important sperm traits in the broadcast spawning marine mussel, Mytilus galloprovincialis. Specifically, we assay components of sperm motility and morphology across two fertilisation environments that simulate either sperm limitation (when there are too few sperm to fertilise all available eggs), or sperm saturation (when there are many more sperm than required for fertilisation, and the risk of polyspermy and embryonic failure is heightened). Our findings reveal that the strength, form, and targets of selection on sperm depend on the prevailing fertilisation environment. In particular, our analyses revealed multiple significant axes of nonlinear selection on sperm motility traits under sperm limitation, but only significant negative directional selection on flagellum length under sperm saturation. These findings highlight the importance of local sperm densities in driving the adaptation of sperm phenotypes, particularly those related to sperm motility, in broadcast spawning invertebrates.

ID = male ID; Block = block number; Treatment: H (high density fertilisation treatment) or L (low density fertilisation treatment); Fert: number of normally developed embryos out of 200; motilecount = number of motile sperm tracked; ALH = amplitude of lateral head movement; BCF = beat-cross frequency; LIN = linearity; STR = straightness; VAP = average-path velocity; VCL = curvilinear velocity; VSL = straight-line velocity; PM = percentage of motile sperm in sample; HW AVE = average sperm head width; AC AVE = average length of acrosomal cap; HL AVE = average sperm head length; FL AVE = average sperm flagellum length; TL AVE = total sperm length.

Some individuals are missing values for either sperm motility or morphological measurements and were removed from analayses.