In species with alternative reproductive tactics, there is much empirical support that sneaker males have larger testes size and greater sperm numbers. However, support for higher sperm performance by sneakers is inconsistent. We used the sand goby (Pomatoschistus minutus) to test whether sperm performance differed between breeding-coloured males (small testes, but large mucus-filled sperm-duct glands; builds a nest lined with a sperm-containing mucus, provides care) and sneaker-morph males (no breeding colouration, large testes, rudimentary sperm-duct glands; no nest, no care). We compared motility (proportion motile sperm), velocity and longevity of sperm, between the two morphs. Furthermore, we compared gene expression of testes, and tested if the sperm-duct gland contents affected sperm performance, and if sperm morphometrics differed between male morphs. We found a clear difference in gene expression of testes between the male morphs with 109 transcripts differentially expressed between the morphs. Notably, several mucin genes were upregulated in breeding-coloured males and two ATP-related genes were upregulated in sneaker-morph males. There was some evidence of higher sperm velocity in sneaker-morph males, but no difference in sperm motility. Presence of the sperm-duct gland contents significantly increased sperm velocity, but equally so for the two morphs. The same was true for sperm motility although the difference was not significant. The sand goby has remarkably long-lived sperm, with only small or no decline in motility and velocity over time (5 min vs. 22 hours), but again, this was equally true for both morphs. Sperm length (head, tail, total) did not differ between morphs, and did not correlate with sperm velocity for either morph. Thus, other than a clear difference in testes gene expression, we found only modest differences between the two male morphs, confirming previous findings that high sperm performance as an adaptation to sperm competition is not a primary target of evolution.

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