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Because sperm is costly and limiting, males are predicted to allocate sperm differentially across matings, according to the level of sperm competition, female reproductive quality, and female novelty. We investigated sperm allocation in the European bitterling (Rhodeus amarus), an externally fertilizing species of fish that spawns and incubates its eggs in the gills of freshwater mussels. We predicted that males would allocate sperm differentially according to the quality and novelty of mussels. Dominant males responded to rivals by increasing both sperm investment and aggression, whereas subordinates responded chiefly through sperm investment. Dominant males invested more sperm in novel mussels in accordance with predictions for a Coolidge effect, the mussel representing a new fertilization opportunity. However, subordinate males were not influenced by mussel novelty but were responsive to order of mating. Males did not allocate sperm according to mussel quality, suggesting that certainty of paternity is a more important variable than offspring survival in shaping bitterling sperm allocation strategies. This study demonstrates strategic ejaculate expenditure in an externally fertilizing species, analogous to that in internal fertilizers, but with sperm investment operating on the level of the site of fertilization rather than the female.