The sex-role reversed pipefish Syngnathus typhle is a member of the Syngnathidae, a family of fishes in which males brood embryos on their body surface. As in most ectotherms, embryonic development is highly temperature-dependent in syngnathids and male brooding periods are extended when water temperatures are reduced. The influence of temperature on reproduction is expected to effectively truncate the breeding season and reduce fecundity in cold waters, potentially enhancing the opportunity for both fecundity and sexual selection. We studied spatial variation in the morphology and reproductive biology of S. typhle in five European populations which vary in latitude and water temperature. Microsatellite analyses indicated that the average number of male mates per population ranged between 1.3 and 3.7. The frequency of multiple mating by males was negatively correlated with the degree of sexual size dimorphism in each population, suggesting that disproportionate increases in female fecundity may be able to compensate for increased male brood pouch capacity. Both sexes were larger and males had an increased brood size where water temperatures during the breeding season were lower. Morphological variation among populations may be mediated by differences in fecundity selection associated with different optimal reproductive strategies in cold and warm water environments.