Temperature is a major environmental factor influencing the morphology of fishes and will gain further importance regarding global warming. However, information on thermal effects on shape remains scare. To this end, this study examined the influence of temperature on the body shape in threespine stickleback (Gasterosteus aculeatus) juveniles. Laboratory bred sticklebacks, stemming from an anadromous population, have been raised at three different temperatures (13, 17 and 21 °C). Effects on

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... and shape were assessed and visualized by using modern geometric morphometric methods. In addition, relative linear measurements were inferred from the landmarks. It has also been tested if temperature affects both sexes in the same way. Significant differences were found between the temperatures. Fish reared at 21 °C were significantly smaller than specimens from other groups. Fish reared at 17 °C were on average the largest and also showed the least variance for shape and size, suggesting a canalized development at this temperature. Major differences between the temperature groups were much smaller heads in fish reared at 13 °C and a reduction in size and length of several bony elements in fish reared at 21 °C. General trends across all temperatures were a reduction in length of the median fins, a decrease in size and length of the operculum and the posterior process, an increase in length of the first dorsal fin, and enlargement of the eyes with increasing temperature. Substantial sexual dimorphism was found, however not for size. Deeper bodies, longer median fins, larger heads, and longer posterior processes of the pectoral girdle characterized male sticklebacks. The patterns of shape differences remained the same throughout all temperature groups and did not bias the findings for thermal effects. All findings are put in context with existing literature and implications for the performance of the fish are discussed.