ABSTRACT Background: Many fishes have evolved long bodies. Decades of research have uncovered substantial variability in body form within the threespine stickleback (Gasterosteus aculeatus) and among its relatives in the Gasterosteoidei, including the evolution of extremely long bodies. Elongation is likely to be associated with the evolution of the axial skeleton but we need studies linking variation in length and the axial skeleton. Objectives: Examine the relationship between body form and vertebral variation in the Gasterosteoidei (the sticklebacks and their closest relatives). Methods: We examined samples of all genera of the Gasterosteidae, Aulorhynchidae, and Hypoptychidae, collected throughout their ranges. We examined body form variation using geometric morphometric methods and the fineness ratio. We obtained total vertebral number and the proportion of precaudal to caudal vertebrae from X-rays of the same specimens. Results: Mean total vertebral number varied from 26.8 in Gasterosteus wheatlandi to 54.5 in Hypoptychus dybowskii. Body shape was significantly related to total vertebral number across taxa, with longer bodied species having more vertebrae. Hypoptychus dybowskii, which has more vertebrae than predicted from its relative body length, is an outlier. The number of precaudal and caudal vertebrae covary linearly across taxa. Again, Hypoptychus dybowskii was an exception, having an excess of precaudal vertebrae for the number of caudal vertebrae that it possesses. Apeltes quadracus was a second exception, having an excess of caudal vertebrae for the number of precaudal vertebrae that it possesses. In summary, changes in the relative proportions of the body are typically accompanied by corresponding changes in vertebral number. Differences in the number of caudal vertebrae appear to be particularly important among some of the morphologically more similar genera like Gasterosteus, Culaea, and Pungitius. Consistent with the hypothesis of pleomerism, vertebral number was also associated with body size, such that larger species have more vertebrae
