Vertebral number covaries with body form and elevation along the western slopes of the Ecuadorian Andes in the Neotropical fish genus Rhoadsia (Teleostei: Characidae)

Vertebral number is adaptively important in fishes and is associated with body shape at broad taxonomic ranks. Less is known about this association within species. Rhoadsia is a deep-bodied characid genus endemic to western Ecuador and northwestern Peru. It includes two species differing in body depth (BD), described from different drainages and elevations. Recently, BD has been found to vary with elevation in parallel between similar to 30 and 1260 m across drainages in western Ecuador. Here, we report that vertebral number also covaries with elevation and with body shape. The latter association was present both across and within sites, suggesting that these traits are phenotypically integrated. The ratio of precaudal to caudal vertebrae also differed significantly among drainages, and variation in vertebral number appeared to be associated primarily with the caudal vertebrae, raising questions about its potential adaptive significance given the functional importance of caudal vertebrae in fishes. Vertebral number was associated with body size at some sites, consistent with geographically localized pleomerism. Disentangling the causative mechanisms at play will probably be complex and represents an important future research direction. This is one of the first known cases of an association between vertebral number and elevation in fishes.