Variation in Vertebral Number in Galaxiid Fishes, How Fishes Swim and a Possible Reason for Pleomerism

Abstract

The number of vertebrae in fishes is widely variable, with this variation having connections to phyletic position, geography, various environmental factors such as temperature and salinity, biome occupied and life history pattern. Variation is sometimes a response to environment, sometimes explicitly adaptive and probably often both. Swimming mode is likely to be influenced by body flexibility, which in turn is influenced by vertebral counts. Since vertebral number is fixed early in ontogeny, there is a predictive element in the ‘choice’ of vertebral number during development that affects later adaptiveness. Pleomerism, the relationship between vertebral number and body size across the diversity of fishes, may be driven by the square/cube relationships between length, cross-sectional area and volume. Pleomerism in diadromous galaxiid fishes probably reflects adaptive advantages achieved during marine juvenile life and in non-diadromous species may reflect the size at which mid-water, shoaling juveniles become to benthic, cryptic, within-substrate behaviours.