• Bergmann’s rule;
  • ecogeography;
  • Galaxiidae;
  • Jordan’s rule;
  • pleomerism;
  • vertebral count;
  • Williston’s rule


Aim  To explore variation in the number of vertebrae in fishes in the context of Jordan’s rule and other ecogeographical rules.

Location  Global.

Methods  The study is based on literature review.

Results  The number of vertebrae varies very widely across the diversity of fishes. Jordan’s rule states that vertebral number increases with latitude, and this is widely attributed to ambient temperatures during ontogeny of individual fishes. However, the number of vertebrae may depend on both the ontogenetic environment and inheritance. Diverse other aspects of fish development and ecology are suggested as influencing vertebral number, including fish size, phyletic position, body shape and swimming mode.

Main conclusions  The number of different factors that influence the number of vertebrae in fishes makes for highly complex patterns of variation, and means that unravelling causes is difficult. The question needs to be addressed at the population/species/species group scale; moreover, the lack of discrimination between environmental and inherited causes of variation adds to the complexity.