Jordan's rule is an ecogeographical rule named after the father of American ichthyology David Starr Jordan describes the inverse relationship between water temperature and meristic characteristics. The most common found relationship is for fin ray, vertebrae, or scale numbers to increase with decreasing temperature.
- McDowall, R. M. (March 2008). "Jordan's and other ecogeographical rules, and the vertebral number in fishes". Journal of Biogeography. 35 (3): 501–508. doi:10.1111/j.1365-2699.2007.01823.x.