Pleomerism, the Widespread Tendency Among Related Fish Species for Vertebral Number to be Correlated with Maximum Body Length

Abstract

Data for 3137 fish species were analyzed for possible correlations, among related species, between mean vertebral number (V) and maximum recorded body length (L). Of 118 families having counts for over 4 species, 90 show a positive correlation; statistically significant correlations are positive in 45 and negative in only 2 families. The relationship can be expressed by the power function V = CLm, in which C and m are constants characteristic of each family or taxon. Among families with a significant correlation between V and L, the median value of m is 0.12 (representing about 10% vertebral increase for each doubling of length) but m ranges widely, from −0.033 to +0.416. Families with high mean vertebral counts tend to have high m values. The phenomenon, termed pleomerism, occurs within genera as well as within families, and sometimes between races, between populations, or even between the sexes. It exists in widely different shapes of fishes (e.g. sharks, mackerels, sea horses, sand lances, and lumpfish), and in caecilians and in sea snakes. Latitudinal gradients in vertebral number (Jordan’s rule) are often reinforced by pleomerism coupled with latitudinal gradients in maximum length, but gradients in vertebral numbers persist even when effect of maximum length is removed. Although its cause is unknown, pleomerism is taxonomically useful in predicting adult sizes of species known only from young specimens, and in unmasking synonymies of "species" based on young of other named species. Vertebral number considered in conjunction with maximum size is more reliable for making taxonomic judgments at all levels than is vertebral number alone. Moreover, if the cause of pleomerism is functional, it may provide information on locomotory mechanisms and on population biology, since mean vertebral number in each population may be matched to that body size at which selection operates most significantly.