Body size determines the number of scales in cyprinid fishes as inferred from hormonal manipulation of developmental rate

Summary In experiment with hormonal manipulation of developmental rate (alteration of thyroid status) of two cyprinid species, Abramis brama and Rutilus rutilus , we found that number of scales strongly correlates with body size at the moment of first scale appearance. If squamation development starts earlier and at a shorter body length, it results in reduction of scale numbers. If squamation development starts later and at a larger body length, the number of scales is increased. Calculated distances between scales at the moment of their appearance are the same in the different experimental groups despite of acceleration or retardation of developmental rate and a corresponding change in the number of scales (decreasing and increasing). It suggests that size of body at the moment of scale initiation determines the number of scales. Induced heterochrony in initiation of squamation is one of the mechanisms, which is evidently responsible for intraspecies variability in scale numbers and might be considered a relevant reason for differences between close‐related species in case of evolutionary fixed heterochronies of scale initiation. Another plausible path for diversity in scale number is a change in distance between forming scales that seems to be more appropriate for species represented phylogenetically more distant lineages.