Evolutionary Allometry and Variation in Body Form in the South American Catfish Genus Corydoras (Callichthyidae)

Abstract

The systematic relationships within the speciose genus Corydoras have been very difficult to elucidate due to similar body forms among species, lack of discrete morphological features, and uncertain homologies of pigmentation patterns. Most taxa have been described on the basis of pigmentation and of proportional differences in body measurements, which often are variable within and among conspecific populations. Multivariate comparisons of body form among species indicate that both meristic and mensural attributes are highly correlated with body size, thus displaying strong evolutionary allometry. Most ratios of measurements, including those often considered diagnostic among taxa, are as strongly dependent on body size as are untransformed measurements; however, some bivariate ratios reflect multivariate patterns of shape variation and, thus, may be useful in descriptive taxonomic studies. Size-independent variation in form seems to be continuous among species, with no apparent morphological gaps with which to distinguish “natural” species groups. Thus, changes in morphology in these fishes are highly conservative and result from subtle, perhaps heterochronic changes in relative growth rates among body structures.