Correspondence between ontogenetic shifts in morphology and habitat use in minnow Phoxinus phoxinus

The morphology of minnows Phoxinus phoxinus from two rivers at the south-east of England was analysed on mensural characters and qualitative traits (occurrence of appendages, pattern of pigmentation and scalation). Eight developmental intervals were identified from the qualitative traits, and bivariate and multivariate analysis revealed that allometric growth occurs occasionally during ontogeny, mainly in earlier developmental periods. Body shape is under the influence of rapid increase during development, mainly due to the quick growth in early developmental intervals. The exclusion of 'general size', remarkable in early developmental intervals, revealed several 'shape' characters that mainly determine minnow morphology by changing the mode of variability during development (e.g. caudal characters, maximum body height, belly length and ventral head length). The tail certainly undergoes the most consistent and most durable change, its characters being the most variable and the most discriminative for developmental periods from larvae to adults. The most prominent alteration in the overall shape development occurs at the transition from larva to juvenile, both regarding the number of characters involved into the change and their variability. This transition takes place at about 28 mm SL, which corresponds to a significant change in microhabitat use in both the rivers Lee and Frome, characterized by shifts by 0+ juveniles to significantly deeper waters than those occupied by larvae, with increased amounts of either submerged filamentous algae (Lee) or vegetation (Frome).