Phenotypic plasticity over genetic diversity: ecomorphological patterns revealed in the eurytopic and threatened Italian endemic freshwater fish Sarmarutilus rubilio (Bonaparte, 1837)

The large number of cryptic species and extensive intraspecific phenotypic plasticity among freshwater fishes pose a challenge to the correct assessment of diversity within these taxa, which is crucial for their conservation. Recently, three different genetic lineages have been identified within the Italian endemic South European roach Sarmarutilus rubilio, a threatened species with a broad ecological niche. Using geometric morphometrics, we tested the existence of morphological differences associated with genetic common ancestry (i.e. between lineages) and/or environmental parameters. We observed streamlined body shapes in sites scarcely altered by human intervention and with fast water flow; on the contrary, we observed deeper body shapes in canals and in one reservoir with slow/still water flow. Our results suggest that morphological diversity in the South European roach is mainly a result of phenotypic plasticity in response to different environmental drivers (i.e. lotic vs. lentic hydrodynamic patterns) rather than different genetic backgrounds.