Adaptive downsizing in the piscivorous cyprinid fish, Opsariichthys uncirostris, facilitates rapid establishment after introduction to a small-scale habitat in Japan

The three-lips (Opsariichthys uncirostris) is a piscivorous cyprinid fish native to the Biwa Lake, central Japan. A recent unintentional introduction of the species has led to its invasion of a novel, smallscale habitat (the Futatsu River) composed of a few riverine stretches connected by irrigation ditches. Comparative observations of fish samples from both habitats provide evidence of disparities in the fish’s size at maturity between the two habitats, with fish in the Futatsu River reaching maturity at smaller body sizes. Achieving a large body size at maturation requires a larger habitat offering a sufficient supply of food. Principle component analysis of body-surface morphometric characteristics revealed that the two populations show differences in both their body shape and growth rates. In the Futatsu River, the growth rates of upper jaw length, lower jaw length, eye diameter, predorsal length, and tail length were accelerated, while the growth rates of head depth, body depth, and tail depth were decelerated, leading to the acquisition of a distinctly slender body. It is plausible that smallscale habitats with less abundant food supplies favor sexual maturation at smaller body sizes and that slender bodies improve swimming performance, thus making such disproportional downsizing a consequence of adaptation to a novel environment. Whether this change is due to phenotypic plasticity or rapid evolution remains unknown.