Growth in response to sustained swimming in young montezumae swordtails, Xiphophorus montezumae

This study tested the effect of sustained swimming on growth, ingestion rate, and morphology of juvenile montezumae swordtails (Xiphophorus montezumae). Because montezurnae swordtails inhabit running freshwater systems, it was expected that moderate exercise would increase feeding and growth rates, promoting also the hydrodynamic form of the fish. Experimental groups were subjected to different levels of sustained exercise by being forced to swim against water currents at four different velocities (0, 4.1, 7.8, and 12.9 cm/s). The results showed that growth-rate decrease in fish forced to swim, while the increase in exercise did not modify the ingestion rate. Thus, it is likely that the extra energy expenditure associated with the swimming cost was not compensated by an increase in food ingestion. Sustained exercise increased the fineness ratio towards a more hydrodynamic form. Morphological traits linked with minimum drag, such as caudal peduncle depth and amplitude of the caudal fin were not modified. Our results suggest that water velocity has an important role determining the physiology and, to a lesser extent, the morphology of young montezumae swordtails.