Repeated patterns in the body shape of distantly related estuarine and riverine cichlids from Northern Middle America

Body shape is a morphological attribute that frequently changes as organisms adapt to environmental fluctuations and optimize the use of available resources. In fish whose distribution includes estuarine and riverine environments, it is common to observe changes in body shape that are related to maneuverability and speed of movement in response to temporal and spatial variation in water flow. Here, through geometric morphometric and linear morphometric analysis, the intraspecific morphological variation of the cichlids Amphilophus trimaculatus, Astatheros macracanthus, and Mayaheros beani was evaluated to determine if there are repeated patterns of variation in body shape associated with estuarine and riverine environments. The three species showed the same trend of morphological variation; in the estuaries, the specimens were generally deeper and robust, with a long head and short caudal peduncle, while river specimens had shallowed and fusiform bodies with a short head and long caudal peduncle. The magnitude of the changes was not the same in the three species, as M. beani showed greater differentiation, and some morphological measures showed changes in opposite directions between the species. These findings indicate that the environment occupied by the species is an important factor in the differentiation of body shape, probably due to water flow, although other factors may determine the magnitude and direction of change in some morphological traits.