Morphometric and parasitological evidence for ontogenetic and geographical dietary shifts in intertidal fishes

Studies on the feeding ecology of intertidal fish assemblages have indicated the existence of three trophic groups: herbivores, omnivores and carnivores. This classification has enabled researchers to establish some ecological relationships among their components. However, temporal and spatial variations in the use of food resources have rarely been addressed. In this study, ontogenetic and geographical variations in the diet of two intertidal fish species: Girella laevifrons and Graus nigra were evaluated through an integrative analysis of their diet, relation between intestinal length/body length, and their parasite fauna. Results of this combined analysis suggest ontogenetic and geographical variations in the diet of G. laevifrons. In G. nigra, no evidence of ontogenetic dietary shifts was found, but marked differences in diet were detected among localities. However, parasitological evidence does not reflect the geographical differences in its diet. Omnivory of G. nigra at two localities (viz Caleta Errázuriz and El Tabo) may be indicative of the opportunistic nature of this species related to the greater availability of macroalgae at these localities. The assessment of intestinal length of G. nigra highlights the importance of considering macroalgal species composition, and not only their abundance as a group, when estimating the quality of the diet of a given fish species. In general, the geographical differences in diet, length of the intestine and parasitofauna of G. laevifrons and G. nigra together suggest that a species' diet is a dynamic feature that may be related to variations in the availability of food resources in the environment.