HUSKEY, S.H.*; TURINGAN, R.G.: Functional and morphological bases of intraspecific variation in the feeding ecomorphology of largemouth bass, Micropterus salmoides

The strength of intraspecific ecomorphological studies is in their ability to focus on adaptive processes within a single species, thus eliminating the inherent confounding effects of phylogeny present in interspecific comparisons. To investigate the effects of environmental differences between two populations of a species that inhabit a broad geographic regime, we examined ontogenetic changes in diet, jaw gape, and jaw lever mechanics between two populations of M. salmoides. We tested the hypothesis that ecomorphology varied among broadly distributed fish populations (i.e. Michigan vs. Florida). Volumetric analysis of gut contents revealed a significant difference in the onset of piscivory between the two populations. Michigan bass were planktivorous through later stages of ontogeny than were their Florida conspecifics. Following a shift to piscivory, Florida bass included a significant amount of alternative prey resources (e.g. grass shrimp) in their diet. In contrast, Michigan bass maintained a diet composed of primarily fish after the onset of piscivory. These differences in diet were coupled with differences in key biomechanical features of the feeding mechanism. For example, Michigan bass had larger gape heights and widths than did Florida bass. However, Florida bass had longer outlevers (i.e. length of the lower jaw) than did Michigan bass. It is apparent that the food habits and the functional morphology of the oral jaws vary among M. salmoides from different environments. The ability of this species, and other broadly distributed species, to alter their ecomorphology in response to environmental differences underlies their ability to occupy broad geographic regimes.