Ecomorphology of Centrarchid Fishes

From the ecologist’s perspective, centrarchid fishes are widely recognized as a model system for investigating the role of phenotypic variation in shaping ecological patterns. To the ichthyologist, this group is considered among the most morphologically and ecologically diverse of North America’s freshwater ichthyofauna. This chapter is intended to bring these perspectives together, highlighting the contributions of studies linking resource use patterns to morphology in order to make sense of the ecological, functional, and morphological diversity exhibited within the Centrarchidae. We review literature on feeding and on locomotion. Historically, the diversity represented within this radiation helped inspire the development of ecomorphology, a research perspective that investigates hypothesized associations between organismal design and ecology. Working independently, Werner (1974, 1977) and Keast (1978, 1985; Keast and Webb 1967) were among the first to point out a general association between head and body form and resource use in centrarchid species. Using bluegill sunfish (Lepomis macrochirus), green sunfish (Lepomis cyanellus), and largemouth bass (Micropterus salmoides) to represent the range of ecological and morphological diversity in centrarchids, Werner and coworkers developed the first mechanistic insights into the implications of variation in body and head morphology. The diversity of form and feeding habits represented by bluegill, largemouth bass, black crappie (Pomoxis nigromaculatus), and rock bass (Ambloplites rupestris) motivated Keast’s proposal that different suites of morphological features confer varying prey capture and habitat use capabilities on these species and that these differences underlie the capacity for these species to coexist in sympatry. The rationale for ecomorphology research can be seen in both Werner’s and Keast’s work: an organism’s morphology affects its capacity to perform an ecologically relevant task, and this performance capacity affects the resources available for its use. This research program was made more explicit (Werner 1977; Mittelbach 1984; Wainwright 1996) by emphasizing that researchers’ ability to explain ecological phenomena through organismal design requires focus on characters whose performance consequences are predictable. This stipulation established a primary role for functional morphology research, which investigates the morphological basis of performance variation. Moreover, the ecomorphology research perspective led to widespread recognition that the choice of an appropriate performance measure is vital to the success of studies that seek to understand the relationship between morphology and resource use. Performance variables range from proximate measures that focus on the mechanical capacities of isolated functional units, such as maximum pharyngeal jaw bite force, to more integrative measures that involve multiple functional units, like prey handling time, which is influenced by the fish’s ability to capture and process prey. In either case, the performance measure should have predictable consequences for resource use. This is not a trivial issue, as the link between any given performance measure and resource use is more frequently assumed than demonstrated. Nevertheless, studies involving centrarchid fishes provide some of the best examples of the ecomorphology research program carried out to completion. The morphological diversity of centrarchid fishes ranges between the forms exhibited by the predominant ecomorphs: piscivore/crayfish predator, zooplanktivore, molluscivore, and insectivore, which possess combinations of head and body characters that are associated with different patterns of resource use. Although these ecomorphs are named according to trophic habits, they are generally associated with habitat use patterns as well. Here, we highlight a set of morphological characters that have well-known consequences for performance and resource use. We focus on mouth gape, degree