Ecomorphology: Experimental Functional Anatomy for Ecological Problems

Abstract

It is generally believed that the functional design of an organism relates to its ecology, yet this ecomorphological paradigm has historically suffered from the lack of a rigorous framework for its implementation. I present a methodology for experimentally exploring the ecological consequences of variation in morphology. The central idea is that morphology influences ecology by limiting the ability of the individual to perform key tasks in its daily life. Inthis scheme the effect of morphological variation on behavioral performance is first tested in laboratory experiments. As the behavioral capability of an individual defines the range of ecological resources that it can potentially make use of (the potential niche), the second step in the scheme involves comparing the potential niche of an individual to actual patterns of resource use (the realized niche). This permits a quantitative assessment of the significance of an organism's maximal capabilities in determining actual patterns of resource use. An example is presented from work on the feeding biology of fishes in the family Labridae (wrasses and parrotfishes). Most labrids feed by crushing shelled prey in their powerful pharyngeal jaws. This example explores the dietary consequences of variation in crushing strength amongand within species. Crushing strength was estimated from biomechanical analyses of the crushingapparatus in several species, and these predictions of relative strength were tested in laboratory feeding experiments with hard-shelled prey. Morphology accurately predicted relative crushing ability, and the final section of the study explored the effect of variation in crushing ability on diet. Within each of three species crushing strength appears to underlie a major ontogentic dietary switch from soft-bodied prey to a diet dominated by hard-shelled prey. In each species this switch occurred at about the same crushing strength, around 5 Newtons (N), in spite of the fact that this crushing strength is achieved by the three species at different body sizes. Diet breadth increases during ontogeny in each species, until a crushing strength of 5 N is achieved, when diet breadth begins to decline. The strongest fishes specialized almost entirely on molluscs and sea urchins. Thus, these labrids take advantage of ontogenetic and interspecific differences in crushing strength by including harder and harder prey in their diet, and ultimately specializing on hard prey types. The specialized organization of the labrid pharyngeal jaws can be viewed as a key innovation that has permitted this lineage of fishes to invade the mollusc eating niche, a relatively empty trophic niche within the highly speciose and diverse communities of coral reef fishes.