Principal Components of Body Shape Variation within an Endemic Radiation of Threespine Stickleback

Abstract

Principal component analysis of superimposition coordinates and partial warp scores (i.e., RWs) are used to explore patterns of body shape variation among 32 samples of lacustrine threespine stickleback Gasterosteus aculeatus, from Cook Inlet, Alaska. Principal components from orthogonal least squares, orthogonal resistant fit, affine least squares and affine resistant fit superimpositions are compared with each other and with the RWs. The pattern of body shape variation described by the first component of all five methods is extremely similar. This pattern describes a body shape with caudally positioned dorsal spines, a poorly developed pelvic girdle and short median fins at one extreme of an axis and a body shape with cranially positioned dorsal spines, a robust pelvis and long median fins at the opposite extreme. This pattern suggests a predator effect, which is supported by differences in first component scores between sticklebacks inhabiting lakes with and without native predatory fish. The second component of the orthogonal analyses and the uniform parameters of the affine analyses describe similar patterns of body form variation. This component describes a body shape with a long snout, shallow body and long caudal peduncle at one extreme of an axis and a body shape with a short snout, deep body, and short, deep caudal peduncle at the opposite extreme. This pattern suggests a foraging effect. Interestingly, a foraging effect is supported only in lakes with native predatory fish. Sticklebacks inhabiting lakes without native predatory fish tend to have body shapes representing open-water foragers regardless of habitat type. '