WARD, A.B.*; BRAINERD, E.L.; Univ. of Massachusetts: Patterns of vertebral column evolution in actinopterygian fishes

Eel-like body forms have evolved multiple times within actinopterygian fishes. The goal of this study is to determine how the axial skeleton of elongated fishes differs from that of closely related, non-elongate species. Elongation of the body axis can occur in several ways: an increase in the number of vertebrae, an increase in the aspect ratio (vertebral length/width) of the vertebrae, or an increase in both. The actinopterygian axial skeleton contains two primary regions: the precaudal (abdominal) region and the caudal region. We hypothesize that: 1) vertebral number and aspect ratio vary independently in the precaudal and caudal regions (some elongate fishes have long bodies and others have long tails); and 2) vertebral number and aspect ratio vary independently of each other. To test these hypotheses, we are examining vertebral column morphology in seven major clades of fishes: Beloniformes, Elopomorpha, Gadiformes, Osteoglossomorpha, Otophysi, Polypteriformes, and Scombroidei. Preliminary results indicate that elongate members of some clades have a larger number of precaudal vertebrae but a similar number of caudal vertebrae when compared with non-elongate members of the same clade. The reverse also occurs, but few lineages show an increase in both regions. A difference in aspect ratio, when it occurs, generally occurs equally in both precaudal and caudal regions. Based on these preliminary results, we hypothesize that natural selection on body axis length acts on three independent characters: number of precaudal vertebrae, number of caudal vertebrae, and aspect ratio of all vertebrae. We further hypothesize that separate developmental modules control the number of somites in each region, whereas one developmental module controls vertebral aspect ratio across the entirety of the axial skeleton. (NSF 0308746)