Meeting Abstract
Extreme body elongation has occurred repeatedly in the evolutionary history of ray-finned fishes. Transitions from disc-shaped or fusiform to highly elongated (eel-like) forms can involve evolutionary changes in head dimensions as well as vertebral number and shape, but it is unknown whether some anatomical characteristics evolve more readily during body shape transformation. In this study, we examine body elongation in blennioid fishes (including combtooth, labrisomid, and tube blennies) and identify two independent origins of extreme body elongation that have occurred through orthogonal anatomical evolutionary trajectories. Within Blenniidae (combtooth blennies), species in the genus Xiphasia have dramatically elongated the caudal region of the body, with increased number of caudal vertebrae and aspect ratio of vertebral centra, but this lineage has experienced little change in head dimensions or abdominal vertebral morphology. In contrast, within Chaenopsidae (tube-blennies), a clade that includes species from the genera Chaenopsis and Lucayablennius has elongated the head and abdominal vertebral region but not the caudal region. These non-overlapping sets of anatomical modifications reveal that elongation can result from divergent evolutionary trajectories, which may reflect differences in the selective factors that shaped diversification. Elongation of the head and abdominal region in the Chaenopsis / Lucayablennius lineage likely enhanced the capture of large prey by ambush predation, whereas the elongation of the caudal region in Xiphasia presumably provided additional body flexibility for crevice-dwelling.
