Why does Gila elegans have a bony tail? A study of swimming morphology convergence

Caudal-fin-based swimming is the primary form of locomotion in most fishes. As a result, many species have developed specializations to enhance performance during steady swimming. Specializations that enable high swimming speeds to be maintained for long periods of time include: a streamlined body, high-aspect-ratio (winglike) caudal fin, a shallow caudal peduncle, and high proportions of slow-twitch ("red") axial muscle. We described the locomotor specializations of a fish species native to the Colorado River and compared those specializations to other fish species from this habitat, as well as to a high-performance marine swimmer. The focal species for this study was the bonytail (Gila elegans), which has a distinct morphology when compared with closely related species from the Southwestern United States. Comparative species used in this study were the roundtail chub (Gila robusta), a closely related species from low-flow habitats; the common carp (Cyprinus carpio), an invasive cyprinid also found in low flow habitats; and the chub mackerel (Scomber japonicus), a model high-performance swimmer from the marine environment. The bonytail had a shallow caudal peduncle and a high-aspect-ratio tail that were similar to those of the chub mackerel. The bonytail also had a more streamlined body than the roundtail chub and the common carp, although not as streamlined as the chub mackerel. The chub mackerel had a significantly higher proportion of red muscle than the other three species, which did not differ from one another. Taken together, the streamlined body, narrow caudal peduncle, and high-aspect-ratio tail of the bonytail suggest that this species has responded to the selection pressures of the historically fast-flowing Colorado River, where flooding events and base flows may have required native species to produce and sustain very high swimming speeds to prevent being washed downstream. (C) 2016 Elsevier GmbH. All rights reserved.