Bonytail, the Arizona Tuna? Convergence in Muscle and Tendon Anatomy in Scombrids and Gila cypha

Authors: Daniel Kimball, Owen Kyle, Michael Minicozzi, Alice Gibb

Year: 2018

Abstract

Bonytail (Gila elegans) is an Arizona native endangered species that have an unusual locomotor morphology with a shallow peduncle and crescent shaped tail. This morphology appears to have evolved as an adaptation to high flow, riverine environments and may reduce drag and increase swimming efficiency. We describe the anatomical features associated with the bones and soft tissues of the caudal peduncle region. We compared the morphology of two closely related native species, Roundtail (Gila robusta) and Humpback Chub (Gila cypha), to that of the Bonytail. We cleared and stained ten individuals from each species to measure the neural and hemal vertebral spine angles (angle the spine creates with the vertebral centra) and normalized spine lengths (spine length/standard length of fish) of the twelve vertebrae proximal to the tail. Bonytail have more acute spine angles in the caudal peduncle region when compared to Humpback and Roundtail. In the soft tissues (muscle and tendon) of the caudal peduncle, we found evidence for two (paired) lateral tendons originating at the anterior axial musculature and inserting along the hypural plates of the tail in Bonytail, which appear to be absent in the other Gila species. The volume of muscle in the peduncle region of Bonytail appears to be reduced when compared to Humpback and Roundtail. Reduced musculature and novel tendons are also seen in scombrid (tunas) fishes, where their long lateral tendons transmit force from the anterior musculature to the tail. For Bonytail, this morphology may be beneficial during historic seasonal flooding events, where a more effective swimming behavior allows them to maintain position in the current with relatively low energy expenditure.