University of Chicago Chicago, Illinois, United States
Abstract Body : Previous analyses of swimming biomechanics have found pelvic fins to have a minimal role in locomotor function, and they therefore have been relatively under-studied. However, pelvic fins play a significant functional role when considering locomotion in benthic environments, with several exceptionally adapted benthic or semi-aquatic fish species recently documented. Here we aim to explore the broader impact of benthic adaptation on pelvic girdles, and to reveal general trends in pelvic adaptation across fish lineages. The central data set presented is a comparative analysis of skeletal and muscular characteristics between benthic-adapted fish and closely related limnetic species, across nine bony fish lineages with independent cases of benthic adaptation, including data from 44 species for skeletal morphology and 23 species for myology, using contrast-enhanced CT scanned specimens.
Results of 3D landmark-based geometric morphometrics applied to the bony girdles of benthic versus limnetic fish species show that distinct clustering is seen in the shape variables in principal component space. Pelvic girdles of benthic fishes share common features such as more robust bone and increased numbers of anterior projections. Analysis of the muscle anatomy highlighted variability in muscle number and structure across species. Important differences were found in the structure of the pelvic musculature, correlated with habitat. For example, the presence or size of the extensor proprius was linked to benthic adaptation, as has previously been suggested. There is a significant difference in the ratio of ventral to dorsal musculature t(7.5) = 7.2, p = 0.0001, with benthic fishes having more ventral than dorsal musculature and the reverse being true for limnetic species. Relative increases in ventral muscle volume indicate the significance of abductive function of the fin in benthic ecologies. Similar functionally relevant adaptations are seen for specific muscles, such as the abductor profundus pelvicus making up a higher proportion of muscle volume for benthic fish in some lineages.
This study reveals diverse arrangements of pelvic musculature and strongly links pelvic morphology to locomotor ecology, contrary to previous assumptions about pelvic fins having limited diversity and functional significance. These findings provide insights into similar adaptations observed during the transition from aquatic to terrestrial environments in early tetrapods and highlights the unexpected importance of pelvic fins in locomotor adaptation.
Funding Sources: GAANN Fellowship in Integrative Neuromechanics, U.S. Department of Education Grant P200A220020. Biological Sciences Division of the University of Chicago
