Meeting Abstract
The diversity of jaw musculoskeletal systems across vertebrates is immense, and evolution of these systems can take place via changes within the skeleton, within the muscle, or in the way that skeletons and muscles connect. We measured 37 anatomical and 5 kinematic variables of the feeding mechanism across five species of sympatric Salish Sea sculpin (Cottoidea) and used a recent sculpin phylogeny (Smith and Busby 2014) to track how the musculoskeletal system evolves. Relating jaw muscle length changes to jaw bone movements (“gearing”) was the focus of our study. We quantified anatomical gearing as the relative distances from the jaw tip and the coronoid process (adductor muscle attachment) to the jaw joint, and kinematic gearing as the amount of gape change for a given amount of jaw muscle length change in feeding sculpins. We found that evolutionary shifts to higher gear ratios were correlated with shifts to shorter muscle fiber lengths for both anatomical (p = 0.0236) and kinematic (p = 0.0295) gearing. The co-evolution of gearing and fiber length results in similar jaw muscle strain magnitudes across species: evolutionary shifts in fiber length do not correlate with changes in strain magnitude (p = 0.6002). Sculpins, therefore, can introduce musculoskeletal variation in their feeding system while avoiding confounding physiological performance limits such as the length-specific ability of muscle to produce high force. Morphological variations such as variable jaw muscle fiber length and gear ratio might be the key to allowing dozens of sculpins to partition distinct ecological niches and coexist sympatrically in the Salish Sea.
