Do fish have necks: measuring 3D motion of the vertebrae and axial muscle dynamics in suction-feeding fishes.

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I will demonstrate how muscles and bones work together to give humans and animals a flexible neck, by studying the hidden "neck" of fish. A neck allows the head to move three-dimensionally, and independently of the limbs and body. Its importance in humans is starkly illustrated by the functional deficits imposed by disorders of the neck, and its origin was a major transformation that spurred the evolution of land-dwelling vertebrates. Yet we know relatively little of how the bones and muscles of the neck interact to provide these essential functions, because their structure and motions are complex and have been impossible to directly visualize or separate from motions of the head and body. With new imaging techniques it is now possible to measure bone and muscle motion in 3D, and fish offer anintriguing model system for investigating these questions. Although fish lack a true, anatomical neck, studies of their feeding suggest the backbone could function as a neck by bending upwards to lift the head away from the body. If fish do have a hidden "neck", it is powered by the body muscles, which extend from head to tail in a complex architecture of muscle fibres. All muscles have a trade-off between how fast they can shorten and how much force they can produce. The orientation of the body muscle fibres and the way that changes as they shorten could allow the muscle to "shift gears" and shorten at different speeds, depending on the force required. This dynamic gearing can occur in human muscles, and may contribute to age-related changes in muscle performance. Directly measuring dynamic fibre orientation and shortening has been very challenging, but new methods for visualizing muscle fibres and measuring their motion may provide key insights into muscle function. I will measure the 3D bending of the backbone in fish and its role in moving the head three-dimensionally and independently of the shoulder girdle and body. I will also examine the architecture and dynamic gearing of the body muscles in this neck region to establish how fibre re-orientation impacts muscle performance. I will carry out this research at the University of Liverpool, using new, X-ray based visualizations to measure 3D bone and muscle motion in three fish with a range of vertebral shapes and hypothesized "neck bending". This work builds on my experience using and developing such 3D imaging tools, and utilizes the University's world-class X-ray filming facility. X-ray video of each fish's head, shoulder girdle, and backbone will be matched with 3D models of these bones, built from CT scans, to create an accurate 3D animation from which I can measure each bone's motion. From these X-ray videos, I will also measure how the body muscles change length and shape during neck motion. By learning contrast-enhanced micro-CT scanning techniques from Dr. Nathan Jeffrey, I can visualize the 3D arrangement of the body muscle fibres. Working with Dr. Karl Bates and Kris D'Aout, I will use these images to add virtual muscle fibres to the bone animations: creating a model of how fibre orientation and muscle shape change affects muscle shortening. The proposed research will change our perspective on the origin of the neck, and provide insights into muscle dynamics. By linking the anatomy and motion of the backbone in living fish, this study will lay the foundation for understanding how the neck may have evolved. It will also help aquaculturists understand and improve feeding performance in commercial trout. My data on shape changes, fibre rotation and shortening of fish muscles can be applied to understanding how human muscles undergo these same dynamics-and how changes these dynamics during ageing may impact health and performance. Lastly, I will use the engaging 3D models, animations, and X-ray videos from this research in outreach programs at the World Museum in Liverpool and local science clubs to inspire the next generation of scientists and innovators.