University of California, Irvine, Biomechanics Lab, Adam Summers

My lab is compiling data on the constraints and opportunities presented by a cartilaginous skeleton to understand the selective pressures that led the chondrichthyan fishes to abandon bone in favor of cartilage. We have established that for many purposed the cartilaginous skeleton is the equal of a bony one without the burden of the high metabolic rate of bone or the large negative buoyancy of the more heavily mineralized material.

The lack of bone in sharks appears to release a constraint on maximum size, allowing sharks to dominate the very large fish niches in the sea.The mineralized cartilage of elasmobranchs is tesselated, that is, the surface of the unmineralized skeletal elements is covered with a layer or layers of small, abutting tiles called tesserae. These tesserae are often tied together with heavy collagenous fibers, though sometimes the joints between them are smooth and without reinforcement. The mineralized tissue of the skeleton appears to respond to applied loads, as when it gets thinker in areas of high stress, or develops ‘trabeculae’ in animals that crush hard prey or exert large forces on their jaws.

Our studies of the skeleton require an understanding of the performance of whole animals, so we are interested int eh biomechanics of routine life in elasmobranchs. For example, we have found that the intermuscular pressures during swimming increase with increasing swimming speed. This implies a gearing system for sharks and also dictates a loading pattern for the vertebrae that can be experimentally verified. The elasmobranchs found routinely in the bight span a wide range of functional niches and have provided a good deal of the material we use for investigating these aspects of the basic comparative physiology and biomechanics of the cartilaginous fishes.

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