Cartilage

I am interested in the relationship between the biochemical constituents of cartilage and the material properties. There is a great deal of variation in cartilage composition, and this variation is underappreciated because of the research bias towards mammalian models. We are looking at the components that make up elasmobranch cartilage such as collagen, proteoglycan, and mineral. We hope this research will lead us to a better understanding of the relationship between biomolocules and material properties.

Hard Prey

In spite of having a skeleton composed entirely of cartilage the chondrichthian fishes evolved durophagy (eating hard prey) four times. These four lineages include the outgroup to all elasmobranchs, the holocephalans, the horn sharks (Heterodontids), one species of hammerhead shark (Sphyrna tiburo), and most of the members of the myliobatid stingray clade. Adaptations for eating hard prey include molariform, or even pavement-like, dentition, large jaw adductors and specialized jaw cartilage.

Big Fish

The largest fishes in the sea have cartilaginous skeletons. There are a number of hypotheses as to why this might be true, though none explain the diversity and number of very large cartilaginous fishes. Patricia Hernandez and I have propsed a functional limitation on size that may be a general explanation for the skewed size distribution. As a fish grows, its weight and the negative buoyancy of its skeleton grows as the third power of length. At the same time the thrust and lift generated by the fins grow with the square of length. Eventually, there will be insufficient lift to counter the sinking force of the skeleton at a certain length. 

Fast Fish

Both marlins (Istiophorids) and mako sharks (Lamnids), two very large, pelagic fishes, can burst swim at very high speed. Swimming quickly requires a stiff body to minimize the wasteful pushing of water to the sides. We have looked into the differences between marlins and makos to see how these skeletons affect the swimming speed and mechanics of fish.

Breathing

Lara Ferry-Graham and I have been investigating the mechanics of respiration in cartilaginous fishes. Aquatic vertebrates are faced with one of two options for respiration: physically moving their gills through the water ("ram"), or actively pumping water over their gills via "buccal pumping". The current model of the mechanics of respiration was described over 40 years ago, and has been incorporated into textbooks and physiological literature alike.

We have used a variety of techniques to approach this question of respiratory mechanics including sonomicrometry, pressure transduction, and direct visualization of the flow patters with an endoscope. Our results suggest that respiration is not as efficient as once presumed. The dominant paradigm does not account for the large amount of back-flow that we see in skates, and two species of shark. We have proposed a new, far less elegant, model of respiration that appears to better predict the flow patterns we observe.

Burrowing

Jim O'Reilly, Dale Ritter, Nate Kley and I are interested in the ways in which limbless amphibians and reptiles burrow. There are only a few ways in which limbless animals move around. The principal modes of locomotion are: 

• swimming undulation: a rhythmic thrashing back and forth, employed by most fishes

• lateral undulation: the animal pushes off against tiny (even microscopic) surface irregularities, used by many snakes, lizards, and caecillians

• concertina locomotion: also known as "inch-worm" locomotion, where part of the body is held still and the rest pushes forwards, used by amphibians and reptiles

Jim and I have explored the ways caecillians use multiple modes of locomotion to burrow. 

Egg Capsules

Tom Koob and I are intersted in the amazing biomechanical, biochemical, and evolutionary aspects of the chondrichthian egg capsule. Egg-laying, or oviparity has evolved at least 4 times in the cartilaginous fishes, and an embryo may spend as long as three years in the capsule before hatching. 

The capsular material is relatively impermeable to oxygen, so the embryo requires a flow of oxygenated water through the capsule as it develops. Small slits open up in the sides of the capsule to allow flow. In the case of skates and chimaeroids the embryo actively pumps water through the capsule by undulating its tail. In sharks the capsule water is passively pumped through the capsule by currents around it. We are investigating the tradeoffs between active and passive flow, the metabolic costs of pumping, the evolution of the capsule shape, and the hydrodynamics of the egg capsule.

SHark Fishing

To study the relationships between the material properties of cartilage and the biochemistry we have to obtain fresh material from a variety of species of cartilaginous fishes. This has led us to visit artisanal fishing camps, and has sparked an interest in the effects that this type of fishing has on shark population biology, local abundances, and the socio-economic impact of the fishery.