Bamfield Marine Sciences Centre

Dr. Chris Wood at BMSC

Sharks are ancient fish called elasmobranchs, and are physiologically very different from modern teleost fish such as salmon or cod. Most species of sharks are in decline, yet surprisingly little is known about their basic physiology. At BMSC, an international team is studying the impact of feeding on the physiology of the dogfish shark (Squalus acanthias). Research to date has shown that in contrast to teleosts, sharks are extremely efficient at absorbing and retaining nitrogen from the food, even though their occasional meals may amount to as much as 10% of their body weight! This occurs in part because feeding activates a series of enzymes that trap the nitrogen and convert it into urea. This urea is used for osmoregulation, raising the osmotic pressure of the body fluids higher than that of the ambient seawater, so that unlike teleosts, they do not need to drink. Excess salt is excreted via the rectal gland in the colon.

The massive secretion of hydrochloric acid into the stomach during the digestion of a meal reciprocally adds base to the bloodstream, resulting in a marked rise in blood pH, the post-feeding “alkaline tide”. Powerful transport mechanisms on the gills excrete this excess base, preventing a potentially fatal alkalosis. The alkalosis serves as a stimulus to metabolism, and helps activate the rectal gland to get rid of excess salt taken in with the meal. Because their osmoregulatory system uses the nitrogenous “waste” urea, sharks may well be N-limited in nature. Based on the rate of urea-N leakage across the gills, a typical meal will provide only enough N – for 5-days, after which muscle wasting occurs. By improved understanding of their feeding physiology, we hope to contribute to the conservation biology of the sharks.