Sharks, which are uniformly accepted to be carnivorous, have guts optimized for digesting a high-protein diet. They consume a wide variety of high-protein/high-lipid diets, which include many teleost fishes that humans depend on as a food and economic resource. Omnivores, on the other hand, also digest plant material, and thus, face the difficulty of digesting foods that are low in protein and lipid, and are sheathed in rigid cell walls. Interestingly, the bonnethead shark (Sphyrna tiburo) is known to consume copious amounts of seagrass (62% of gut content mass in some populations), yet maintains a gut that morphologically reflects its carnivorous ancestry. Hence, in comparison to other sharks, bonnethead sharks may have adjusted their gut function to utilize plant material. The objective of this project is to investigate S. tiburo nutritional physiology to understand how they are capable of digesting seagrass. Bonnethead sharks were held in captivity and fed a 90% seagrass diet equaling 5% of their body weight daily for a total of three weeks.
By growing seagrass for the feeding trials in chambers with an atmosphere enriched in 13CO2, the seagrass tissues become labeled with 13C. Weekly blood drawings from the sharks consuming the labeled seagrass will show (via stable isotope analysis) whether they are assimilating nutritional components of the labeled seagrass.What an animal consumes and what an animal digests and assimilates could be very different. The activities of various digestive enzymes, such as amylase and cellulase (including beta-glucosidase), were measured in order to determine if bonnetheads have the biochemical tools capable in aiding with seagrass digetion. Growth rate, along with digestibility of seagrass macronutrients (carbohydrates, lipids, and proteins) was also measured in captive adult bonnetheads, and data analysis is in progress. Preliminary results show that 56% of the total organic matter in seagrass is digested by S. tiburo. Since seagrass is approximately 60% fibrous material (cellulose), it is clear that S. tiburo are retaining some of the nutrients in seagrass. This project could provide groundbreaking evidence that sharks, animals that were previously thought to be solely carnivorous, can benefit from the digestion of seagrass, which would lead one to re-evaluate the role of S. tiburo in its environment.