A new study presented at the Society of Integrative and Comparative Biology’s annual meeting showed that bonnethead sharks can survive on a vegetarian diet.
The research by Samantha Leigh, a doctoral candidate at the University of California-Irvine, examined the stomach contents for the species and found that over 60 percent of the material in their stomachs of younger specimens was plant-based.
Bonnethead sharks are a close genetic relative of hammerhead sharks that can reach lengths of up to five feet and are found from the northeast United States to the Gulf of Mexico and Brazil, and from southern California to Ecuador. It feeds primarily on crustaceans, consisting mostly of blue crabs, as well as shrimp, mollusks, and smaller fish. Bonnethead sharks are the only species found to display a vegetarian feeding behavior.
To test her hypothesis, Leigh and her colleagues fed captive sharks a diet primarily consisting of seagrass for three weeks with the remainder of their diet consisting of squid. Blood tests revealed that the sharks were digesting the plants and extracting nutrients from it thanks to a digestive enzyme called β-glucosidase, which can help break down cellulose material found in seagrass and other plants.
According to the study:
S. tiburo were held in captivity and fed a 90% seagrass diet equaling 5% of their body weight daily for three weeks. By growing the seagrass in a separate tank containing enriched 13CO2, the seagrass tissues became labeled with 13C. Weekly blood draws from the sharks consuming the labeled seagrass show (via stable isotope analysis) that they are assimilating carbon from the labeled seagrass.
“Whether or not the ingestion of seagrass is incidental, these results provide explicit evidence that bonnethead sharks, animals previously thought to be solely carnivorous, can benefit from the digestion of seagrass, which leads one to re-evaluate the ecological role of S. tiburo in its coastal habitats,” Leigh concluded in her findings.