A Shark with an Omnivorous Diet? New Study Reveals First Known Omnivorous Shark Species

A new study published in the Proceedings of the Royal Society B: Biological Sciences has revealed the first known shark species to be omnivorous. Bonnethead shark (Sphyrna tiburo) is the smallest member of the hammerheads, weighing only 13 lbs (5.8 kg) and growing to approximately 5 feet (1.5 m) when adults (Skomal, 2016). And these little sharks are making history!

A Bonnethead or Shovelhead Shark, Sphyrna tiburo, swims over sea grass in Florida Bay in Everglades National Park, Monroe County, Florida, United States.
O’Neil, M.P. (Photographer). (n.d.). Boonethead shark over seagrass [Digital Image]. Retrieved from https://tepbac.com/

After reports came in that bonnethead sharks were observed eating seagrasses (Thalassia testudinum), a team of scientists at the University of California in Irvine and Florida International University in Miami decided to investigate the feeding habitats and digestion composition. Bonnethead sharks are found commonly throughout temperate and tropical shallow waters of the Gulf of Mexico, Eastern Pacific, the Western Atlantic. It was previously thought that the majority of their diet consisted of small crustaceans like crabs, shrimps, and snails, along with small bony fishes (Tricase, et al., 1997; Skomal, 2016). The reported consumption of the flowering sea grasses were accidental and taken as part of foraging on these bottom dwelling species.

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Flannery, A. (Photographer). (2016 May 14). Sea grass bed in the Bahamas [Digital Image].

The research team wondered how much of the bonnethead’s diet consisted of plant material, and whether or not the material was being digested, with its nutrients retained. Lead author and team researcher Samantha Leigh told The Guardian, “I wanted to see how much of this seagrass diet the sharks could digest, because what an animal consumes is not necessarily the same as what it digests and retains nutrients from” (Sample, 2018). To do this, the team transplanted sea grasses from Florida Bay to their lab, where they introduced the plants the Carbon 13 isotope. As the sea grasses grew and soaked up the isotope, it gave the plants a distinctive chemical signature, which the team could detect throughout the sharks. Next, the team fed 5 bonnethead sharks a 3 week diet consisting of 90% seagrass and 10% squid.

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Grimster, S. (Photographer). (n.d.). Idiosepius notoides under blade of seagrass [Digital Image]. Retrieved from http://portphillipmarinelife.net.au/

Because boinnetheads have developed teeth made for gripping and crushing crustaceans, for the sharks to breakdown the plant material they must rely on their stomach acids to breakdown the complex plant cells into usable enzymes. By running a series of tests on the sharks, including an examination of their fecal matter as well as blood and liver tissue samples, the team discovered that all the bonnethead sharks were  breaking down the plant material into enzymes. They were also retaining high levels of them! In fact, more than half of the organic material in the seagrass was being digested by the sharks. These are the similar figures as the green sea turtles! This demonstrates the sharks are not only able to successfully digest the seagrass, but are also retaining the nutrients to support themselves (Leigh, Papastamatiou, & German, 2018).

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Flannery, A. (2018 May 10). Sea Turtle Munching on Sea Grass [Digital Image].

So what does this discovery mean? The ecological role of the bonnethead shark within seagrass communities must be reevaluated. It is possible that these sharks do for their communities than previously thought. This study also challenges our perception of other shark species and their roles in the environment. It is entirely possible that the bonnethead shark is not the only species to be omnivorous. I absolutely love that our perception of sharks is constantly changing! The more we learn, the more we realize we still have such a long way to go to truly understand them. They never cease to amaze me.

The new Ocean For Sharks Shop is open! There’s handmade ocean inspired plush animals, canvas paintings, and of course my children’s book, Winifred the Wondrous Whale Shark, available in print and PDF. Be sure to stop by. Remember proceeds benefit shark research and conservation with a donation to Project AWARE!

Remember you can make a difference for marine ecosystems by contacting your Congress man or woman and telling them that you care about the quality of our waters! Sharks, rays, and other marine organisms cannot speak or be represented in Congress. They need your voice. Get involved and stand up for sharks. Until next time finatics!

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Featured Image Source

Javier, L. (Photographer). (2102 March 11) Bonnethead Shark at the Aquarium of the Pacific at Rainbow Harbor in Long Beach, CA [Digital Image]. Retrieved from https://www.flickr.com/

Literature Cited

Leigh, S. C., Papastamatiou, Y. P., & German, D. P. (2018). Seagrass digestion by a notorious ‘carnivore.’ Proceedings of the Royal Society B: Biological Sciences, 285(1886), 20181583.

Sample, I. (2018, September 4). First known omnivorous shark species identified. Retrieved from https://www.theguardian.com/environment/2018/sep/05/bonnethead-omnivorous-shark-species-identified

Skomal, G. (2016). The Shark Handbook: The Essential Guide for Understanding the Sharks of the World. (2nd ed.). Kennebunkport, ME: Cider Mill Press.

Tricas, T. C., Deacon, K., Last, P., McCosker, J. E., Walker, T. I., & Taylor, L. (1997). The Nature Company Guides: Sharks and Rays. (L. Taylor, Ed.). Hong Kong: The Nature Company, Time Life Books.

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