The world’s first shark that eats vegetables – a very surprising headline. Despite feeling pretty well informed with all things marine I had never heard of this species of shark. A member of the hammerhead genus, the Bonnethead Shark (Sphyrna tiburo) is a very abundant, small species found in estuaries and bays on the Atlantic and Pacific Ocean coasts of the Americas and the Gulf of Mexico. Its first claim to fame is being the only shark species known to display sexual dimorphism in the morphology of the head, but now is also the very first shark to be known as an omnivore.
the Bonnethead Shark (Sphyrna tiburo) Wikipedia
After reports of the fish chomping on seagrass, a very important type marine plant that forms coastal meadows providing refuge to a wide range of marine organisms, researchers from Florida International University in Miami and the University of California in Irvine, decided to investigate the sharks’ dietary habits and recently reported their findings in Proceedings of the Royal Society B.
“It has been assumed by most that this consumption was incidental and that it provided no nutritional value,” said Samantha Leigh, a researcher on the team. “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.”
So, there are reports that bonnetheads consume seagrass, but there are numerous questions to consider. First, the consumption could be incidental since the sharks are eating critters living in the seagrass meadows? Second, are they gaining any nutritional benefit from the grasses since they don’t have the proper teeth to masticate the vegetation? How much of their diet actually consists of seagrass?
To assess the bonnethead’s omnivorous habits the researchers first gathered seagrass from Florida Bay and transplanted it back at the lab. As the seagrasses took root sodium bicarbonate powder with a specific carbon isotope (13C-labelled) was added to the water. As the grasses grew it was taken up by the grasses giving it a distinct isotopic signature that could be detected. Next the team caught some bonnethead sharks and allowed them to acclimate in the lab. Luckily, these are small species ranging from 2 to 3 feet. The sharks were feed a diet of seagrass and squid for three weeks, and after the sharks were all plumped up the team ran a series of tests.
Primary investigator Samantha Leigh handles a bonnethead shark in Irvine, Calif. Leigh (Yannis P. Papstamatiou/University of California Irvine via Associated Press)
Digestibility analyses and digestive enzyme assays showed despite the absence of the kind of teeth best suited for chewing, the cartilaginous fish successfully digested the seagrass with enzymes to break down the plants starches and cellulose. The fish probably rely heavily upon strong stomach acids to break down the plants’ cells walls so the cellulose-component-degrading enzymes detected in the hindguts can then work on the plant material. Interestingly, using stable isotope analyses, in total more than half of the organic material contained in the seagrass was digested by the sharks, putting their abilities at a similar level to fully herbivorous juvenile green sea turtles.
Other tests found high levels of the seagrass carbon isotope, imparted by the sodium bicarbonate powder, in the sharks’ blood and liver tissues, directly demonstrating that the digested seagrass was being used to build and maintain the animals. How neat is that? Debunked the idea that all sharks are exclusively carnivorous beasts and that bonnethead’s not only eat plants, but that seagrasses can make up as much as 60% of their diet.
These findings beg to ask how well we really know the feeding habitats of other marine species. Seagrass ecosystems are systems which need protection are they are often well used habitats for countless species and crucial to certain life-history stages. As researchers we must now also incorporate how these sharks and potentially other species fit into the complex webs.
To read this neat article and learn more please see:
http://rspb.royalsocietypublishing.org/content/285/1886/20181583
Leigh, S. C., Papastamatiou, Y. P., & German, D. P. (2018). Seagrass digestion by a notorious ‘carnivore’. Proc. R. Soc. B, 285(1886), 20181583.
