A small coastal shark called the bonnethead shark (Sphyrna tiburo) eats copious amounts of seagrass (Thalassia testudinum) and has adaptations in its digestive system to process vegetation, according to new research.
Leigh et al investigated the digestive function of bonnethead sharks in order to determine whether they can digest and assimilate nutrients from seagrass. Image credit: Mills Baker / CC BY 2.0.
The bonnethead shark is a member of the hammerhead shark genus Sphyrna in the family Sphyrnidae.
This species is commonly found in shallow estuaries and bays over seagrass, mud and sandy bottoms at depths from 33 to 263 feet (10-80 m).
It ranges from New England, where it is rare, to the Gulf of Mexico and Brazil and from southern California to Ecuador. It is common in the inshore waters of the Carolinas and Georgia in summer, and off Florida and in the Gulf of Mexico in spring, summer, and fall.
On average, bonnethead sharks are about 2-3 feet (61-91 cm) long, with a maximum size of about 5 feet (1.5 m). Females tend to be larger than males. The body is grey-brown above and lighter on the underside.
The first evidence for plant-eating bonnethead sharks came from an unusual discovery published in 2007.
Dana Bethea, a research ecologist with NOAA Fisheries in Florida, and colleagues examined the stomach contents of bonnethead sharks in the Gulf of Mexico, and were surprised to find that more than half of the material they had ingested was seagrass.
However, it was unclear if the sharks were actually consuming grass and extracting nutrients from it, or just accidentally swallowing it as they hunted for crabs and shrimp hiding in the vegetation.
This work inspired University of California Irvine researcher Samantha Leigh and co-authors to investigate the ability of bonnetheads to digest plants.
“We captured several individuals and brought them into the laboratory at Florida International University, where they were fed a diet of 90% seagrass for several weeks,” the scientists said.
“The seagrass had been labeled with stable isotope carbon-13, so when the sharks consumed it, we could test for a signature of carbon-13 in the sharks’ tissues and see if nutrients from the seagrass were actually taken up into the body.”
“We also collected the shark’s feces, to see how much of the seagrass nutrients (such as carbohydrates, proteins, etc) was simply excreted undigested.”
“Further, we looked at digestive enzymes in the intestines of the bonnetheads, to see if they even have any ability to break down plant material.”
“A purely carnivorous animal should have no mechanism to digest plants, but if the bonnethead sharks eat seagrass regularly, they should have enzymes for this purpose.”
The results were conclusive: carbon-13 from the labeled seagrass was found in the shark’s blood, so they were fully digesting and incorporating nutrients from the grass into their bodies, not just excreting it as waste.
Out of the total grass consumed, about half was actually digested and broken down by the gut, and half was excreted undigested.
The team also found that the sharks had the digestive enzyme b-glucosidase in their guts, which breaks down cellulose, an important component of plant matter.
This is the first finding of plant-specific digestive enzymes in sharks.
On top of this, the bonnetheads seemed perfectly content on their 90% vegetarian diet.
“We observed no negative health effects, and the sharks even gained weight during the study,” the researchers noted.
“While in the wild bonnethead sharks would likely eat less than 90% seagrass, the ability to thrive on such a high plant diet is further support for their ability to obtain nutrients from seagrass.”
“Adaptations for omnivory may allow the bonnetheads to be generalists as opposed to strictly predators, giving them flexibility to consume both plants and protein,” Leigh said.
“We always think of sharks as these apex predators, but here is this shark that is not really acting like an apex predator at all… but more like an omnivore.”
It is still unknown whether the sharks intentionally consume grass in the wild by grazing or if they ingest plants accidentally and have adapted a digestive mechanism to take advantage of that.
“A greater concern is what may happen to bonnethead sharks if these seagrass meadows, which are currently threatened, are destroyed,” the authors said.
“While bonnetheads are not currently endangered, this research indicates seagrass is an important part of their diet, in addition to their habitat.”
Leigh and colleagues reported their findings January 7 at the 2018 Annual Meeting of the Society for Integrative and Comparative Biology in San Francisco, CA.
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S.C. Leigh et al. Omnivorous Sharks? An Analysis of Bonnethead Shark Digestive Physiology Provides Evidence for Seagrass Digestion and Assimilation. SICB Annual Meeting 2018, abstract # 109-1
