Sharks are archetype killers—often the apex predators of their environment, known for ripping prey to shreds. However, the bonnethead shark (Sphyrna tiburo) swims its own path. Scientists have discovered that it is an omnivore, the first known instance of a shark eating plants.
It's not just a nibble, either. The bonnethead, which tends to live around the coasts of southern American regions like Florida, California, Mexico, and Brazil, doesn't turn to plants only when there's no meat around. According to the study, published in the Proceedings of the Royal Society B, upwards of 62.1 percent of a bonnethead shark's gut content mass is made up of seagrass.
It was well known to science that bonnetheads ingest seagrass. What researchers didn't know was the intention behind the act. To figure out the answer, scientists captured five wild bonnethead sharks from the Florida Keys and placed them in an aquatic laboratory.
The lab featured a diet of 90 percent seagrass and 10 percent squid for the sharks. The water was dosed with arbon-13, a traceable isotope, that became integrated into the seagrass. If and when the sharks would eat the seagrass, it could be traced.
Carnivores can physically eat plants, no its not unusual to find some inside them, but digestion is another issue. However, when the scientists kept all five sharks on a seagrass diet, it actually increased their weight. The adult sharks were eating plant fiber just like they were baby green sea turtles, who are omnivorous growing up but switch over to plants exclusively in adulthood.
"Bonnetheads have a digestive system that is very similar to other closely-related species that are definitely strictly carnivorous, so the fact that they are acting like omnivores is truly remarkable," Samantha Leigh, a study co-author and an expert in ecology and evolutionary biology at the University of California, Irvine, to the AFP.
Up next for the researchers is getting a better understanding of the evolution of bonnetheads. If they have similar digestive tracks to other sharks, how did they end up eating plants? This is one shark problem that cannot be punched away.