The general image of a shark tends to be – rightly or wrongly – one of a ferocious predator chowing down on hunks of bloody meat. While for most shark species this savage reputation holds largely untrue, some young sharks off the coast of Florida are seemingly taking things one step further and filling their stomachs with seagrass instead.
Presenting their work at the Society for Integrative and Comparative Biology meeting this week, researchers have compelling evidence that young bonnethead sharks in the coastal waters off the southern United States are eating and digesting seagrass, making them the world’s only known omnivorous shark.
But testing whether or not the little sharks are actually digesting seagrass in the wild is a little tricky, to say the least. To circumnavigate this, the researchers captured a few young bonnethead sharks from the seagrass meadows in the Florida Keys and took them to their facility at Florida International University’s labs.
While here, they fed the young sharks a plant-heavy diet, consisting of 90 percent seagrass and 10 percent squid. During this time, the team analyzed the poop from the sharks and found to their surprise that the creatures were able to digest an impressive 58.6 percent of the plant matter that they consumed, putting them on par with young sea turtles who spend their entire lives chowing down on the seagrass.
What isn’t certain at the moment is whether or not these young sharks are slurping up the grass on purpose or whether it is happening by accident as they learn the ropes of how to be a successful predator. For example, they may be accidentally scooping up the plants as they target a morsel of fish. This theory is backed up somewhat by the fact that the amount of seagrass found in the sharks' stomachs decreases as they age, suggesting that as they hone their hunting skills, they consume less plant matter.
Either way, this doesn’t take away from the fact that the young sharks are still digesting it, even though as predators they don’t normally have the enzymes necessary to break down the tough cellulose found in the cell walls of seagrass.
In order to be considered a genuine omnivore, the sharks need to be getting some nutritional value from the grass, and it seems that is exactly what they are doing. The researchers think it likely that the animals have a specific microbiome in their gut that, a bit like you and me, allows them to break the grass down. This is shown by the fact that carbon isotopes found in the seagrass can also be detected in the blood of the young sharks.
Regardless of whether or not these sharks are doing it on purpose, they seem to have turned eating grass to their advantage, making them truly the world's first known omnivorous shark.