The voracious reputation of sharks might soon change as marine biologists uncover that most coral reef sharks eat pray smaller than a cheeseburger.

Black-tip reef shark. Image via wikimedia

Scientists from James Cook University’s ARC Centre of Excellence for Coral Reef Studies were curious to see exactly what sharks have for dinner, so they did something both awesome and yucky at the same time: they caught some, then examined the content of their stomachs to identify their last meal.

Lead author Dr. Ashley Frisch remembers that what they’d most often find was a huge chunk of nothing.

“We were surprised to find a broad range of small prey items such as fish, molluscs, sea snakes, crabs and more often than not, nothing at all,” Dr Frisch said.

“These results suggest that reef sharks eat small meals infrequently and opportunistically.”

To find out what their eating habits were over a longer period, the team conducted chemical analyses on samples of shark body tissue. The results were very surprising.

“Although black-tip, white-tip and grey reef sharks have long been thought of as top predators, we found that the chemical structure of the sharks’ body tissue actually matched closely with that of large reef fishes such as groupers, snappers and emperors,” Dr Frisch added.

“This result tells us that reef sharks and large fishes have a similar diet, but they don’t eat each other. So rather than eating big fish, reef sharks are eating like big fish.”

Co-author of the study, Dr. Justin Rizzari said that their work allows scientists to better model coral reefs food webs, and serves as a reminder that the large, conspicuous predators aren’t always the top of the food chain. Understanding ‘who eats who’ in coral reefs is important in helping scientists better predict how changes in one population impact another, allowing them to better preserve such environments.

“We now know that reef sharks are an important link in the food chain, but they are not the last link in the food chain. In most cases, the top predators are tiger sharks, hammerhead sharks, or people,” Dr Rizzari said.

“Coral reef ecosystems are very complex. The more we look, the more we realize that each and every species plays an important role. Sharks are no exception. They help to keep coral reefs healthy and should be managed wisely.”

With coral reefs around the world in decline and humans killing an estimated 100 million sharks every year, understanding the exact role sharks play in coral reef ecosystems is more urgent now than ever.

The full paper, titled “Reassessing the trophic role of reef sharks as apex predators on coral reefs” has been published online in the journal Coral Reefs, and is available here.