IT'S no Bruce from Finding Nemo but the bonnethead shark comes close to being a vegetarian.

Researchers claim the small relative of the hammerhead is the first known omnivorous shark.

The coastal bonnethead shark, which was previously thought to be solely carnivorous, enjoys eating seagrass, with up to 60 per cent of their guts full of plant material.

The study authors, from the United States, were unsure if the shark could actually digest the seagrass.

But they soon discovered enzymes in the animal's digestive system that mean they are capable of processing seagrass similarly to juvenile green sea turtles, and even better than pandas.

Pandas, which are herbivores with a carnivorous gut, show about 20 per cent organic matter digestibility of bamboo.

"Remarkably, the bonnethead's digestibility of organic matter is comparable to juvenile green sea turtles," they authors write.

"As green sea turtles mature, they become almost entirely herbivorous, and their digestibility of seagrass increases to 65 per cent.

"Bonnetheads are capable of digesting components of seagrass, with similar effectiveness to omnivores, making them the only shark species known to have the ability to digest plant material."

The scientists from the University of California-Irvine and Florida International University said not all carnivores could digest plant material efficiently, contrary to what Finding Nemo might have you believe.

In the popular Pixar movie Bruce is the leader of the Fish-Friendly Sharks support group.

Scientists fed lab sharks with seagrass and saw they all gained weight and more than met their energetic demands.

Sharks also have highly acidic stomachs and because they lack the secondary jaws that many herbivorous species use to chew plant material, the scientists believe their highly acidic shark stomach could help them digest seagrass.

Bonnethead sharks also have special teeth that are presumed to be for crushing hard prey but they think these teeth may help with seagrass chewing, which could also aid in the digestive process.

The researchers said the sheer abundance of bonnethead sharks in coastal communities

- about 4.9 million alone in the Atlantic and Gulf of Mexico coastal waters of the USA -combined with their eating habits suggest that we need to re-evaluate the role that bonnetheads play in seagrass meadows.

Seagrass meadows are critical ecosystems that provide habitat for thousands of fish species, filter the surrounding water, act as a sink for atmospheric CO2, and produce large quantities of oxygen.

"Understanding how the consumption and digestion habits of bonnethead sharks impacts

seagrass ecosystems is important as these omnivores may stabilise food web dynamics and even play a role in nutrient redistribution and transport," they said.

"This is critical to effectively formulating conservation efforts."