51动漫

The bonnethead shark, a relative of the hammerhead, absorbs nutrients from the seagrass it eats, making it the first known omnivorous shark.

Biologists had previously noticed that bonnetheads consume copious amounts of seagrass in addition to crustaceans and other shellfish. But because the bonnethead鈥檚 digestive system looks almost identical to other meat-eating sharks 鈥� and so seems to be best suited to deal with a high protein diet 鈥� scientists always assumed the seagrass ingestion was accidental.

To better understand these sharks鈥� diet, Samantha Leigh at University of California Irvine and her colleagues fed five wild-caught sharks with a diet consisting of 90 per cent seagrass and 10 per cent squid for three weeks.

Secondary jaw

After dissecting the sharks鈥� digestive tracts at the end of the third week, researchers found the activity of enzymes for carbohydrates digestion was high, and comparable to that seen in fish with an omnivorous or even herbivorous diet. Leigh says these enzymes play a key role in helping bonnethead sharks break down seagrass because sharks don鈥檛 have聽the聽聽that many herbivores fish use for chewing up plants.

But bonnethead sharks don鈥檛 just digest seagrass: Leigh and her colleagues found that they also assimilate the nutrients it contains. By infusing the seagrass with a carbon-13 isotope marker, the researchers could track its components to the sharks鈥� blood and livers. The isotope鈥檚 concentration in shark tissues increased with seagrass ingestion.

The sharks absorbed over 50 per cent of seagrass鈥檚 organic matter 鈥� carbohydrates, proteins and vitamins 鈥� making them relatively efficient plant-eaters.

鈥淭he bonnethead shark has a gut that looks carnivorous, but is acting omnivorous,鈥� Leigh says. 鈥淭his means we need to re-classify what it means to have an 鈥榦mnivorous鈥� digestive morphology. Also, as omnivores, it鈥檚 likely that the bonnethead play a role in stabilising food web dynamics and transporting nutrients throughout the seagrass meadows.鈥�