Small-headed hammerheads turned out to be omnivorous and able to successfully digest plant foods.
The very name of sharks has become synonymous with a fierce predator, but new observations from California biologists show that this is not always true. Small hammerfish, widespread off the American coast of the Atlantic and Pacific Ocean, were able to feed on plant foods when needed. Scientists talk about the omnivorous nature of these sharks in an article published in the journal Proceedings of the Royal Society B.
Some strictly carnivorous animals are quite capable of eating some plants that turn up at hand, but are unable to digest them due to the too simple, short intestines, adapted only for the digestion of protein-rich animal food. True omnivores should be able to assimilate plant fiber, but no one expected this from sharks, so the discovery by Samantha Leigh and her colleagues at the University of California, Irvine became a real sensation.
Small-headed hammerheads (Sphyrna tiburo) reach a length of just over a meter and weigh up to 10 kilograms, feeding on shellfish and fish. Back in 2007, scientists first noticed their suspicious habit of chewing on bottom seaweed. In some cases, the stomach contents of these fish consisted of more than 60 percent plants. Since then, it was believed that sharks only absorb the plant mass without obtaining nutrients from it: for example, accidentally swallowing during an active hunt for squid hiding among the thickets.
However, Samantha Lee and her co-authors decided to test this in experiments. To begin with, they grew enough seaweed, which they fed with baking soda containing a weakly radioactive carbon-13. These plants made up 90 percent of the food on which five sharks caught in Florida Bay were kept for three weeks. Only 10 percent of the fish's diet came from shellfish. The authors analyzed the composition of their secretions, and then euthanized the sharks and examined their gastrointestinal tract.
This work showed that hammerheads are indeed able to absorb nutrients from plant foods and have the necessary cellular and biochemical mechanisms for this. Carbon-13 was found in their blood and liver tissue, showing that they successfully digest plant fibers. According to scientists, the fish were able to extract up to half of the substances contained in the algae. Biochemical analysis has shown that they also have the enzymes necessary to be truly omnivorous sharks.
By the way, the find points not only to new dietary abilities of hammer fish, but also makes us take a fresh look at food and ecological interactions between living organisms inhabiting the warm coastal waters of the Pacific Ocean and the Atlantic. Flowering algae, which these sharks feed so actively, form the basis of local ecosystems, and hammerheads, as it turned out, can make a significant contribution to their dynamics, feeding not only on herbivorous organisms, but also on the algae themselves.
Sergey Vasiliev
Promotional video: