The shark’s exceptional hunter attributes are imbued in the collective imagination. But, as we all know, in any family, there is a “particular” cousin that stands out from the others. In sharks, this distant relative could well be the hammerhead shark (Sphyrna tiburo), the first omnivore in this family of formidable predators.
Close cousin of the true hammerhead shark (Sphyrna mokarran), the tiburo is one of the most common sharks in the world. Up to 5 million of them can be found along the US coast, both in the Pacific Ocean and in the Atlantic, from southern Brazil to the northern United States.
Since its discovery, the species, whose size varies between 80 cm and 1.5 m, was considered carnivorous. After all, this shark had been observed several times feeding on small fish and various crustaceans. However, a strange behavior piqued the curiosity of researchers at the University of California and the Florida International University.
Some people have reported seeing the shark eating copious quantities of seagrass plants, flowering plants growing in coastal seas that are not the same family as seaweeds. Initially, the researchers believed that these sharks ate these plants accidentally while attempting to catch prey that were hidden there, and did not derive any nutritional value from them.
The american scientists, however, wanted to check how the digestive system of these animals reacted to the presence of this vegetable accompaniment and if this consumption was really accidental. So, to test if the sharks could sustain their living from the plants, the researchers went to collect large quantities of plants from seagrass beds. After transplanting them into the laboratory, they exposed these plants to baking soda, to give them a distinctive isotopic signature easily recognizable in laboratory tests.
The team then returned to capture five specimens of tiburo hammerhead sharks, which they fed for three weeks with bites made of 90% grass and 10% squid. This laboratory video below published by the journal Science shows a tiburo hammerhead shark catching a meal composed mainly of seaweed.
At the end of this period, the researchers analyzed the digestive system and feces of the animals. They found that the sharks had extracted more than 50% of the nutrients found in plants, almost twice as much as in the panda, another animal with carnivorous attributes, but feeding exclusively on plants.
These sharks do not have, either, the teeth necessary to grind this type of vegetation. To overcome this lack, their digestive system is equipped with digestive enzymes capable of degrading plant cells, allowing them to assimilate the nutrients found there.
Traces of the isotopes present in the baking soda added to the grass at the beginning of the experiment were also found in the blood and liver of the animals, showing that the nutrients participate in their metabolism.
In all, the study reveals that plants make up to 60% of the diet in this species. If these results were confirmed, it would make tiburo the first type of omnivorous shark. At present, no other shark species are on the list of potential omnivore candidates. However, not all shark species consume fish. For example, the largest species of shark, the whale shark, which can measure between 4 m and 14 m and weigh tens of tonnes, feeds mainly on plankton and krill. Although microscopic, these living creatures still remain animals, so that the whale shark can not be considered a vegetarian.
The study also shows that it is problematic to equate the behavior of an animal species with that of other species that are close to it without directly checking its habits.