Bonnethead -The Grass-Eating Shark.
- bioworld090
- Jan 9, 2023
- 3 min read
Updated: Jan 20, 2023
Image Credit: Florida Fish and Wildlife
Sharks are highly efficient hunters and are usually considered the top predatory fish of the oceans. With more than 500 species, sharks vary in their predatory behavior and dietary preferences but one common thing among them is their sustenance on an exclusively carnivorous diet. However, there is one species of shark that is very different from all the other shark species in terms of its sustenance on a carnivorous diet. This species of shark, known as the bonnethead or shovelhead, can not only sustain on a carnivorous diet but also on a herbivorous diet. It is the only species of shark that is an omnivore.
The bonnethead shark (Sphyrna tiburo) is a small-sized shark in the genus Sphyrna. This genus also includes the hammerhead and winghead sharks. Bonnetheads typically live in warm and shallow waters of the Atlantic and Pacific oceans. These sharks are usually 2-3.5 ft in length and weigh 4.5-7 kg. The maximum length and weight they can reach are 5 ft and 11 kg respectively.
THE OMNIVORE DIET
The bonnethead shark primarily feeds on crustaceans like crabs and shrimps and other preferences include mollusks and small fish. Apart from this carnivore diet, it also consumes seagrass. This is evident from the seagrass found in their stomach.
But could this ingestion of seagrass be just accidental as bonnetheads live in waters where seagrass also grows?
The ingestion of seagrass by bonnethead sharks is possibly not accidental because the amount of seagrass ingested by bonnethead sharks is huge. The seagrass can make up to 62% of their diet.
The other factor that denies the possibility of accidental ingestion of seagrass by bonnethead sharks is that these sharks can digest seagrass and absorb nutrients from it with moderate efficiency (50 ± 2% digestibility of seagrass organic matter) to use for cellular processes.
THE ADAPTATIONS THAT POSSIBLY AID IN SEAGRASS DIGESTION IN BONNETHEAD SHARKS:
The highly acidic (pH 1-2) stomach of sharks could possibly aid in the digestion of seagrass by weakening the cell wall and plasma membrane of the seagrass.
Bonnethead sharks possess molariform teeth which they use to crush the hard shells of the crustaceans they eat. It is believed that the molariform teeth may also be capable of masticating or chewing seagrass which could aid in seagrass digestion.
EVIDENCE OF SEAGRASS DIGESTION:
Bonnethead sharks can digest the seagrass they consume and it is evident from the following:
The elevated activities of protein and lipid degrading enzymes in the stomach of bonnethead sharks.
Elevated levels of β-glucosidase in the hindgut of bonnethead sharks.
EVIDENCE OF ASSIMILATION OF SEAGRASS MOLECULES:
Assimilation is basically absorbing vitamins, minerals, nutrients, and other molecules from the digested food and utilizing it for various purposes such as obtaining energy, building up new tissues, and repairing old tissues.
The following two points indicate that bonnethead sharks are capable of utilizing digested seagrass.
The sharks used in the study were given a diet of 90% seagrass and 10% squid. All the sharks, despite being on an almost herbivorous diet, gained weight. This shows that bonnethead sharks can utilize the digested seagrass.
Increase in the 13C signature in the blood and liver tissues of the bonnethead sharks.
[13C or carbon 13 is an isotope of carbon and is used as an isotopic signature in isotopic labeling (a technique used to track the passage of an isotope through a reaction, metabolic pathway, or cell). Here, 13C is used to label the seagrass that was fed to the bonnethead sharks used in the study. At the end of the study, it is used to find if the sharks could assimilate the digested seagrass by looking at the levels of 13C in the blood and liver tissues.]
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