Fish
Bonnethead Shark
Sphyrna tiburo · also called bonnethead shark, bonnet shark, shovelhead shark
The bonnethead is the runt of the hammerhead family, and the odd one out in another way too: it is the only shark ever confirmed to genuinely digest plants. Alongside crabs and shrimp dug from the sand, this small, sociable shark eats and absorbs real nutrition from seagrass.
Overview
Bonnethead sharks live in shallow bays, estuaries, and seagrass flats of the western Atlantic, Gulf of Mexico, and eastern Pacific. At up to 1.5 m, they are the smallest of the nine hammerhead species, with a smoothly rounded, shovel-shaped head. They form social schools of hundreds, more so than most sharks.
The only omnivorous shark
Seagrass can make up half of a bonnethead’s stomach contents, and studies show the shark actively digests it with enzymes and absorbs nutrients — not just incidental swallowing, but genuine omnivory, unique among sharks.
The hammer as a metal detector
Like other hammerheads, the wide head spreads out electroreceptor pores over a larger area, helping it detect prey buried in sand.
A shark that can reproduce alone
In 2001, a captive female with no access to a male gave birth via parthenogenesis — genetically confirmed asexual reproduction, a documented backup option in sharks.
Status
The bonnethead is Least Concern globally but declining in places from shrimp-trawl bycatch and the loss of seagrass nursery habitat.
Bonnethead Shark: common questions
Is the bonnethead shark really the only omnivorous shark?
Based on current evidence, yes, at least in the sense of actively digesting and gaining nutrition from plant material. A 2018 study fed bonnetheads a diet that was over 90 percent seagrass and tracked biomarkers showing the sharks digested and absorbed nutrients from it using digestive enzymes, rather than the seagrass simply passing through undigested as incidental gut content, which is what happens when other sharks occasionally swallow plant matter along with prey. Combined with field studies showing seagrass regularly makes up a large fraction of wild bonnethead stomach contents, this makes the bonnethead the first shark species confirmed to be a genuine, functional omnivore rather than a strict carnivore that sometimes eats plants by accident.
Why does the bonnethead have a shovel-shaped head instead of a typical hammerhead shape?
All hammerhead sharks have a laterally expanded head called a cephalofoil, and the bonnethead has the smallest and most smoothly rounded version, shaped more like a shovel, spade, or bonnet than the sharply angled, almost rectangular hammer of species like the great or scalloped hammerhead. As in its larger relatives, the wide head spreads the shark's electroreceptive sensory pores over a broader area, improving its ability to detect the weak electrical signals given off by crabs, shrimp, and other prey hidden under sand, and also improves manoeuvrability. The more rounded, less extreme shape is thought to reflect the bonnethead's smaller size and its specialisation on close-range bottom foraging in shallow water rather than the wide-area open-water hunting some larger hammerheads do.
Can a bonnethead shark reproduce without a mate?
Yes, in at least one well-documented case. In 2001, a female bonnethead shark that had been housed for years in a US aquarium with only female sharks, with no possibility of contact with a male, gave birth to a pup. Genetic testing confirmed the pup had no paternal DNA and was produced by parthenogenesis, a form of asexual reproduction in which an egg develops without fertilisation, something previously known in some reptiles and other animals but which this case helped confirm can also occur in sharks. It is not the shark's normal reproductive strategy, which is sexual with live birth of litters after mating, but it shows the capability exists as a biological fallback, potentially useful for a female isolated from mates in the wild.
How does the bonnethead compare to the great hammerhead?
They share the basic hammerhead body plan but differ enormously in size, diet, temperament, and conservation status. The great hammerhead can exceed 5 to 6 metres and is a powerful, wide-ranging, strictly carnivorous predator that famously specialises on stingrays and is classified as Critically Endangered due to heavy fishing pressure, largely for the shark-fin trade. The bonnethead tops out around 1.5 metres, is a mild-mannered, highly social, omnivorous forager of shallow bays and seagrass beds, poses no danger to people, and is currently assessed as Least Concern globally, though with some regional declines. The two illustrate how varied the nine hammerhead species are despite sharing the same distinctive head shape.