The bonnethead shark lives well in captivity, which has given researchers plenty of opportunity to learn more about it.
The smallest of the nine hammerhead species, the bonnethead has a distinct appearance. Not only is it small, but its head is smooth and spade-like and doesn’t really resemble a hammer at all.
It’s not only its appearance that sets it apart from other hammerheads but also its diet.
Let’s find out what makes the bonnethead so intriguing and why it’s the only semi-vegetarian shark in existence.
What do Bonnethead Sharks Look Like?
Bonnethead sharks are easily identified by their smooth, spade-shaped heads. The bonnethead’s eyes are situated on either side of its flattened head, giving it a 360° view of the world.
Where the eyes are located, there are two evenly rounded lobes on either side of the head. These increase the bonnethead’s field of vision even further, creating an overlap of binocular vision in the center.
The bonnethead is one of the few shark species to be sexually dimorphic, meaning the males are physically different from the females.
Where the shark’s head meets its body, the male bonnethead has a distinct notch or bulge that’s absent in females. This bulge only appears at sexual maturity, when the male’s clasper cartilages also start to elongate.
The bonnethead varies from light brown to dark grey on its back, with a light, creamy-colored underside. This countershading provides camouflage in the ocean and is a feature nearly all sharks share.
The bonnethead has a compact body, a long first dorsal fin, and short, powerful pectoral fins. It’s the smallest of all the hammerhead species and is commonly mistaken for a baby of one of the larger species.
Bonnethead Shark’s Taxonomy
As a species of hammerhead, bonnethead sharks belong to the Sphyrnidae family. Each member of this family shares the same distinctive flattened head structure, known as a cephalofoil.
The Sphyrnidae family is part of the largest order of sharks, the Carcharhiniformes. Although there are over 200 different species of shark within this order, there’s not a great deal of variation.
They are all ground sharks with two dorsal fins, five gill slits, and nictitating eyelids.
Bonnethead Sharks’ Characteristics
Bonnethead sharks are small, with the females averaging between 2.3 to 3.3 feet long and males slightly smaller.
Although they are most active during the day, bonnethead sharks never stop moving. If they did, they would instantly sink.
Hammerhead sharks “are among the most negatively buoyant of marine vertebrates” and rely on continuous motion to keep themselves afloat.
All hammerheads have a special power bestowed on them by their strangely shaped heads. With their eyes situated so far apart, hammerheads have a greater field of vision than almost any other creature.
Not only can they see 360° horizontally around themselves, but they can also rotate their eyes and move their heads so they can see above and below as well.
Despite being so far apart, the bonnethead’s line sight still overlaps in the middle, creating a 13° window of “binocular or 3D vision.”
This gives them excellent depth perception, which enables them to accurately judge the distance between themselves and their prey.
Although the bonnethead’s pectoral fins are short, they’re longer than those of other hammerheads and strong enough to propel them through the water.
Other hammerheads swim on their sides, using their tall dorsal fins to offset their negative buoyancy.
Bonnethead Shark’s Life Cycle
Bonnethead sharks have the shortest gestation period of any shark species, lasting just four to five months.
In most instances, the male bonnethead fertilizes the female’s eggs via internal fertilization, which can be a violent event.
The female bonnethead may not become pregnant straightaway, sometimes storing the sperm for up to four months before the “actual fertilization of the eggs.”
Female bonnethead sharks are also capable of asexual reproduction. This phenomenon, known as parthenogenesis, occurs when an egg develops into an embryo without being fertilized by a sperm.
During the gestation period, the bonnethead embryos are nourished via the placenta and a “fluid-filled, acellular egg capsule” that surrounds the fertilized eggs and embryos.
At the end of the gestation period the bonnethead shark gives birth to a litter of up to 14 pups. Each pup measures approximately 12 to 13 inches long and is independent and free swimming from the moment it’s born.
Bonnethead sharks give birth in shallow water close to shore, where their pups have a better chance of survival.
Off the Gulf of Mexico, bonnetheads give birth in late summer or early autumn, having mated in early spring.
Female bonnethead sharks take longer to reach sexual maturity than males, but they also have a longer life expectancy. Studies indicate that female bonnetheads can live to nearly 18 years old, while males are unlikely to live beyond 16 years.
Where do Bonnethead Sharks Live?
Bonnethead sharks live in subtropical waters in the Atlantic and Pacific Oceans. They also occur in the Caribbean and the Gulf of Mexico.
Bonnethead sharks prefer warmer waters over 70°F and migrate seasonally to avoid cold currents.
During the summer, they frequent coastal waters around Georgia and North and South Carolina. In the spring and autumn, they head south, following the changes in water temperature.
Bonnetheads prefer inshore areas to open oceans and are often found in estuaries and sandy bays. They also utilize habitats over mud and sand flats and reef habitats.
Despite being a marine species, bonnethead sharks “will readily travel in and out of brackish estuaries with the ebbing and flooding of tides.” They aren’t euryhaline like bull sharks, however, and can’t survive in freshwater systems for any sustained length of time.
Bonnethead sharks typically occur in shallow water less than 82 feet deep but have been found at depths up to 260 feet.
Bonnethead Shark Behavior
Bonnethead sharks are extremely sociable creatures and spend most of their time in small groups. These groups tend to be sexually segregated and have a rigid social hierarchy based on size.
Most groups contain between five to 15 individuals, although congregations of thousands have been observed, especially during migration.
Bonnethead sharks aren’t aggressive, nor are they territorial. They utilize various methods of communication, including various postures and patterns of movement. Bonnethead sharks also release “a specialized type of cerebrospinal fluid” that lets other bonnetheads know it’s in the area.
Even when competing for food, bonnetheads show any “intraspecific aggression.” Instead, they use avoidance strategies and increase their maneuverability to secure themselves a decent meal.
Although not aggressive, bonnethead sharks do appear to use specific postures and patterns of movement to establish and maintain their dominance within the group.
Researchers concluded that sharks hunching their backs or displaying “patterns associated with exaggerated lateral, head, and tail movement” should be treated with caution as these postures were associated with aggression.
Bonnethead sharks are considered harmless to humans, although there has been one unprovoked attack by this species.
What do Bonnethead Sharks Eat?
Bonnethead sharks have small, sharp teeth in the front of their mouths and large, flattened teeth at the back. This combination is ideal for capturing and consuming crabs, which make up a large portion of the bonnethead’s diet.
A study of bonnethead sharks off the coast of South Carolina found that “the diet was dominated by crabs, primarily portunids,” although males appear to eat fewer crabs than females.
The most unusual thing about the bonnethead’s diet is that it’s the only shark capable of eating and digesting plant matter.
Although some argue that seagrasses “may only occur in stomachs as accidental ingestion,” studies indicate that bonnethead sharks can thrive on a flexitarian diet consisting of “90 percent seagrass and 10 percent squid.”
The bonnethead is the only shark species known to have a “plant-specific digestive enzyme” that enables them to break down cellulose.
Not all bonnethead sharks enjoy a vegetarian diet, however, and some even “occupy habitats without seagrasses,” suggesting that they “are not an essential part of the diet.”
A recent study by the research division of the Florida Wildlife Conservation Commission examined the feeding ecology of bonnethead sharks in Florida, Texas, Alabama, and South Carolina.
Researchers concluded that there was little evidence to “support the concept that bonnetheads may play a more substantial role as nutrient vectors in seagrass ecosystems than previously recognized.”
What Hunts Bonnethead Sharks?
Lemon and tiger sharks both utilize similar habitats to the bonnethead, posing a potential threat. Both these species are big and hungry enough to predate the small bonnethead.
There is no indication that bonnetheads cannibalize others of the same species, although some claim that the female bonnethead loses her appetite when she gives birth to stop her from eating her babies.
Over the years, bonnethead sharks have been heavily targeted by commercial fishing operations, but the short gestation period and large litters make this shark species more resilient to overfishing than most others.
The bonnethead is currently classified as a species of “least concern” by the IUCN.
Conclusion
Bonnethead sharks have several characteristics that distinguish them from other sharks. They’re the only shark species known to feed on plant matter in the form of seagrass and one of the few capable of asexual reproduction.
Far from being the solitary man-eaters, many envisage sharks to be bonnetheads are non-aggressive and highly social.
The bonnethead is one of the few sharks to survive in captivity, which has allowed researchers to study it in more depth than highly migratory species like the great white.
Nevertheless, our understanding of bonnethead behavior in the wild remains somewhat limited, especially in terms of the ecological role it plays in its preferred habitats.
Nicky is a British adventurer and animal lover who spends her time exploring the natural world and writing about her experiences. Whether on horseback, underwater, running, hiking or just standing with a fishing rod in hand, she embraces everything her adopted home of South Africa has to offer.