The Bonnethead Shark (Sphyrna tiburo) looks like a miniature version of the Hammerhead Shark, and indeed it is the smallest of the ten species in the hammerhead family. Bonnetheads average about 3.5 feet long and can sometimes reach 5 feet. They are a common shark in shallow warm waters on both coasts, ranging seasonally from Brazil around the Gulf of Mexico all the way up to Massachusetts, and from Ecuador to California. In Florida, they can be found in bays and estuaries for most of the year, though they tend to move further south during the winter. I occasionally encounter them while kayaking in Tampa Bay.
Unlike most sharks, who eat fish, Bonnetheads feed mostly on crustaceans and invertebrates, using their large flattened teeth to crush the shells. Like most sharks, they have electro-sensory organs in their snouts that can detect the electric field generated by a living organism, which they use to detect prey buried in the sediment.
Bonnetheads are gregarious and usually move in schools of a dozen or so individuals, though at times hundreds of them may congregate together, particularly when migrating to warmer waters. Individuals release small amounts of cerebro-spinal fluid into the water, which seems to be an attractant and allows the sharks to find each other. Like most sharks, Bonnetheads must swim constantly, both to keep a flow of water passing over their gills so they can breathe, and because they are negatively buoyant and sink when not moving.
Like all sharks, Bonnetheads have tiny teeth embedded in their skin, called “denticles”, which give them a rough sandpapery feel. The denticles may help the shark swim more efficiently by influencing the flow of water over the skin and reducing drag.
The most striking aspect of the Bonnetheads is, of course, their “bonnet”–their heads are elongated to the sides, with the eyes perched at each end. No one is quite sure why the Hammerheads are shaped like this–it has been hypothesized that it aids in vision, that it gives greater sensitivity to the electro-sensory organs, or that it acts like a hydroplane to help with swimming and directional changes. The Bonnetheads have the smallest “hammerheads” proportionately than any other members of the family, and while the front edge of the other Hammerheads is a straight line, the Bonnetheads form a gentle outward curve that gives them a distinct appearance.
Male Bonnetheads are generally smaller than the females–they can be recognized by the small bulge at the front edge of their”bonnet” (caused by an enlarged rostrum) and by their long “claspers” underneath the tail. The sharks mate in the spring. After mating, the females move to shallow areas near shore. They do not lay eggs, but give birth to live young–the eggs are retained inside the body where they hatch, and the embryos are nourished by the female through an attachment between the embryo’s yolk sac and the lining of the female’s uterus. After a relatively short gestation period of five months, the mother gives birth, usually 4-10 pups, in late summer. The newborn Bonnetheads are about a foot long. They find food and protection among mangrove roots until they are big enough to move into more open water. In areas that stay warm all year round, they stay within a small area; other populations migrate throughout the year to stay in warm waters. Bonnetheads reach sexual maturity at around age two, at a length of 2 to 2.5 feet. In captivity they have lived as long as 6-7 years. In the wild, the primary predator on adult Bonnetheads seems to be larger species of shark and other large predatory fish.
Although closely related to the much-feared Great Hammerhead (Sphyrna mokarran), the Bonnethead is shy and inoffensive towards humans. It is so docile that Bonnetheads are often found in “touch tanks” at public aquariums. Bonnetheads are often the target of sport fishers–thousands are taken in Florida every year. The populations can tolerate this because of their high rate of reproduction, and Bonnetheads are one of the few shark species that are not listed as threatened or endangered.