It's Wet Beast Wednesday and this week's topic is Sphyrna tiburo, the bonnethead shark. This is a small and wildly adorable species of hammerhead shark that has some very unique features not found in other sharks.
As with most sharks, females are larger than males, reaching an average of about 0.7 to 1 meter (2.3 - 3.3 feet) in length with the longest recorded specimen being 1.5 meters (5 feet). They live in shallow waters, preferring to live in seagrass beds and muddy or sandy sediment. They are found on both coasts of the Americas in warm seas, though they can be found as far north as New England or northern California in rare cases. They migrate south during the winter. Bonnetheads are social and usually live in groups of 5 - 15 members, but can occasionally school in the hundreds or thousands, usually during migration. Scientists have identified at least 18 social behaviors, often used to display dominance or compete for mates. Bonnetheads are very timid and flee from humans. Only one attack on a human has been recorded and it left minor injuries.
One of the unique features of the bonnethead is the fact that they are the only shark species to have sexual dimorphism of the head. Sexual dimorphism is when the males and females of a species have distinct physical features. This is common with sharks, with males usually being smaller than females and having distinct reproductive organs known as claspers. Bonnetheads have an additional dimorphic feature on the cephalofoil (that's the hammer head of the hammerhead sharks). Females have a smooth cephalofoil while males have a distinct bulge on the back edges. in addition, the cehalofoil of the bonnethead is smaller and more rounded than in other hammerheads. Other hammerheads don't use their pectoral fins very much and rely on using their cephalofoil to achieve pitch (up and down movement) and yaw (side to side movement). Because bonnetheads have smaller cephalofoils, they must rely on their pectoral fins more than other hammerheads, and theirs are therefore proportionately larger and stronger than the fins of other hammerheads. By hammerhead standards, the bonnethead is the guy with massive biceps.
The other major unique feature of bonnetheads is their diet. Bonnetheads eat mainly crustaceans, molluscs, and small fish, but also eat a large amount of seagrass. this makes them the only omnivorous sharks, with all others being strict carnivores. Scientists thought that the seagrass was used to protect their gut from shark shells, but recent studies have shown that they do actually digest around 50% of the seagrass and have enzymes in the hindgut that can break down cellulose, implying they do gain nutrition from seagrass.
Bonnetheads are one of several shark species capable of asexual reproduction. A female at the Henry Doorly Zoo in Nebraska produced a pup through through parthenogenesis. The pup's DNA was identical to the mother's. Bonnetheads also have the shortest gestation period of any shark at only 5-6 months and give live birth.
Bonnetheads were once classified as "Least Concern" by the IUCN. this led to it being highly targeted in fisheries, both commercial and recreational. In 2020, it was reclassified as "Endangered" due to massive population declines in the Caribbean, Central and South Atlantic, and most of their Pacific range, largely due to overfishing. As of late 2021, the IUCN now considered them the be "Largely Depleted, meaning their population is currently much smaller than it was historically.
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