Bonnethead shark

Sphyrna tiburo

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Type: Fish - Shark Litter size: 4 to 16 pups Other common names: Bonnet shark, bonnet nose shark, shovelhead shark Life span: 16 to 18 years Diet description: Crabs, shrimps, bivalves, snails, some small bony fishes, seagrasses Max length: 1.5 metres Habitat and range: Found in the western Atlantic from Rhodes Island to Brazil, and in the Pacific from California to Ecuador. Typically in coastal habitats, especially in estuaries, shallow bays and reefs, and often over muddy and sandy substrates. Relative size: Image IUCN status: Endangered (EN) - Decreasing population Endangered (EN)
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Identification

Bonnetheads are one of the smaller species of hammerhead at only 1.5 m, but they still have the distinct hammer-shaped head, or cepalophoil, of this species. In bonnetheads, the hammer is particularly round and smooth, and it is proportionately not as wide as some other species. Although generally pale grey in colour, they can have dark spots along their sides. Males and females also have slightly differently shaped hammers – in fact, they are the only known shark species where the sex of the individual can be told just by the shape of its head.

Photo © Idealphotographer | Shutterstock
Photo © Idealphotographer | Shutterstock

Special behaviour

Bonnetheads are equipped with almost 360-degree vision and excellent depth perception, which helps them to detect prey. They have also recently shown to be sensitive to the earth’s magnetic field, and likely use this for navigation during migration. They are quite social sharks, often found in groups of up to 15.

Reproduction

Bonnetheads are viviparous, meaning they nourish their pups in a uterus using a placenta and give birth to live young. Females can store sperm for up to four months prior to fertilisation, and when it is time to pup (after a 5- to 6-month gestation) will move to shallow bays and estuaries. Up to 14 pups can be born in a litter, with each measuring 0.3-0.4 m.

Habitat and geographical range

They are found in the western Atlantic from Rhodes Island to Brazil, and in the Pacific from California to Ecuador. They are typically found in coastal habitats, especially in estuaries, shallow bays and reefs, and often over muddy and sandy substrates.

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Diet

Bonnetheads have a varied diet that includes crustaceans such as crab and shrimp, bivalves and other molluscs including snails, and various bony fish. Most curious, however, is their ability to feed on, and digest, seagrass.

Threats

As with most sharks, the primary threat for bonnetheads is overfishing. They are particularly vulnerable to being caught in gillnets due to their habitat selection and head shape, and they present as considerable bycatch in shrimp trawlers. They are also a popular catch in recreational hook and line fisheries, especially in the northwest Atlantic, and are highly exploited in the southwest Atlantic as a result of largely unmanaged trawl, gillnet and longline fishing. In Venezuela, they are reported to be the fourth most commonly caught shark. Similar intense exploitation in this species’ Pacific range has led to its local demise in certain areas such as the Gulf of California. Another considerable threat is habitat degradation, due to coastal development and increasing removal of their mangrove habitat to make space for shrimp aquaculture.

Relationship with humans

Bonnetheads are primarily exploited in fisheries, both commercial and recreational, and continue to lose habitat as a result of human activity (e.g. aquaculture). Due to their murky coastal habitats, they are not typically targeted by ecotourism operators.

Photo © Ken Griffiths| Shutterstock
Photo © Ken Griffiths| Shutterstock

References

David A. Ebert. et al, 2021, Sharks of the World: A Complete Guide.

IUCN Red List of Threatened Species, Bonnethead Shark: Sphyrna tiburo

Florida Museum, 2018, Sphyrna tiburo

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