Beyond the Hammer: Discovering the Secrets of Sphyrnidae
Hammerhead sharks are among the ocean’s most fascinating animals, instantly recognisable by their wide, hammer-shaped heads, but there are many interesting facts about them beyond their head shape!
The omnivore shark
The Bonnethead (Sphyrna tiburo), also known as the Shovelhead, is one of the smallest members of the hammerhead family. The species is abundant in the North Atlantic coast and the Gulf of Mexico and is recognised for several unusual traits.
Bonnetheads are omnivores! Both adults and juveniles feed mainly on crabs and small fishes, sweeping the seafloor and moving their head from side to side like a metal detector to find subtle electric signals of prey buried in the sediment. Interestingly, they also eat large amounts of seagrass, which recent studies have shown to be essential for their development. In fact, seagrass makes up about 26% of the diet of adults and juveniles, whereas newborns rely much more heavily on seagrass, which can represent up to 62% of their early diet! These findings highlight the crucial role of seagrass in the growth of bonnethead sharks throughout their life stages and confirms they are an omnivore shark!
Bonnethead shark (Sphyrna tiburo) - By Yinan Chen, CC0
The Golden Shark Mystery
The Golden Hammerhead (Sphyrna tudes) is another small species of the Sphyrnidae family. Golden Hammerheads live in the Caribbean Sea of Venezuela and Colombia and are usually found in coastal waters at depths of 9–40 metres over muddy bottoms.
The unusual golden coloration of Sphyrna tudes puzzled scientists and observers for some time! Research demonstrates that the golden colour of this shark is a result of their diet. A carotenoid pigment is passed through the food chain: juveniles acquire their yellow-orange colour from shrimp, which intensifies in subadults and fades as they switch to a fish diet upon reaching adulthood. Scientists believe that adults likely obtain pigments from catfish and their eggs.
Deep divers
Scalloped Hammerheads (Sphyrna lewini ) usually live between the surface and 200 metres deep and prefer tropical to subtropical waters worldwide. In Australia, they can be found from Sydney, along the northern coast, down to Perth in Western Australia.
Scalloped Hammerheads (Sphyrna lewini) aren’t just surface swimmers, they can perform very deep dives (up to 1,240 metres!). At this depth the water temperature can drop below 5 °C and oxygen levels are extremely low. How do Scalloped Hammerheads survive these harsh conditions? They close their gill slits to maintain body temperature, functionally “holding their breath” while they explore the deep ocean in their search for food.
Scalloped Hammerheads (Sphyrna lewini) - By Simon Pierce, CC BY-NC 4.0
The Hammerhead’s Strike
The Great Hammerhead (Sphyrna mokarran) is the largest species in the Sphyrnidae family, reaching an impressive length of 6.1 metres! They live in tropical regions of the Atlantic, Pacific, and Indian Oceans, and can be found in both inshore and offshore waters, including several locations around Australia.
This marine giant uses its incredible strength against its prey in a strikingly literal way. True to its name, these sharks have been seen repeatedly using their hammer-shaped heads (or cephalofoil) like a hammer hitting a nail, knocking prey to the seafloor before eating them. This behaviour has been observed when hunting their favourite prey, stingrays.
Great Hammerhead (Sphyrna mokarran) - By Simon Pierce, CC BY-NC 4.0