Scalloped Hammerhead
Sphyrna lewini
The shark with a head like a wide, flat wing, and one of the sharks in the most trouble.
A scalloped hammerhead is a fish, with gills, a skeleton of cartilage instead of bone, and a tail that stands on its edge. What sets it apart is its head: wide, flat and stretched out sideways, with an eye at each end and a notch in the middle of the front edge. By day, scalloped hammerheads often gather in big groups around islands and underwater mountains. At night, they head out to hunt.
On the conservation status
Critically Endangered on the IUCN Red List since 2018. Its numbers have fallen steeply almost everywhere, mostly because it is caught for its fins and caught by accident in nets and on fishing lines. It is also protected under the US Endangered Species Act.
- Length
- Most 2–2.5 m (7–8 ft), up to 4.3 m
- Weight
- Up to about 150 kg (335 lb)
- Lifespan
- 30 years or more
- Diet
- Fish, squid, octopus, rays, crabs and shrimp
- Litter
- Around 12–41 pups
- Depth
- From the surface down to 1,240 m
The good stuff.
A head that senses
Every shark can sense the faint electricity that living animals give off, using small pores on its head. A hammerhead's wide head spreads those pores over a bigger area. Tests suggest it can find hidden prey, like a fish buried in sand, from farther to the side than a shark with an ordinary head.
Named for its notches
The front edge of its head isn't smooth. It has a notch in the middle, with a gentle curve on each side. The scallops in its name come from that wavy outline.
Schools by day, hunters by night
Scalloped hammerheads are often seen in groups over underwater mountains and around islands, then spread out into open water at night. At Cocos Island off Costa Rica, scientists tagged 84 of them and picked up most of their signals between about 6 am and 6 pm.
Cleaning stations
Around Cocos Island there are spots on the reef where small fish pick pests and dead skin off passing sharks. Scalloped hammerheads visit them regularly.
Nurseries in the shallows
Newborn pups, about 45–50 cm long, spend their first couple of years in shallow water near the coast. That keeps them out of the way of big predators, but it also puts them right where fishing nets are.
Slow to replace
A mother carries her pups for about 9 to 12 months before they're born, and a litter is a dozen or a few dozen. A fish that grows up slowly and has young slowly can't bounce back quickly after heavy fishing.
Wait, really?
- ✦
Its stomach has been found holding up to 50 or more stingray spines. Stingrays are on the menu.
- ✦
From above it's brownish-grey to olive, and underneath it's pale. A fish looking up sees a pale belly against the bright surface; one looking down sees a dark back against the deep.
- ✦
It lives in warm seas all over the world, from the coasts of Brazil and Australia to Hawaii and Japan.
- ✦
Cocos Island, the Galápagos Islands and Malpelo Island form a triangle in the eastern Pacific. Hammerheads travel between them.
What it sounds like
Silent. Sharks have no known calls of any kind.
Where you might see one
Cocos Island in Costa Rica, the Galápagos Islands in Ecuador, and Malpelo Island in Colombia.
The trouble
- Fishing for fins, mainly for the shark fin trade
- Getting caught by accident in nets and on longlines
- Heavy fishing in the shallow nurseries where pups grow up
- Slow growth and a few pups at a time, so losses add up
One real thing
Most hammerheads are lost to the fin trade. Never buy shark-fin soup or anything made from shark fins, and tell others why.
More ways to helpYour field notes
Read about the scalloped hammerhead and add it to your passport. Then show what you learned with the field check, and collect its trading card.
Your passport is saved on this device only. It's never sent anywhere.
Field check
A question about what you just read
One question from the scalloped hammerhead's page. Get it right to collect its trading card. If not, you'll see the answer and where to find it, and can try another.