The Scalloped Hammerhead Shark (Sphyrna lewini).
© K. Amsler
Scalloped Hammerhead Sharks (Sphyrna lewini) are probably the most commonly found
species of hammerheads located in coastal regions, appearing in very shallow waters
such as estuaries and inlets. Their distribution in the water reaches from the
surface down to a depth of approx. 275 m. The young, however, remain mostly in
shallow waters along the shore to avoid the danger of falling into the mouths of
predators. At certain times of the year and places, and during certain phases of
their lives, scalloped hammerheads form very large schools, sometimes counting
hundreds of individuals, but they also swim the oceans alone. Some populations remain
stationary, others clearly wander, migrating in the direction of the poles in summer.
Some sexually-related migrations have also been observed, e.g. females who undertake
migrations during particular periods of their sexual development.
The scalloped hammerhead shark belongs to the large hammerhead species, and like all
representatives of this family, has the typically formed "hammer" consisting of a
central dent and an arched front edge (hence the name). Another typical
characteristic is the free end tip of the second dorsal fin which almost reaches the
tail fin. Their coloring is mainly olive, bronze or light brown with a white belly.
The edges of the fins are usually darker on young animals but becomes lighter as they
Mature females can reach a length of more than 4 meters, the average length is,
however, less. Males reach sexual maturity at a length of about 160 cm, females when
they reach approx. 210 cm. The pups measure approx. 50 cm at birth.
This hammerhead species feeds mostly on fish such as sardines, herring and mackerels,
occasionally also on invertebrates such as octopuses. Large scalloped hammerhead
sharks also eat small-sized shark species such as the Atlantic sharpnose shark
(Rhizoprionodon terraenovae) or the blacktip reef shark (Carcharhinus melanopterus).
Scalloped hammerheads bear their young alive and have an egg yolk placenta. Pregnancy
lasts between 9 and 10 months. Depending on their size, the females give birth to
between 15 and 30 pups. The "hammer" is made of cartilage and is very soft when the
young are born so as to easen the birth process. Young scalloped hammerheads grow
relatively slowly when compared to other shark species.
Scalloped hammerhead sharks are found practically around the world in the coastal
regions of tropical, subtropical and moderate climate zones.
As already mentioned, this shark species tends to form huge schools whose function is
presumed to be manifold and may, among other things, concern feeding habits and
reproduction. Although many studies also consider this behavior to be a group
protective function, this is somewhat questionable since the animals have practically
no natural enemies after reaching full maturity. Groups of scalloped hammerheads
prefer staying in regions which have pinnacles or sea mounts which reach from great
depths practically to the water's surface. Latest research also shows that these
sharks can make use of the earth's magnetic field during their migrations.
Although these sharks have definitely been involved in accidents, they are not really
considered dangerous in the sense of being aggressive. Since they often appear in
estuaries where visibility is very limited and where the influence of fresh water
does not allow an optimal reaction of their electrical sensors (ampullae of
Lorenzini), any accidents with humans are more likely a defensive reaction when
surprised or frightened.
When it comes to size and appearance three additional species resemble the
scalloped hammerhead and are commonly found in the latter's area of distribution.
These include the great hammerhead (Sphyrna mokarran), the smooth hammerhead (Sphyrna
zygaena) and the whitefin hammerhead (Sphyrna couardi). They can be differentiated by
the form of their "hammers", the first dorsal fins and by color. Identification
problems with the whitefin hammerhead are only possible in the latter's area of
distribution which is limited to the Ivory Coast.
May be published only by indicating the source: Shark Info / Dr. Erich K. Ritter