Essay/Term paper: Hammerhead sharks

Essay, term paper, research paper:  Science Reports

Hammerhead Sharks


Marine Science/ Per. 1


Sharks are one of the most feared sea animals. They live in oceans
across the world but are most common in tropical waters. There are over three
hundred fifty species of sharks. They can be broadly categorized into the
following four groups: Squalomorphii, Squatinomorphii, Batoidea, and
Galeomorphii. The shark family Sphyrnidae that includes the Hammerheads are
part of the Galeomorphic classification. They are probably the most easily
recognizable of all the sharks. The Hammerheads are among the strangest looking
sharks. As the name indicates they have a flattened head which resembles the
head of a hammer. Their eyes and nostrils are at the ends of the hammer. There
are many species of Hammerheads. There are eight living species of hammerheads.
The following four are the main categories:

1. Scalloped hammerhead (Sphyrna lewini)-Pectoral fins are tipped with black
this grey shark. The maximum length is about 12 feet.

2. Bonnethead (Spyrna tiburo)-With a head shaped like a shovel the bonnethead
rarely grows more than four feet long. This shark is commonly seen inshore.

3. Smooth hammerhead (Sphyrna zygaena)-Bronze with dusky fin tips, it can grow
to thirteen feet.

4. Great hammerhead (Sphyrna mokarran)-Attaining a length of a possible 18 feet,
this is the largest and most dangerous of all the hammerheads.

One of the most interesting things about the hammerheads is the unique
shape of their heads. Ever since scientists started to study the hammerhead
they have speculated about the use of the hammer. The hammer is a complex
structure and probably serves more than one function. The most important
function of the hammer according to scientists is increased electroreceptive
area and it's sensory perception. This means that the hammerhead has a
remarkable sensory ability to detect the small electrical auras surrounding all
living creatures. Under certain conditions, such as in searching for wounded
animals, the electrical activity increases helping the hammerhead to feed. It
is also believed that the hammerhead may be able to use the Earth's magnetic
field as a source for navigation. Some hammerheads migrate a lot and may rely
on this built in compass sense to guide them in the open ocean. Another use for
the hammer is to enhance maneuverability. The hammer's similarity to a
hydrofoil seems to explain its usefulness for maneuverability and improved lift.
However, this theory has not been tested.
Sharks generally have a small brain in comparison to their body weight.
Among sharks hammerheads have a relatively large brain-body weight ratio.
Sharks differ form most other fish in several ways. Sharks have a boneless
skeleton made of cartilage that is a tough elastic substance. Most sharks have
a rounded body shaped like a torpedo. This shape helps them swim efficiently.
Hammerheads are especially good swimmers because of the hydrodynamic function of
their head.
All sharks are carnivorous. Most eat live fish, including other sharks.
Most sharks eat their prey whole, or tear off large chunks of flesh at a time.
They also eat dying animals. Hammerheads have definite food preferences. Their
elongated head may help them locate the prey they prefer. The Great Hammerhead
likes to eat stingrays. This was observed when the stomach contents of a
hammerhead were examined and stingray spines were found. Stingrays are usually
difficult to detect because they are partially buried in the sediment. Yet, the
hammerhead is capable of finding them because they can swim close to the bottom
swinging their heads in a wide arc like a metal detector.
Sharks reproduce internally. Unlike most fish sharks eggs are
fertilized internally. The male shark has two organs called claspers which
release sperm into the female where it fertilizes the egg. In many sharks the
eggs hatch inside the female, and the pups are born alive. Other species of
sharks lay their eggs outside. The hammerhead female has an internal pregnancy
in which a placenta is formed around the embryo. The gestation period for most
placental sharks is between nine and twelve months. The placenta appears about
two to three months after ovulation when the embryos have consumed their yolk.
Eggs are ovulated at intervals of a day or so, which explains why their may be
considerable variations in the developmental ages of pups in a litter. It's not
unusual to find embryos that have died during development.
Hammerhead sharks tend to form schools of fifty to two hundred. They
tend to congregate and swim at special sea mounts. Sea mounts are underwater
mountains. In these sea mounts there are many other fish attracted by rich
algae and invertebrate larvae. The hammerheads have no interest in these fish.
So why do they gather at these underwater mountains? Recent research seems to
indicate that hammerheads go there for mating purposes. Observations in these
sea mounts show that the majority of hammerheads there are female. This
indicates that its easy for the male to find a mate. However, researchers were
surprised to find that there were many immature female hammerheads at the sea
mounts. This led them to believe that in addition to reproduction there must be
other reasons for coming to the sea mounts. It is believed that the sea mounts
serve as navigational centers. Each evening the hammerheads begin a ten to
fifteen mile swim away from the mount, always returning by dawn or the following
day. It seems that they spend the night at distant deep water feeding grounds.
The young females participate in these long distance swims. The sea mount
serves as a navigational center helping them find their way back. The nightly
swim help the young find nutritious food which helps them in their growth.

Bibliography:

Klimley, Peter, "Hammerhead City", Natural History, Oct. 1995, pp 33-38. Martin,
Richard, "Why the Hammerhead?", Sea Frontiers, May-June 1989, pp. 142-145. Moss,
Sanford, Sharks, Prentice-Hall, 1984. World Book Encyclopedia, Sharks, World
Book Inc., 1988.