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:
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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 its 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 u
sefulness 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 from 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 there 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.
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.