Biogeography of Hammerhead Shark

Description of Species:
The Smooth Hammerhead is in the family Sphyrnidae, which includes the Bonnethead shark and the Scoophead shark. These animals are smooth and aerodynamic making the ocean an ideal place for them to live. What separates the Smooth Hammerhead from the others is that the anterior portion of the shark’s head lacks a central indentation, which is present in all other types of hammerheads. Also it has nasal grooves so long that they extend to the middle lobe of the head. The head is laterally expanded with the inner side of the head between nasal grooves, divided into three lobes. Another difference is the first dorsal fins rear tip is disconnected from the torso.

The teeth of the Smooth Hammerhead are smooth edged cusps, usually oblique, with a notch on their outer edges. Like all other sharks the teeth are in rows with the replacements immediately behind the functioning teeth. There is a never ending supply of these teeth. This method of rotation has not changed during the entire evolution of the sharks. Although the shapes and sizes of the teeth have changed with evolution.

The color of this type of shark is similar to most sharks. It is deep olive to brownish gray along its top. This color fades into white towards the underbelly of the hammerhead. This shark has occasionally been known to have black tipped pectoral fins. The maximum size of the mighty Sphyrna zygaena is 370-396 cm (12- 13 ft.).(Castro 1983).

It is the flattened wide head that makes the Hammerhead Shark so distinguishable from its relatives. There are a few theories as to why this head is shaped the way it is. One is for swimming purposes, it is believed that the shape of the head is helpful for turning and also for lift. This would make it easier for the fish to swim and also make it more maneuverable. A second belief on the function of the flattened head of the hammerhead involves the Ampullae of Lorenzini. On the body of all sharks is a system of pores most densely located around the head and mouth. These pores are surface openings for long, jelly-filled tubes that connect to a few congregations of ampullary organs located in the heads of all elasmobranches (Moss 1984). These are sensory receptors which are connected to the brain. Each of these ampulla produces about five sensory nerves which pass to the brain. It was once believed that these were to sense either temperature or hydrostatic pressure. Today it is known that they are an electro receptive system that is sensitive to small electric fields, responding to voltage gradients of less than .01 microvolt per centimeter (Moss 1984). Once it was known that fish emit electrical fields around them, the significance of the ampulle of lorenzini can be seen. The smooth hammerhead uses these sensors to capture its prey. It enables these sharks to find hidden prey and to accurately know where to strike. If a ray is buried on the sea floor these ampulla enable the shark to find the prey, without seeing it, and to capture it before it becomes visible. Sharks are acutely tuned to their environments. Endowed with good eyesight, an excellent sense of olfaction, good hearing and mechanoreception, these fish more than any other group of fish display a broad spectrum of sensory excellence. Perhaps this is what has made the shark a lasting animal in the evolution of species.

Natural History:
The Smooth Hammerhead (also known as the Hammerhead Shark) can often be seen on the surface of the water with its upper fins exposed. For sustenance it feeds on stingrays in the south, skates in the north, and numerous bony fishes, crabs and shrimp. It also feeds on sharks, including it’s own species. The Smooth hammerhead migrates along the coasts northward in summer and southward in winter. They have been known to swim in schools of over a hundred individuals.

Internal fertilization is a key adaptation for energy-intensive reproduction. Unlike other fish, shark sperm is deposited within the reproductive tract of the female. This is advantageous because it makes the fertilization of each of the few eggs more likely and provides the sperm with a carefully regulated environment. In the male sharks the intermittent organs (analogous to the mammalian penis) are known as claspers. From the remains of fossils it has been determined that internal fertilization is a very old process in sharks, for sharks it occured hundreds of millions of years ago. As for the rest of the internal anatomy of male sharks, it includes two testes, one functional, which produce sperm, and a duct system to provide the fluid components of the semen, package the sperm, and conduct them to the outside. The testes are suspended well forward in the body cavity. The sperm that is produced is different than the sperm we humans produce. Instead of a tadpole shape they are corkscrew shaped which enables them to change direction by changing the rotation of their spin. The clasper tip which is responsible for exchanging the semen into the female is complex. It opens like a flower, exposing cartilaginous hooks and spurs that dig into the walls of the female oviduct, anchoring the clasper during copulation. (Moss 1984)

The ovaries of the female hammerheads are suspended from mesenteries well forward in the body cavity in a position analogous to that of the testes in the male. In the hammerheads only one ovary is developed and functional (moss 1984). When the development is complete the eggs burst from the ovaries into the body cavity. These female sharks have sperm stored inside them from the last copulation. This enables the eggs to immediately be fertilized. In hammerheads, after the rupture of the fragile shell , the embryo establishes a placental connection with the endometrium of the maternal uterus. This shark placenta is functionally similar to that found in mammals. In the placenta maternal blood and that of the fetus are able to exchange oxygen, nutrients, and waste products across the thinnest possible membranous barrier. Observations of courtship and mating among sharks has been rare. Most of what is believed has been inferred, and not actually seen.

In order to trace the evolution of a species, fossils are used to create a timeline. In the case of the shark, these timelines are often missing important information. The reason for this is that sharks lack hard parts that are needed to fossilize. Teeth, fin spines, and placoid scales make up most of the fossil remains of sharks (Moss 1984). On rare occasions prints of skulls, jaws, fins, and axial skeletons have been found. These have helped paint a picture of what early Elasmobranches may have been like.

The evidence that has been gathered points to an origin of the class Chondrichthyes about 400 million years ago. Chondrichthyes are not ancestral to the bony fish, but instead share a common ancestor (moss 1984). 360 million years ago in the late Devonian period a number of Elasmobranch genera already existed. These early relatives included Xenacanthus and Cladoselache. These are the earliest recognizable sharks and they had terminal mouths with long upper jaws firmly attached to the skull. They appear to be fast moving, predacious sharks (Moss 1984). These sharks existed at a time of momentous evolutionary events, the bony fish were then giving rise to the earliest Amphibians, which shortly after produced the first reptiles. Which means these earliest sharks have an evolutionary history not much longer than Amphibians. Below is a cladogram which shows the relatives of all sharks. The Elasmobranchii are the sharks and all species above in the diagram are related.

It was during the late Paleozoic and Mesozoic age that many sharks achieved new levels of organization. These principal adaptations included a pectoral fin skeleton, the development of an anal fin, and the addition of new ventral elements to the vertebral column. At this time in the Mesozoic ( between 225 and 65 million years ago), when the dinosaurs were having their fling, and while both birds and mammals were first appearing, the Elasmobranches were undergoing a lot of experimenting

Deuterostomata
|--Chaetognatha
|--Lophophorata
`--+--+--Hemichordata
   |  `--Echinodermata
   `--Chordata
      |?-Urochordata
      `--+--Cephalochordata
         `--Craniata

      CRANIATA
      |--Myxini (=Hyperotreti)
      `--Vertebrata
         |?-Euconodonta
         |--Hyperoartia
         `--+--Thelodonti
            `--+--Galeaspida
               `--+?-Anaspida
                  `--+?-Pituriaspida
                     |--Osteostraci
                     `--Gnathostomata
                        |--Placodermi
                        `--+--Chondrichthyes
                            |--Holocephali
`                              --Elasmobranchi

The Smooth Hammerhead Shark is found in many places throughout the world. It inhabits warm temperate waters and avoids tropical waters. It has been seen on the fringes of tropical waters but prefers temperate areas. On the East Coast of North America the range of the Hammerhead is from Nova Scotia in the summer all the way to Florida during the winter (Castro 1983). It remains in the Northern waters until the Autumn temperature reaches 67 degrees F. In the Eastern Atlantic it is found from the British Isles to Co^te d’Ivorie. It is also found throughout the Mediterranean Sea. In the Indo-Pacific Ocean the smooth hammerhead is found from Sri Lanka to South Africa and from Siberia to Australia including Hawaii and New Zealand (Lineaweaver 1970). On the west coast the Hammerhead is present from central California to the Gulf of California. This is a stenotopic species with a continuous distribution. The limiting factor in the distribution of the Sphyrna zygaena is temperature. It can only tolerate temperate waters and so it must migrate with the seasons.

The importance of Sphyrna zygaena to humans has been no surprise. Humans exploit all animals they can profit from. During World War II this species was exploited for it’s liver. The liver of this species is especially high in vitamin A. So much that one gram of oil contains fifty thousand units of vitamin A (Gilbert 1967). Not until after World War II when humans became able to create a synthetic form of vitamin A, did the demand for the Hammerhead liver dwindle. The hammerheads are still at risk from being harmed by humans for other reasons. The fins of all Sphyrna zygaena’s that are between ten and fifteen feet are used for shark fin soup. The Smooth Hammerhead is also in demand for its Cod like dry meat. Fortunately for the shark it’s skin is too thin for it to be desired by humans. This is fortunate because the more uses humans have for animals, the more threatened that animal becomes. At this point Hammerheads are not threatened or endangered. There are no laws protecting their populations. Those are the direct threats humans pose to the Smooth Hammerhead, but even more important are the indirect threats towards it’s habitat.

The ocean that provides life for all on earth is being threatened by the actions of humans. Through our excessive use of fossil fuels, such as coal and oil we are heating the atmosphere at an alarming rate. This is causing our oceans to heat up as well. The Hammerhead Shark migrates with the seasonal change of water temperature. The heating of the oceans will force the Hammerhead into waters that previously due to global warming were too cold for the shark. And the waters that they are used to swimming in will become too warm for the shark. The food that the Hammerhead eats will also be affected by this warming of the oceans. The Hammerhead and its prey will need to relocate..

The human race has historically been known to use the ocean as a bottomless garbage. We dump garbage, radioactive materials, sewage, and furniture right into the ocean. The runoff from land deposits all sorts of oils and chemicals into the ocean. The amounts are hard to measure and even harder to control. The effects of this are equally as difficult to measure, and we may not know how much harm we are doing until it is too late. It just might be too late for the Hammerhead, too late for all other sea fairing creatures, and too late for the great mother ocean.

Castro, Jose. Sharks of North American Waters. Texas A+M University Press, Texas. 1983.

Gilbert P, Mathewson, Rall. Sharks, Skates, And Rays. Johns Hopkins Press, Baltimore.1967.

Lineweaver, Thomas. The Natural History of Sharks. Lyons and Burford, New York. 1970.

Moss, Sanford. Sharks. An introduction for the amateur naturalist. Prentice-Hall inc. New Jersey. 1984.