Hammerhead Shark – Family Sphyrnidae
A hammerhead shark is any of the ten species that make up the Sphyrnidae family. This family belongs to the order Carcharhiniformes and has two genera: Eusphyra and Sphyrna, the first with one species and the latter with the remaining nine.
All hammerhead sharks are easily recognized by the unique and striking head they have, which is the origin of their name in English (Hammerhead shark). The most popular species are the great hammerhead shark (Sphyrna mokarran), the Scalloped bonnethead (Sphyrna corona), the smooth hammerhead shark (Sphyrna zygaena) and the scalloped hammerhead shark (Sphyrna lewini).
Hammerhead sharks have a unique T-shaped head similar to that of a hammer, as you can see. For many years the shape of their head has been the cause of theories and studies. The latest and most accepted theory is that it evolved in such way so that the position of the eyes improves the animal’s vision.
They can see above and below them at the same time.
In effect, hammerhead sharks have a 360 ° view, that is, they can see above and below them at the same time, which helps them to find food. This vision plus and efficient vertebrae structure that allows efficient movements and a wider distribution of the Lorenzini ampules over a large head area give them a good advantage as predators.
Other physical characteristics of the hammerhead sharks are similar to another shark species. They have nostrils at the tip of their head and large eyes at each side of it. The mouth is small, contains serrated teeth and locates under the head.
They have two dorsal fins, the first larger than the other. They have a different color in the belly than the dorsal area which is useful to camouflage with the bottom of the sea, because while the ventral region is a light, the dorsal area presents a light gray or greenish color.
There is a broad range of measures among all species of this family with long between 0.9 and 6 meters, and weight that ranges between 3 and 580 kilograms.
DISTRIBUTION AND HABITAT
Different species of hammerhead sharks inhabit temperate and tropical waters around the world, along coastal lines and continental shelves. They dwell the Mesopelagic zone and up to 80 meters deep. Their most common habitats are reefs of shallow waters and occasionally visit brackish waters.
Hammerhead sharks are carnivorous and feed on a variety of prey. Their diet includes bony fish, squid, octopus, crustaceans and their favorite food: rays. Occasionally they practice cannibalism.
When it regards to eating, these sharks hunt alone. Thanks to their electroreceptor organs and to their improved vision, they can detect and catch rays that hide beneath the sand of the ocean floor.
They are viviparous which give birth to live offspring. These sharks reproduce once a year by internal fertilization, and the number of offspring that a female deliver has a direct relation to its size. At greater weight and length, more pups.
To mate, a male of a school selects a female, and then introduces its clasper in the oviduct to transfer his sperm. Once pregnant, the female carries the eggs inside for 8-10 months which feed through the yolk sac. Subsequently, 12 to 50 offspring are born with a length of 18 centimeters and a soft, rounded head. Pups do not receive parental care, but they huddle together in warm waters, departing when they can defend themselves.
They are considered dangerous to humans, but they are not particularly aggressive.
This family of sharks has an unusual behavior by forming groups or “schools” of up to 500 members. Few species of sharks do this. Each group has a social structure that determines hierarchical dominance according to size, age, and sex. Sharks usually stay in schools during the day and separate at night. It is not known why they group, but apparently, the practice provides them protection against larger predators.
Some species of hammerhead sharks often migrate during the summer months, when they travel to cooler waters. Not all species of this shark family like shallow waters. Some, like the great hammerhead shark, stay in deep waters.
They are considered dangerous to humans, but they are not particularly aggressive. Most hammerhead shark species are very small and harmless.
THREATS AND CONSERVATION
Some populations of hammerhead sharks are stable and are not considered endangered. In contrast, the species Sphyrna mokarran and Sphyrna lewini are in danger of extinction, and the species Sphyrna tudes and Sphyrna zygaena are “Vulnerable,” according to the Red List of the International Union for the Conservation of Nature.
These species reached this status because of overfishing and illegal trade, although there are not appropriate sanctions or conservation plans for the most endangered species of hammerhead sharks. Nevertheless, some countries like Ecuador and Costa Rica have denounced this and applied regulatory measures for their capture.
According to the IUCN Red List of Threatened Species, this is the status of species of hammerhead shark:
Whitefin hammerhead (Sphyrna couardi) – Not evaluated
Carolina hammerhead (Sphyrna gilberti) – Not evaluated
Scoophead (Sphyrna media) – Data deficient
Bonnethead (Sphyrna tiburo) – Least concern
Scalloped bonnethead (Sphyrna corona) – Near threatened
Smooth hammerhead (Sphyrna zygaena) – Vulnerable
Golden hammerhead (Sphyrna tudes) – Vulnerable
Great hammerhead (Sphyrna mokarran) – Endangered
Winghead shark (Eusphyra blochii) – Endangered
Scalloped hammerhead (Sphyrna lewini) – Endangered
A. Peter Klimley. The Biology of Sharks and Rays. University of Chicago. 2013
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