(E. Griffith & C. H. Smith, 1834)
|Range of the scalloped hammerhead|
The scalloped hammerhead (Sphyrna lewini) is a species of hammerhead shark, and part of the family Sphyrnidae. Originally known as Zygaena lewini, the name of its species was later renamed to its current name. The Greek word sphyrna translates into "hammer" in English, referring to the shape of this shark's head. The most distinguishing characteristic of this shark, as in all hammerheads, is the 'hammer' on its head. The shark's eyes and nostrils are at the tips of the extensions. This is a fairly large hammerhead, though is smaller than both the Great and Smooth Hammerheads.
This shark is also known as the bronze, kidney-headed or southern hammerhead. It primarily lives in warm temperate and tropical coastal waters all around the globe between latitudes 46° N and 36° S, down to a depth of 500 metres (1,600 ft). It is the most common of all hammerheads.
The scalloped hammerhead was first named Zygaena lewini and then renamed Sphyrna lewini by Edward Griffith and Hamilton Smith in 1834. It has also been named Cestracion leeuwenii by Day in 1865, Zygaena erythraea by Klunzinger in 1871, Cestracion oceanica by Garman in 1913, and Sphyrna diplana by Springer in 1941. Sphyrna comes from the Greek and translates into hammer.
On average, males measure 1.5 to 1.8 m (4.9 to 5.9 ft) and weigh approximately 29 kg (64 lb) when they attain sexual maturity, whereas the larger females measure 2.5 m (8.2 ft) and weigh 80 kg (180 lb) on average at sexual maturity. The maximum length of the scalloped hammerhead is 4.3 m (14 ft) and the maximum weight 152.4 kg (336 lb), per FishBase. A female caught off of Miami was found to have measured 3.26 m (10.7 ft) and reportedly weighed 200 kg (440 lb), though was in a gravid state at that point.
These sharks have a very high metabolic rate, governing behavior in the acquirement of food. These sharks occupy tertiary trophic levels. The scalloped hammerhead shark, like many other species, will use the shore as a breeding ground. Due to high metabolic rates, young scalloped hammerhead sharks need a lot of food, or they will starve.
The scalloped hammerhead is a coastal pelagic species, it occurs over continental and insular shelves and in nearby deeper water. It is found in warm temperate and tropical waters, worldwide from 46° north to 36° south. It can be found down to depths of over 500 metres (1,600 ft) but is most often found above 25 metres (82 ft). During the day they are more often found close to shore and at night they hunt further offshore. Adults occur alone, in pairs or in small schools while young sharks occur in larger schools.
This shark is often seen during the day in big schools, sometimes numbering hundreds. This is most likely because it is easier for the scalloped hammerhead shark to obtain food in a group than alone. This behavior allows for them to catch larger and trickier prey, as commonly seen. The younger the sharks, the closer to the surface they tend to be, while the adults are found much deeper in the ocean. They are not considered dangerous and are normally not aggressive towards humans.
The female scalloped hammerhead undergo migration offshore at a smaller size than males. This is because the larger classes of the hammerhead, such as those from 100 to 140 cm long, travel deeper down. Males and females differ in that males are observed to stay deeper than female sharks in general.
As expected, males are more aggressive than females, leading to an increased average size in comparison to the female. Through observation, researchers have found that sexual maturity will only occur once the scalloped hammerhead became 240 cm in total or longer. Physically, the matured females have considerably wider uteri than their maturing counterparts. Research results show that there is a lack of mating scars on matured females. Unlike females, males are recorded to reach sexual maturity at a much smaller size.
The male to female ratio of the scalloped hammerhead is 1:1.29. Research tends to suggest that females are capable of giving birth annually. This is usually held in the summer, as can be expected by the type of animal they are. For both males and females, the sexual organs correlate to mating potential, as seen in many others.
Scalloped hammerhead sharks have a homing behavior in order to navigate in the ocean. They move in the night and use the environment as a map, similar to a human reading a topographical map. By experimentation in tagging these sharks, one could test for any guidance in a shark’s movement. These sharks utilize a point to point type of school swimming, and do not favor going too deep where temperature changes hitchhike with current speed and directional change.
The scalloped hammerhead utilizes deep-water to survive as safety and feeding. Although they have high metabolic rates, they have a tendency to be sedentary and allow currents to carry them as they swim. As a result, this causes the scalloped hammerhead to be selective where they swim and the depth at which they tend to stay at. The scalloped hammerhead has a tendency to eat cephalopods.
The gestation period is reported to be around 12 months. Compared to other species, the scalloped hammerhead produces large litters, and, this is most likely due to high infant mortality. Like most sharks, parental care is not seen. Nursery grounds for this species are predictable and repeated over the years, and it is recorded that they are very faithful to their natal sites. This is an interesting fact because their natal sites still cause high infant mortality. There is a lack of resources for the young to all survive. As a result, only the fittest will grow into maturity. Also, should a population get depleted, they will recover through reproduction and not immigration. A study done in Taiwan scalloped hammerheads reinforce what is said by other researches. Through graphs done on dismembered hammerheads that the length and size have a high correlation to each other. In addition, the weight is indeed not the major factor in this species. This species do not seem to attack each other even in periods of starvation. In addition, the scalloped hammerheads have migratory behaviors. As a result, there is deprivation from migration as well as young growth. While the Taiwan scalloped hammerheads seem to have an earlier maturity rate, it is still reported that they are slow to mature.
This shark feeds primarily on fish such as sardines, mackerel and herring, and occasionally on cephalopods such as squid and octopus. Larger specimens may also feed on smaller species of shark such as the blacktip reef shark, Carcharhinus melanopterus.
As of 2008, the scalloped hammerhead is on the "globally endangered" species list. Research has shown that in parts of the Atlantic Ocean, scalloped hammerhead populations have declined by over 95% in the past 30 years. Among the reasons for this drop off are over-fishing and the rise in demand for shark fins. Researchers attribute this growth in demand to the increase in shark fins as an expensive delicacy (such as in shark fin soup) and are calling for a ban on the practice of Shark finning, a practice in which the shark's fins are cut off and the rest of the animal is thrown back in the water to die. Hammerheads are among the most commonly caught sharks for finning.
South African Journal of Marine Science Vol. 20, Iss. 1, 1998
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