Scalloped hammerhead

The **scalloped hammerhead** (*Sphyrna lewini*), also known as the **bronze**, **kidney-headed**, or **southern hammerhead,** is a species of Hamvvvmerhead shark. The scalloped hammerhead typically lives in warm, tropical, or temperate waters, and can be found in many locations around the globe between 46°N and 36°S. It is the most common of all hammerheads.

The IUCN lists this species as "Endangered", due to low populations after an estimated 95% population drop over the past 30 years in certain parts of their range. This can mostly be attributed to the shark fin trade.

## Description

On average, males measure between 1.5 and 1.8 meters (4.9 to 5.9 feet) in length and weigh on average 29kg (64lbs). Females are larger, measuring an average of 2.5 meters (8.2 feet) and weighing 180 pounds at sexual maturity. One female caught off Miami was measured at 3.26 m (10.7 ft) and reportedly weighed 200 kg (440 lb), though the shark was pregnant at the time.

The scalloped hammerhead is a sister species to the Carolina hammerhead, only being discernible from its relative by counting the number of vertebrae. These sharks have a very high metabolic rate, governing behavior in acquiring food. These sharks occupy tertiary trophic levels. The scalloped hammerhead shark, like many other species, uses the shore as a breeding ground. Due to high metabolic rates, young scalloped hammerhead sharks need a lot of food, or they will starve.These sharks are fairly large for a hammerhead species, although not as large as either the Great hammerhead or the smooth hammerhead.

## Taxonomy

In 1834, the species was renamed *Sphyrna lewini,* after it had previously been known as *Zygaena lewini.* It was given a total of four different names (see above for details). The Carolina hammerhead (*Sphyrna gilberti*) is its sister species. The only notable difference between the Carolina hammerhead and the Scalloped hammerhead is the number of vertebrae. One former species, *Sphyrna couardi,* is now considered synonymous with *Sphyrna lewini*.

## Ecology and diet

Due to their size and metabolism, the scalloped hammerhead occupies a tertiary trophic level, which is directly under level 5, known popularly as "Apex Predators". They generally feed on fish, and sometimes cephalopods such as sardines, mackerel, and herring, and the occasional squid and octopus. Very large specimens have even been observed consuming smaller shark species such as the blacktip reef shark.

## Distribution and habitat

The scalloped hammerhead generally lives in waters above 25 meters (82 feet) deep, however they have been observed over 500 meters (1600 feet). In the day, they are generally in shallower water, and they tend to hunt deeper at night. They live in warm, temperate, and tropical waters, mostly in the open ocean. These sharks will also gather at seamounts

## Reproduction

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 they are faithful to their natal sites. Their natal sites still cause high infant mortality; a lack of resources prevents all the young from surviving. As a result, only the fittest grow to maturity. Also, should a population get depleted, it recovers through reproduction and not immigration. This species does not seem to attack each other even in periods of starvation. In addition, scalloped hammerheads have migratory behaviors. As a result, deprivation results from migration and young growth. While the Taiwan scalloped hammerhead populations seems to have an earlier maturity rate, it is still reported to be slow to mature. Nursery grounds have been located for this species in the Galapagos Islands.

## Behavior

### Schooling behavior

These sharks are often seen during the night, day, and morning in big schools, sometimes numbering hundreds, most likely because large groups can obtain food easier than singles or small groups, especially 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. Indeed, hammerheads are generally skittish and have been known to flee from divers due to being startled by their air bubbles.

### Sexual Dimorphism

The female scalloped hammerheads undergo migration offshore at a smaller size than males 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.

Sexual maturity generally occurs once the scalloped hammerhead attains 240 cm in total or longer. Physically, the mature females have considerably wider uteri than their maturing counterparts. A lack of mating scars has been found on mature females. Unlike females, males reach sexual maturity at a much smaller size.

The male-to-female ratio of the scalloped hammerhead is 1:1.29. Females probably are capable of giving birth annually, usually in the summer.

### Navigation Abilities

Scalloped hammerhead sharks have a homing behavior 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 use 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 uses 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.

## Human interaction

As of 2008, the scalloped hammerhead is on the "globally endangered" species list. In parts of the Atlantic Ocean, their populations have declined by over 95% in the past 30 years. Among the reasons for this drop off are overfishing 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 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.