Scalloped Hammerhead

The scalloped hammerhead (Sphyrna lewini) is a species of hammerhead shark in the family Sphyrnidae, formerly known as Zygaena lewini. The distinguishing characteristic of this shark is its hammer-shaped head, which is where the Greek word sphyrna, meaning "hammer" in English, comes from. The shark's eyes and nostrils are located at the tips of the extensions on its head. While it is a fairly large hammerhead, it is smaller than both the great and smooth hammerheads.

Also referred to as the bronze, kinky-headed, or southern hammerhead, this species primarily inhabits warm, temperate, and tropical coastal waters worldwide, between latitudes 46°N and 36°S, at depths of up to 500 m (1,600 ft). It is the most common species among all hammerheads.

The key distinguishing feature of the scalloped hammerhead is the central indentation on the anterior margin of its head, with indentations on either side, giving it a "scalloped" appearance. It has a large first dorsal fin that is slightly hooked and a smaller second dorsal fin. Scalloped hammerheads are usually gray, grayish-brown, bronze, or olive in color, with a white underside for countershading. Males typically measure 1.5 to 1.8 m (4.9 to 5.9 ft) and weigh around 29 kg (64 lb) when sexually mature, while larger females measure 2.5 m (8.2 ft) and weigh approximately 36.2 kg (80 lb) at sexual maturity. The maximum recorded length of a scalloped hammerhead is 4.3 m (14 ft) with a weight of 152.4 kg (336 lb).

Scalloped hammerheads have a high metabolic rate, which influences their foraging behavior. They occupy tertiary trophic levels and utilize the shore as a breeding ground. Young scalloped hammerheads have particularly high food requirements, or they risk starvation.

Sphyrna lewini is recognized for its large and complex brain, which allows for cognitive capabilities, social intelligence, sensorimotor functions, migrations, habitat relationships, and successful prey capture. Social intelligence is crucial for aggregative behavior, enabling the species to reproduce with genetically superior individuals and increase its population more effectively. These sharks have small, triangular teeth with smooth edges, although larger individuals may possess slightly serrated teeth.

The scalloped hammerhead is a coastal pelagic species that can be found over continental and insular shelves, as well as nearby deeper waters. It has a global distribution in warm temperate and tropical waters, ranging from 46°N to 36°S. It can occur at depths exceeding 500 m (1,600 ft), but is frequently found above 25 m (82 ft). During the day, scalloped hammerheads are often near the shore, while they venture further offshore to hunt at night. Adults can be solitary, form pairs, or gather in small schools, while young sharks are found in larger schools. Juveniles and pups thrive in shallow coastal waters such as bays and mangroves, where they find shelter from predators and nutrient-rich environments. Research has shown that adult scalloped hammerheads migrate from the open ocean surrounding 🇨🇨 Cocos Island (🇦🇺 Australia) to the tropical fjord of Golfo Dulce (🇨🇷 Costa Rica) on the Pacific Coast of 🇨🇷 Costa Rica. Female sharks give birth to live young in the mangroves, where juveniles remain for approximately three years before returning to 🇨🇨 Cocos Island (🇦🇺 Australia) to feed.

Scalloped hammerheads are commonly observed in large schools, numbering in the hundreds, during night, day, and morning periods. This behavior makes it easier for them to obtain food, particularly larger and more elusive prey. Younger sharks tend to stay close to the surface, while adults are found at greater depths in the ocean. They are not considered dangerous to humans and are typically non-aggressive.

Female scalloped hammerheads migrate offshore at a smaller size compared to males, as larger individuals tend to occupy deeper waters. Males and females display differences in their depth preferences, with males generally staying at deeper depths.

Sexual maturity is typically reached when scalloped hammerheads reach a total length of 240 cm (7.9 ft) or longer. Mature females have noticeably wider uteri compared to those still maturing. Mating scars are absent on mature females, unlike males who reach sexual maturity at smaller sizes.

The male-to-female ratio of scalloped hammerheads is 1:1.29, and females are likely capable of giving birth annually, usually during the summer.

Scalloped hammerheads exhibit a homing behavior to navigate in the ocean and use their surroundings as a map. By tagging these sharks and studying their movements, researchers can assess any guidance or patterns in their behavior. They swim in a point-to-point manner as a school and tend to avoid deep waters where changes in temperature, current speed, and direction occur.

Survival and feeding for scalloped hammerheads rely on their ability to navigate deep water. Despite their high metabolic rates, they are known to be relatively sedentary and allow currents to carry them as they swim. Consequently, these sharks are selective in their swimming locations and depth preferences. They also make use of Earth's magnetic field.

The hammerhead's unique head shape is advantageous when it comes to capturing prey. Its ability to bury itself into the seafloor and immobilize stingrays is facilitated by the wide head and special sensory cells. Scalloped hammerheads have demonstrated successful detection techniques for fish.

The gestation period is estimated to be around 12 months, and scalloped hammerheads give birth to live young. Compared to other species, they produce larger litters (12–41 pups), likely due to high infant mortality rates. Parental care is not observed among sharks. Nursery grounds for this species are predictable and revisited annually, with females remaining faithful to their natal sites. Despite the nursery sites, high infant mortality rates persist due to limited resources, allowing only the fittest individuals to reach maturity. In depleted populations, recovery is achieved through reproduction rather than immigration. Scalloped hammerheads do not display aggression towards each other, even during periods of food scarcity. Additionally, they exhibit migratory behaviors, leading to challenges in nutrition and growth during migration. While 🇹🇼 Taiwan scalloped hammerheads appear to mature earlier, they are still considered slow to reach sexual maturity.

Scalloped hammerheads are opportunistic predators and generalists, consuming whatever prey is available and abundant in their area. There is no difference in diet between male and female sharks, but larger individuals are known to feed on larger prey compared to smaller, juvenile hammerheads. Juveniles primarily prey on inshore fish, while adults consume both inshore fish and larger organisms found in deeper waters. Their diet consists primarily of sardines, mackerel, and herring, with occasional cephalopods such as squid and octopus. Larger sharks may also prey on smaller shark species like the blacktip reef shark, Carcharhinus melanopterus.