The hammerhead sharks are excellent predators that belong to the order Carcharhiniformes and family Sphyrnidae. Members of this family of sharks have unique head structures that extend to form a hammer-shaped (T-Shaped) cephalofoil that have different functions including sensing, water maneuvering, and manipulating prey. This family has two genera; Eusphyra and Sphyrna. Hammerheads are distant relatives of the mid-Tertiary period carcharhinid sharks. Today, species of the hammerhead include Eusphyra blochii (Winghead shark), Sphyrna corona (Scalloped bonnethead), Sphyrna couardi (Whitefin hammerhead), Sphyrna gilberti (Carolina hammerhead), Sphyrna lewini (Scalloped hammerhead), Sphyrna media (Scoophead), Sphyrna mokarran (Great hammerhead), Sphyrna tiburo (Bonnethead), Sphyrna tudes (Smalleye hammerhead), and Sphyrna zygaena (Smooth hammerhead). Due to the hammerheads non-combative nature, most of the species face extinction, especially from human fishing activities. This article covers the general features, behaviors, and diet of the whole Sphyrnidae family.
5. Physical Description
On average, the length of hammerhead sharks ranges from 3 to 19.7 feet; the smallest of this family weighs 6.6 pounds while the biggest ever seen weighed 1,278.7 pounds. Most of the members are gray in color and have some greenish tint with white bellies. This color balance enables the sharks to perfectly blend with the ocean since a prey viewing from the bottom will not see it sneaking in for a meal. The cephalofoil laterally project outwards, thus giving the shark the hammerhead shape, and as a sensory organ, the ability to easily notice electric field created by prey or threats from miles away. All the hammerhead sharks have excessively small mouths, in comparison to other sharks, located at the bottom of the cephalofoil with serrated triangular teeth. Hammerheads have wide-set eyes on the outer edges of the hammer adapted to give a wider and better visual range compared to other sharks. The eyes can rotate for a 360-degree vision and one eye can see below while the other sees up simultaneously, however, the positioning of the eyes create a blind spot in front of the nose. The nostrils expand further enhancing their ability to follow a scent and locate prey. Other features include two extra tall dorsal fins of different sizes. Most hammerheads have a lifespan of between 25 and 35 years, though observations are currently indicating that more and more of this shark family live longer than 35 years.
Hammerhead sharks often solitary hunters, swimming close to the bottom of the ocean, and studying prey’s behavior before striking with the help of their electroreceptors. The T-shaped heads of these carnivores are important in pinning down and weakening their targets, that includes smaller bony fish, squid, octopuses, crustaceans, and stingrays, which happen to be their favorite meal. A common belief is that they hunt smaller prey because of their small mouths. Some larger species like the Great Hammerhead practice cannibalism, as they eat other smaller hammerheads and their own young. Enhanced vision and receptors on the head come in handy as “metal detectors” and help the sharks to detect stingrays that hide beneath sand at the floor of the ocean. With enhanced vision and smell, combined with the ability to make sudden sharp turns while maintaining balance, hammerheads easily catch even the most elusive of prey.
3. Habitat and Range
Hammerhead sharks reside in the warm tropical waters along continental shelves and coastlines of the world; however, they usually migrate in mass to cooler waters during summers and to warmer waters during winters. They live within the Mesopelagic zone and a little deeper (up to 80 meters deep) but are sometimes found within very shallow, salty waters. However, the great hammerhead lives entirely in deep waters. Massive schools of hammerheads roam close to Cocos Island off Costa Rica, Malpelo Island in Colombia, and Molokai in Hawaii as well as the waters off the shores of eastern and southern Africa. Whenever there is warm El Nino environment, these sharks usually spread out in schools to cover hundreds of miles.
Just like their distinct appearance, hammerheads do not share all social and behavioral traits of other sharks as they do not act aggressively towards human beings but only become defensive or attack when provoked. Rarely do they attack without giving warning signals and are therefore safe for divers. Of all the species, only the great hammerhead is dangerous to human beings. Only a few attacks from this family of sharks are on record for many years. During the day, they swim in schools (mostly between 100 and 500 members) but become solitary hunters at night. Solitary hunting works to their advantage as it increases camouflage and the element of surprise. Close observation reveals that each school has a social pattern of dominance according to age, sex, and size. Additionally, being in schools during the day gives them security in numbers, thus protecting themselves against larger predators. Behavioral analyses show that these species think and act as individuals and not as a group.
Female hammerheads play “hard to get” by swimming in schools, thus having the security in numbers to select preferred males for mating. Consequently, shark tourists usually travel to the Great Barrier Reef or Galapagos Island to witness this phenomenon. During mating, a male shark chooses a female from within the school, battles her until she gives in, then inserts its clasper into the oviduct for the transfer of sperms and fertilization. All hammerheads produce once a year through internal fertilization, with the offspring developing inside their bodies (viviparous). The size of the offspring is dependent on the mother’ weight and length. Pregnant sharks have a gestation period of 8-10 months, and eventually give birth to between 20 and 50 soft pups of approximately seven inches in length and with round heads. During the gestation period, developing embryos feed through a yolk. When birth nears, usually during spring or summer, the females move to shallow warm waters where there is minimal wave activity. After birth, hammerheads do not give parental care to pups, which group together in warm water until such a time that they are able to defend and fend for themselves.