Hammerhead Sharks
Of the nine known species of hammerheads, three can be dangerous to humans: the scalloped, great, and smooth hammerheads. Hammerheads range in size from 2–6 m long, and all species have projections on both sides of the head that give it a resemblance to a flattened hammer. The shark's eyes and nostrils are at the tips of the extensions. The hammer shape of the head was once thought to act as a wing, allowing the shark to execute sharp turns without loss of stability. However, it is the special design of its vertebrae that allows it to make the turns correctly, more so than the head. The hammer shaped head also provides lift, as hammerheads are one of the most negatively buoyant of all sharks. Like all sharks, hammerheads have electrolocation sensory pores. By distributing the receptors over a wider area, hammerheads can sweep for prey more effectively. The hammer-shaped head also gives these sharks larger nasal tract, increasing the chance of finding a particle in the water by at least 10 times as compared to other 'classical' sharks. The hammerheads seem closely related to the carcharhinid sharks (and the Winghead shark who has the largest hammer) and evolved during the mid-Tertiary period. The rest of the hammerhead sharks evolved one at a time from the original Winghead shark, each with a smaller hammer.
Reproduction
Hammerhead shark mating courtship is a very violent affair. The male will bite the female until she acquiesces, allowing mating to occur. Unlike many other shark species, the hammerhead shark has internal fertilization occurring much like in mammals. A safe environment for the sperm to unite with the egg is created and the embryo develops within the female inside a placenta and is fed through an umbilical cord. The gestation period is 10–12 months. On average 12-20 pups are born once a year. However the number of pups may be an underestimate, as one shark near Florida was found to be carrying a record 55 pups! Once the pups are born the parents do not stay with them and they are left to fend for themselves.
Hammerhead Shark Classification
Order: Carcharhiniformes
Family: Sphyrnidae
Genus: Eusphyra
Species: blochii - The Winghead Shark
Genus: Sphyrna
Species: corona - Scalloped Bonnethead
Species: couardi - White Fin Hammerhead
Species: Lenini - Scalloped Hammerhead, Bronze Hammerhead, Kidney-headed Shark
Species: media - Scoophead
Species: mokarran - Great Hammerhead
Species: tiburo - Bonnethead, Shovelhead, Bonnet Shark
Species: tudes - Smalleye Hammerhead
Species: zygaena - Smooth Hammerhead, Black Hammerhead
Great Hammerhead Shark
Scientific Name: Sphyrna mokarran
The great hammerhead shark has a wide, thick head with the eyes at the margins. The head is indented at the center of the "hammer," which is almost rectangular in shape. This shark is gray-brown to gray on top changing to olive near the bottom, with an off-white belly. The first dorsal fin is very large and pointed. Teeth are triangular with extremely serrated edges. The average great hammerhead shark is up to 3.5 m long and the largest reported was 6 m long. These large sharks average about over 230 kg but can weigh as much as 450 kg. The smooth and great hammerheads are easily confused, as both are very large in size.
Encounters
The great hammerhead is found in tropical and sub-tropical waters worldwide. It usually swims along the coastlines and lives over the continental shelves and the adjacent drop-off (the upper part of the mesopelagic zone) to depths of about 80 meters. The great hammerhead migrates seasonally to cooler waters during the summer.
Diet
The great hammerhead is a fierce predator with a good sense of smell that helps it find its prey. They eat fish, including rays, and other sharks (including their own species), squid, octopus, and crustaceans. Stingrays seem to be a particular favorite of the great hammerhead. It kills the ray by using its "hammer" to pin the stingray down while it takes bites from the ray's wings. The great hammerhead has a defense against the venom of sting rays. Hammers are solo hunters.
Reproduction
The great hammerheads are vivparous, giving birth to live young. The litters consist of 20-40 pups and they are about 70 cm long at birth. Females mature at about 2.10–2.50 m and males at about 2.25–2.70 m.
Attacks Many of the hammerheads are harmless to people, but a few species like the great hammerhead, can be very dangerous. When encountered by divers, the great hammerhead is usually quite shy and normally not aggressive towards them. The International Shark Attack File classifies the great hammerhead as the least dangerous shark with only one case of a provoked attack. However hammerheads are in 7th place for dangerous sharks and this may include the great hammer species, so it is considered dangerous but not extremely aggressive.
Smooth Hammerhead Shark
Scientific Name: Sphyma zygaena
Within the hammerhead family there are several species that are differentiated by subtle differences in the shape of the cephalophoil (head). The smooth hammerhead is the only one that appears in Canadian waters and is characterized by a head that lacks a central indentation, hence the name. This shark is white underneath turning to brownish gray or a deep olive colour on top. The teeth, which are similar on both the upper and lower jaw, are triangular and smooth-edged. Adults of this species can attain lengths up to 3.7 to 4 meters and can weigh in excess of 400 kg.
Encounters
The smooth hammerhead shark is distributed worldwide. They occur in all warm temperate waters and are very rare in tropical waters. They prefer inshore waters of depths less than 20 meters. These sharks occur at the surface in the open ocean and often form enormous schools of more than 100 in the summer during their migration to cooler waters. This shark can be found in the western and eastern North Atlantic and in the Indian Ocean. In the western Atlantic the smooth hammerhead ranges from Nova Scotia to Florida.
The smooth hammerhead feeds on schooling fish such as herring and menhaden. Other animals in their diet include other sharks (its own species included), shrimp, skates, crustaceans and cephalopods.
ReproductionThe smooth hammerhead is a viviparous species that gives birth to a large litter, between 20 to 40 pups. The pups are usually 50cm long at birth. Individuals become sexually mature at 2.3 meters.
Distinguishing Characteristics
· Hammer-shaped head that lacks a central indentation
Scalloped, Bronze, Kidney-headed, Sounthern Hammerhead
Encounters The scalloped hammer primarily lives in warm temperate and tropical coastal waters all around the globe down to a depth of 300 m. It is the most common of all hammerheads. The most distinguishing characteristic of this shark as in all hammer heads is the 'hammer' on its head. The shark's eyes and nostrils are at the tips of the extensions. The max length of the scalloped hammerhead is 430 cm and the maximum weight 150 Kg. This shark is often seen in the day in big schools, sometimes numbering hundreds. They are considered dangerous but are normally not aggressive to humans. In fact most incidents with humans are probably defensive after the shark was surprised or frightened.
DietThis shark feeds primarily on fish such as sardines, mackerel and herring, and occasionally on squid and octopus. Larger specimens may also feed on smaller species of shark such as the blacktip.
Conservation Near threatened
Bonnethead or Shovelhead
Characterized by a broad, smooth, spade like head, they have the smallest cephalafoil (hammerhead) of all Sphyrna. It is colored gray-brown above and lighter on the underside, it is a timid and a harmless shark. On average, bonnetheads are about 1 m long, being the smallest hammerhead shark around.The bonnethead is an active tropical shark that swims in small groups of 5-15 individuals, but sometimes migrating schools of hundreds and even thousands of these sharks have been reported. Bonnetheads swim continuously, traveling long distances everyday, following the changes in water temperature. This species must stay in motion at all times in order to force its gills open to receive oxygen. They will sink if they do not keep moving since hammerhead sharks are among the most negatively bouyant fish. The bonnethead shark uses a special body fluid, called "cerebrospinal fluid” or "Cl-excess", to let others know it is in the area.
The pectoral fins on most fish control pitching (up-and-down motion of the body), yawing (the side-to-side motion) and rolling. Most hammerheads do not yaw or roll and achieve pitch by using their cephalofoil. The smaller cephalofoil of a bonnethead shark is not as successful and they therefore have to rely on the combination of cephalofoil and their large pectoral fins for most of their motility. Compared to other hammerheads, bonnetheads have larger and more developed pectoral fins and are the only species of hammerhead to actively use pectoral fins for swimming.
EncountersThis species lives in the Western Hemisphere where the water is usually warmer than 70 degrees F. It ranges from New England, where it is rare, to the Gulf of Mexico and Brazil, and from southern California to Ecuador. In the winter, the bonnethead is found closer to the equator, where the water is warmer.
DietPrimarily crustaceans, consisting mostly of blue crabs, but also shrimp, mollusks, and small fishes. Seagrasses have also been found in its stomach contents. Their feeding behavior involves swimming across the seafloor, moving their bonnet in arc patterns as a metal detector, looking for minute electro-magnetic disturbances produced by crabs and other creatures hiding in the sediment. Upon discovery, they sharply turn around and bite into the sediment where the disturbance was detected. If a crab is caught, bonnethead uses its teeth to grind its carapace and then uses suction in order to swallow the crab whole. To accommodate for many animals that it feeds on, bonnethead shark has small, sharp teeth in the front of the mouth (for grabbing soft prey) and flat, broad molars in the back (for crushing hard-shelled prey).
ReproductionThe bonnethead is viviparous, which means females produce eggs that are retained and nourished in the reproductive system until the young are mature enough to be released to the outside. They reach sexual maturity at about 30 inches. The pups are born in late summer and early fall, measuring 12-13 inches.
Bonnetheads are the only known sharks to exhibit sexual dimorphism i.e. male and female adult bonnetheads look different from each other. As adults, female bonnethead sharks are characterized by a broadly rounded head morphology whereas males possess a distinct bulge along the anterior margin of the cephalofoil. This bulge is formed by the elongation of the rostralcartilages of the males at the onset of sexual maturity and corresponds temporally with the elongation of the clasper cartilages.
Smalleye, Golden Hammerhead
The smalley or golden hammerhead is a small subtropical hammerhead shark found in the southwest Atlantic from Venezuela to Uruguay, in the Mediterranean and in the eastern Pacific.
It is a dark yellowish-brown color which have given it the name golden hammerhead. It is found to at least a depth of 12 m.
Very little is known about this shark.