A Brief Tour of the Fuvahmulah Hammerheads

You shouldn`t be a shark specialist to recognize the hammerhead shark. These creatures stand out strongly from the entire biomass of marine fauna. They are frighten some peole and looks like something terrible, for another people they`are magnificent, so these people are go to the other side of the planet for meetiing these fishes, others consider them a valuable sports trophy. But what exactly are hammerhead sharks? In this article, a marine biologist - Nikita Kornilov and an experienced instructor of our dive center - Petr Britanov will tell you facts and personal observations about these animals.

Today, there are known only 9 species in the family of hammerhead sharks (Sphyrna).

On our island of Fuvahmulah, there are two species of these sharks, the giant hammerhead shark (Sphyrna makkaran) and the bronze hammerhead shark (Sphyrna lewini), which will be discussed in this article.

Scalloped hammerhead shark (Sphyrna lewini)

Let's talk about the scalloped hammerhead shark at first. These animals can descend to a depth of 1000 m, but are also found at shallow depths of 25 m. But it`s not so easy to meet them.

Hammerhead sharks are easy to recognize due to the characteristic shape of the head, which this fish got its name. For a long time there was no clear understanding of why she needed such a head shape, and even with the largest distance between the eyes in the animal world. Hence one of the myths that these sharks do not see well and therefore bite everything. This is not entirely true: hammerhead sharks use peripheral vision and turn their heads while swimming, thereby increasing the viewing angle to 360 degrees. Moreover, no deaths from hammerhead shark attacks have been documented, although there have been cases of attacks and collisions with people resulting in serious injuries. Hammers are considered potentially dangerous more because of their size.

These creatures migrate across all the oceans, they form numerous flocks and migrate towards the pole in summer. But they can also swimming alone. Probably exactly this type of shark that you have seen in documentaries or pictures where hundreds of hammerhead sharks arec swimming. By the way on these photographs all animals are females. Moreover, they have a very strict hierarchy, in which younger and inexperienced individuals, even immature ones, are driven away in to center of the pack by larger and more experienced females. And this is not social protection, or parental behavior, since bites and pushes into vulnerable places of "small" sharks, as well as dominant behavior with a demonstration of the belly and their size, have been recorded more than once. At the same time, there is a main female in the pack - the Matriarch, as a rule, the largest animal. There are also scouts who swim away from the pack and warn of predators or other threats with their body movements. However, for what reason the hammers gather in flocks is still unknown. This is not related to the food supply, since scalloped hammerheads hunt alone and on single prey species - mainly rays, smaller coastal sharks, bony fish and squids. It was previously thought that this was due to reproduction, but current observations and studies of flocks of these sharks say that even already pregnant females and immature individuals who do not need to find a partner swim in these flocks. However, from time to time, males swim next to the flock and look for a females for mating, having found her, the couple sails to the side, where they perform this act. By the way, the video of this process in bronze hammerhead sharks was first filmed by ordinary divers in 2017 on Cocos island.

The male, when meeting a female, demonstrates courtship behavior by displaying his size and genitals - claspers, swimming sideways to the left or right of female. She, in turn, can either swim away, or reciprocate and show her belly in response, hinting at a large number of potential offspring. After such an exchange of pleasantries, the male pursues the female and trying to grab her pectoral fin with his teeth. At the same time, the female may try to swim away, to which the male will respond with an increase in speed - this is a test for strength and endurance, good genes! Grasping the female by her fin, the male thus fixes it and fertilizes it with one of the two genital organs - claspers. Sharks have internal fertilization and the process itself lasts 3-5 minutes, after which the lovers disperse.

Hammerhead sharks have a very interesting feature associated with childbirth. Most sharks reproduce by live birth, when the embryos develop in egg capsules inside the female's body and feed on the yolk to which they are associated. When the yolk is absorbed sharks are born.

But hammerheads are completely different. They have a placental connection between the mother and the embryo. That is, the sharks in womb feed through the umbilical cord from the mother's body, with the help of an organ that works on the principle of the placenta, although it is not such in the literal sense, therefore it is called the placental connection. This distinguishes them not only against the background of other sharks, but animals in general. After all, apart from them, only mammals have a similar mechanism. Threshing sharks are born 30-50 cm long, one female bears up to 40 pupы , but usually gives birth to about 20 and bears at least 12 months. By the way, when the sharks are ready for childbirth, they gnaw their seven umbilical cords in the womb. Brutal! After birth, the cubs are left to themselves, the mother does not take care of them in any way.

On average, males reach sexual maturity with a length of 1.5-1.8 m and a weight of 30 kg, and females, which are generally larger, with a length of 2.5 m and a weight of about 80 kg. The maximum recorded length of the bronze hammerhead shark is 4.3 m and a weight of 152.4 kg.

Great hammerhead shark (Sphyrna makkaran)
The Great hammerhead shark can be found in all common habitats - the tropical waters of the Atlantic, Pacific and Indian oceans, but its numbers never reach a large number.

This is the largest shark of the whole family, their maximum recorded weight is 530 kg, and the length is 6.1 m. A serious predator in all respects. There is unconfirmed evidence of larger specimens.

The Great hammerhead shark has a straight head shape, almost a square rather than a semicircle. It can also be recognized by its characteristic crescent-shaped dorsal fin. These fish live in tropical and warm temperate waters of all oceans.

These sharks are solitary, they do not gather in groups, moreover, they prey on other smaller sharks, such as blacktip reef sharks. Often they can feed and live together and without aggression with other large species, such as tiger, lemon and bull sharks.

Like other hammerheads, the giant preys on bony fish, squid, but their favorite prey stingrays are stingrays and bracken. A venomous stingray barb lodged in the jaws or head of a hammerhead shark is common.

Giant hammerhead sharks mainly hunt at dawn or dusk. They move their heads above the seabed to spot stingrays buried in the sand. By the way, about the head - why do these sharks have such a head shape ?! There are three established reasons and benefits for this:

1. Like others, hammerhead sharks have electroreceptors on their heads - ampoules of Lorenzini, with the help of which all sharks capture electromagnetic radiation from the work of the heart or nervous system of prey at close distances. The larger the surface area, the more such receptors can be located on it, and the more accurate and successful the hunt, especially for those who are buried in the sand. It is believed that such an abundance of receptors also helps the shark in orienting itself in the ocean using magnetic fields. This opinion has developed because cases have been repeatedly described from ichthyologists involved in marking sharks that when they caught an animal, they put a tracking sensor on it, they specially took it away, and released the fish at a distance away from the place of fishing, so as not to catch it again, then all the same again caught the same individuals.

2. In addition, this head shape provides excellent visibility. Yes, yes, they see perfectly, despite the lack of a direct look - binocular vision. The fact is that in hammers, the eye on each side of the head sees almost 180 degrees, and in order to compensate for the blind zone, the shark simply constantly shakes its head from side to side, thus it sees the entire space around it. This helps both in hunting prey and in escaping predators. At the same time, all hammerhead sharks have a tapetum - a layer of cells that reflects light. Other nocturnal predators, such as cats, crocodiles, and so on, have the same. When you shine in the eyes of one of them at night and see the "shine" of the eyes, this is exactly this layer. It helps to capture the smallest particles of light and therefore the predator sees perfectly even at night.

3. Another reason to have such a head becomes visible if you look at her profile from the side. It resembles an airplane wing, and actually works like a rudder and stabilizer. The hydrodynamics of the hammerhead shark's head is so effective at this that its profile is taken as the basis for designing submarine rudders. And the predator itself can, thanks to this shape and hydrodynamics, turn around literally on the heel, and more than once.

The hunting of the giant hammer has been well studied. When a predator finds prey, it kills or stifles it with a head blow and bites the stingray from different sides by the fins, thereby stunning it and then paralyzing it. The hammer literally corresponds to the name of the tool after which it is named. It is a completely different story with eagle rays, which the shark tries to kill with one or two bites, first damaging its fins and then grabbing it head first, pressing it to the bottom with its hammer.

Like other hammers, the giant is a viviparous species with a placental connection of the embryo from the mother. Pregnancy lasts 11-12 months, in one litter there are an average of 20-40 puppies. But females give birth every 2-3 years. The maximum lifespan of these sharks is 50 years, with an average of 20-30 years.