Sharks may well be the most feared and misinterpreted of all predators on our planet. Fuelled by the movie 'Jaws' (1975) and its sequels, subsequent publications, movies and documentaries, the picture of sharks as blood thirsty monsters has been firmly rooted in our heads.
In real life, the majority of the about 500 shark species described today is not even physically capable of causing serious damage to a human in the water, let alone interested in approaching one.
On the contrary, attempting to closely encounter sharks in their natural environment can easily turn into a challenging and time-consuming quest, not only because of their declining numbers, but because they avoid humans and their activities whenever possible.
The Red Sea is home to some of the more tolerant or even inquisitive species; close encounters may occur e.g. with oceanic whitetip sharks (Carcharhinus longimanus), silky sharks (C. falciformis), scalloped hammerhead sharks (Sphyrna lewini), grey reef sharks (C. amblyrhynchos) and even the elusive pelagic thresher shark (Alopias pelagicus).
Observing these large predators in the open sea is a fascinating experience.
The key to understanding, interpreting and eventually predicting their behaviour is knowing how sharks perceive their world; what senses they have at their disposal and what signals they are prone to react to.
This basic knowledge can avoid some of the major misconceptions about shark behaviour, which are stubbornly clouding any objective assessment of these animals.
Different shark species or families may have specific adaptations or alterations to their sensory systems; given here is a general overview.
The furthest reaching sense that will alert a shark to a potentially interesting situation is its hearing. The only external signs of this sense are two small pores behind the eye on either side of the head, leading into the inner ear. The hearing range is about 10 – 1500 Hz (humans: 20 – 20.000 Hz, dolphins: 200 – 150.000 Hz), with most responses shown to irregular pulsating sounds lower than 400 Hz. This enables predatory sharks to pick up, for example, sounds created by the erratic flapping movements of injured or dying fish. Depending on the strength of the signal, low-frequency sound waves might well travel several kilometers under water before being significantly attenuated.
Skin flaps are directing water into the paired nostrils on either side of the shark's snout, leading it past extensive olfactory epitheliums packed with chemoreceptors. These receptors will recognise a whole array of specific molecules in the water, allowing the brain to analyse the composition of the scent trail and identify the underlying source, especially if it belongs to its prey or indicates a conspecific's readiness to mate. The acuteness of the sharks sense of smell is legendary, under controlled conditions in pools they have responded to one part of tuna extract or blood in one million parts of water! Under natural conditions in the open sea, a lot of factors can cause a chemical trail to be diluted or break up, so the exact range of this sense is likely to vary with situation.
The eyes of sharks are complex, highly evolved vertebrate eyes, and about ten times better adapted to low light conditions than human eyes. Species living in clear or shallow water additionally have colour vision, with photo-receptors that are most sensitive to blue-green wavelengths. To protect the vulnerable lenses, some sharks have a third eyelid, the nictitating membrane, to be drawn over the eye just before attacking prey or closely approaching objects; others roll their eyeball under the tissue surrounding the eye away from potential injury.
(Lateral line system). All along both sides of the body sharks have a system of water filled pores and connected canals, whose pressure-sensitive haircells detect differences in water pressure and acceleration. This information is related to the brain and allows the shark to identify and locate prey organisms and avoid potential predators. Depending on the strength of the water movement and pressure changes, the lateral line system is reported to function over a distance of about two body lengths to more than 100 m.
(Ampullae of Lorenzini). Little dark pores all over the sharks' snout and lower jaw mark the exit points of channels connected to small sacs, the so-called Ampullae of Lorenzini. They are filled with mucus and are sensitive to small electric currents, which helps to detect the minute electric activity any living organism produces, as well as allowing the sharks to use the magnetic field of the earth for orientation. Again, information on this sense's maximum range differs, in this case from 2 m to about 20-30 cm. Hammerhead sharks with their flattened, widened heads and enlarged sensory area are the most efficient in their use of this sense.
This contact sense relies on open nerve endings under the sharks' skin that react to direct physical pressure. Some sharks, especially bottom-dwellers, have barbels around their mouths giving them additional sensitivity when probing the sand for food.
This second contact sense is formed by taste buds in the sharks' mouth & gullet. They detect the same principal tastes as humans (sweet, salty, bitter & sour), and sharks tend to nudge and mouth objects to decide if they are edible.
Objective assessments of shark behaviour throughout the last few decades have revealed surprising signs of social interactions and even cooperation; sharks have developed fascinating physiological solutions to be able to inhabit basically all available marine habitats; they learn and adapt their strategies to different environmental situations.