lorenzini – BioGeoPlanet

I've researched sharks for many years and some of my most popular presentations feature sharks and their amazing abilities. Sharks have senses that seem magical since we humans have nothing of the kind. Sharks have seven senses including two that humans do not possess 1. electroreception for electric fields, and 2. lateral lines to detect variations in water pressure. The other five senses are sight, smell, hearing, touch, and taste. Sharks have senses so acute that they can smell one drop of blood 0.25 mile (0.4 km) away and detect an electric field as tiny as 125 microvolts (millionths of a volt). Let’s begin with the two most intriguing senses – both of which are totally foreign to us humans: electroreception and hydrodynamic reception. Read on to discover how these complimentary senses help sharks locate and devour prey and avoid predators. Electroreception It might not seem evident at first, but the ability to sense electrical fields could prove remarkably handy if you are a shark searching for food in the vast ocean realms. The electro-sensory system of a shark is so sensitive that it can detect the tiny electrical currents generated by the muscles of a fish swimming nearby, a great advantage for those trying to find prey in the pitch-black. This ability would be like a magical sixth sense to us, considering that a shark can detect the electrical signals given off by the beating of a fish’s heart (or a human heart for that matter). Consider the implications! As soon as you jump into the ocean, every shark in the vicinity knows you are there. There’s no hiding behind a boat or chunk of coral. If your heart is beating or your muscles are twitching, sharks will sense the electrical current. Sharks have by far the highest level of electrical sensitivity of any animal on the planet. The electrical sensitivity of a shark is equivalent to being able to detect the electric field of a flashlight battery connected to electrodes placed 10,000 miles (16,000 kilometres) apart in the ocean. Seems like a stretch, doesn’t it! Believe me, this is based on extensive research (see King et al. 2018). In the 1970s, Adrianus Kalmijn was one of the first scientists to confirm suspicions that sharks could perceive weak bioelectric fields, through his innovative studies. How does Electroreception Work? If you look closely at the face of a shark, you’ll notice hundreds of tiny black dots. If you were to swim closer until you were face-to-face with the shark (watch those teeth!) you could see that each dot is a pore in the shark’s skin. Electroreceptor pores on the snout of a Leopard Shark. Photo by Albert Kok on Wikimedia CC-by-03 These pores are electroreceptors - small jelly-filled tubes that are directly exposed to sea water. Deep at the end of each pore there’s a bottle-shaped cell referred to as the ampullae of Lorenzini, after the Italian anatomist who first described them in 1678. What Lorenzini Found: A Network of Tubes Each pore leads to a long transparent tube filled with jelly.Some of the tubes are small and delicate, while others are nearly the size of spaghetti.Tubes can be short or several inches in length.Deep within the shark’s head, the tubes congregate in several large masses of clear jelly. Diagram showing network of electroreceptors on the face of a shark. Wikimedia public domain The jelly inside each tube is highly conductive, and electric charges travelling through the tube create voltage differences between the pore opening and the ampulla. These voltage differences activate sensory nerves in the ampulla that send signals to the brain, where the information from thousands of receptors is integrated and interpreted. When an electric charge enters a pore, it travels through the highly conductive gel in the canal to the ampulla where special cells detect the current and send a message along nerves to the brain. Graphic credit: PBS Digital Studios Here’s an excellent video showing how sharks can use their electroreceptors to detect electrical fields generated by prey, even when hidden in the sand. I find it surprising that the sensory mechanism for electroreception involves modified hair cells. I wouldn’t associate hair cells with electricity, but there you go. These modified hair cells line the deepest part of each ampulla. Most sensory hair cells respond to bending, but these cells are different: they respond to local reversals in electrical polarity. A net negative charge inside an ampulla causes an electrical change in each hair cell, triggering a signal to the brain. It is interesting to note that the ampullae of Lorenzini are extensions of the lateral line system, my next topic of discussion in this article. Both electroreceptors and hydrodynamic receptors incorporate modified hair cells as the key functional unit. Who knew that fish had hair cells to begin with!? Now we’re beginning to understand how important modified hair cells have been to the success of sharks over hundreds of millions of years of natural selection! Sharks, like most fish, have adapted hair cells as the functional unit used to sense sound, vibration, and electrical stimuli. Lateral Lines and Hydrodynamic Receptors Hydrodynamic sensors are not unique to sharks - nearly all fish have networks of sensors along each side of their bodies known as lateral lines. These lines of receptors are highly sensitive to changes in water pressure. Even though comparatively little is known about the function and roles of the lateral line system in sharks and rays, recent findings are highlighting the importance of this mysterious sensory system in the life of a shark. Lateral line system of a shark. In general, lateral lines have an important role to play in most aspects of a shark’s life: from how they swim and orient themselves in the water, how they find and capture prey, and how they avoid crashing into obstacles or even other sharks. As one example of the usefulness of this seemingly magical sensory mechanism, sharks can use the lateral line receptors to track fleeing prey by following the vortices left behind in the water. How does the System of Lateral Lines Work? Here’s a quick overview of what you need to know about this fascinating topic. Fish have unique hydrodynamic receptors in the system of lateral lines that detect motion, vibration, and pressure gradients in the water. Once you know what to look for, the lateral lines are readily apparent on most species of fish as a distinct line that runs along each side of the fish from head to tail. Lateral lines perform the job of hydrodynamic detectors, responding to even the slightest variations in water flow and pressure. Fish use the lateral lines to avoid obstacles and to move efficiently relative water currents. Hydrodynamic sensitivity also helps to optimize feeding behavior when locating, tracking, and targeting prey. The hydrodynamic receptors - referred to as neuromasts - are small receptor organs in the fish’s skin that have hundreds or even thousands of sensory hairs arranged inside the organ. Neuromasts are primarily arranged in two main tubes that constitute the lateral lines. When water flows past a neuromast, it bends the sensory hairs inside which generates a stimulus that is transmitted to the brain. Researchers have differentiated between two types of neuromasts or sensory units associated with the lateral line system (Ristroph et al., 2015 and Gardiner & Atema 2014). The better known of these sensory units are those that are embedded within the lateral line canals – called the canal neuromasts. Difference between Canal Neuromasts and Superficial Neuromasts CANAL NEUROMASTS are located inside the lateral line canal and are sensitive to pressure gradients between the pores that perforate the canal wall. As water pulses through the pores, these sensors monitor the pressure differences. In addition, there are freestanding sensors, referred to as SUPERFICIAL NEUROMASTS, found on the surface of the skin and dispersed over large portions of the body surface. In sharks, superficial neuromasts are distributed along the head and body in shallow pits on the surface of the skin – these are called pit organs. These freestanding sensors are primarily tuned to changes in water velocity since they are exposed and not hidden inside the canal. Why Do Sharks have Distinct Receptors for Water Pressure and Water Velocity? Why would a shark need two sets of sensors associated with the movement of water? It begins to make sense when you consider that sharks are usually swimming through a current, even if it is just created by their own swimming motion. The freestanding sensors are constantly monitoring changes in water velocity and nearly constantly sending signals to the brain. But what if a prey species suddenly swims by in the dark? How would the shark detect its prey given the constant firing of the freestanding sensors, somewhat akin to background “noise”? This is where the pressure sensors or canal neuromasts would kick in. In contrast to the freestanding sensors, canal neuromasts are hardly affected by background water currents due to their position within the lateral line canals. Vibrations associated with the movements of potential prey or predator would be picked up by the lateral lines as pressure changes that would stand out as “anomalies” against the background flow of water currents. BANG! Even the tiniest differences in water pressure would alert the shark to the presence of potential food, or a potential threat, swimming nearby. Imagine how useful this would be in murky water or in the pitch black of the deep oceans. By comparison just imagine putting ourselves in the same environment of dark or murky water with our five limited senses! We wouldn’t be able to see much and our skin wouldn’t be capable of discerning tiny pressure differences in the water. We would basically drift around in our bubble of ignorance wondering what was happening around us – until… we felt the skin of some creature brushing up against us! Or the razor edge of some creature's teeth…! Smell Sharks have an acute sense of smell that is used as their primary means of finding prey, especially at a distance. To give us some idea of the importance of the sense of smell consider that as much as two thirds of the weight of a shark's brain can be dedicated to smell. Brain of the lemon shark showing the size of the olfactory bulbs relative to the rest of the shark’s brain (Meredith et al, 2013) Sharks have two forward-facing nostrils positioned on the underside of the snout. Water enters the nasal passage and moves past folds of skin covered with sensory cells. Unlike humans, sharks use their nostrils strictly for smelling and not for breathing, since the nostrils are not connected to the mouth or gills. Sense of smell also plays an important role in reproduction and mating behavior of sharks. Males are highly sensitive to female pheromones and can use their olfactory organs to find and track potential mates. Some sharks can detect their prey at concentrations as low as one part per 10 billion – equivalent to one drop of blood or bodily fluid in an Olympic-sized swimming pool! Sharks have a directional sense of smell (like our sense of hearing) with the ability to differentiate between the strength of stimuli in each nostril, as well as the ability to detect tiny differences in timing. Acutely sensitive olfactory organs give sharks the ability to home in on prey from a long distance. Smell can be used as an early trigger, long before some of the other senses begin to receive signals. Sight Most sharks have excellent vision although eyesight varies from species to species. In general, the eyes are very similar to human eyes, with cornea , lens, retina, iris, pupil. The shark’s vision is up to 10 times more acute than ours in clear water. As far as hunting goes, the sense of sight is only effective at relatively close quarters of less than 100 feet or 30 meters. As far as we know, there are about twenty species of shark that can detect color with eyes that contain rods and cones. But not all sharks have the same cones, bull sharks, for example, cannot see in colour. Like many animals adapted to living in low light conditions, sharks have a ‘tapetum lucidum’. This is a reflective layer of mirrored crystals that lies behind the retina, allowing nocturnal and deep-water species to hunt more effectively. By adjusting the crystals to reflect light back onto the retina the shark can amplify the strength of an image. Hearing Sharks have a good sense of hearing, especially at lower frequencies of 10 to 1,000 Hertz. By way of comparison, human hearing is in the 20 to 20,000 Hertz range, with most every day sounds around 250 - 6,000 Hertz. Human ears are more sensitive to high-frequency sounds than the ears of a shark. Since sound travels far and fast underwater, hearing is a good way of gathering sensory information in the vast ocean depths. When hearing is combined with the sense of smell, sharks are well equipped to find any weak or injured animals within a large area. Sharks don’t have external ears, but they do have small holes on either side of their head, behind the eyes. These lead to the inner ears where three cartilaginous tubes are filled with fluid and lined with hair cells. (Not again with the modified hair cells!) Sound waves make the tiny hairs vibrate which sends signals to the brain. In addition to the sense of hearing, the three semicircular canals in the shark’s inner ear provide the shark with information related to balance. Taste Sharks have taste organs that seem to play a different role compared to our sense of taste, with an important association between biting and tasting. While human taste buds sit on the tongue, sharks have taste organs on the jaws near the teeth. The highest concentration of taste buds is directly behind the last rows of teeth in both the upper and lower jaws. It appears that sharks might have to take a bite before getting to taste their food! What’s the best way of sampling food if you’re a shark! Take a bite! This helps to explain why sharks are known to take ‘test bites’ when assessing potential prey for palatability. This also might help explain the relatively high survival rate of shark attack victims! Touch This is a touchy subject when it comes to sharks. They have armoured skin with tiny teeth called dermal denticles covering the entire body… so where can they have a sense of touch? Well, sharks do indeed have nerve endings under their skin. Some sharks, like the nurse shark, even combine the senses of taste and touch in whisker-like feelers called nasal barbels. Sharks even have pressure sensitive nerves in their teeth! They can use their teeth to learn more about an object. I don’t want to be one of those objects! Seems like we’re getting back into ‘test bite’ territory. As well as direct touch, sharks have what some people refer to as distant touch – which is in fact pressure sensitivity through the lateral lines (covered elsewhere in this article). Help Celebrate Amazing Sharks The combination of seven senses that I’ve covered in this article make the shark an incomparable predator. If you were to sit down and take a couple of million years to design the ultimate killing and eating machine living in the oceans you would likely come up with a shark. Sharks are among the greatest success stories of our planet, with more than 3,000 species having evolved over the past 400 million years. They are more ancient than dinosaurs, and where here long before trees or flowering plants appeared. Sharks have proven themselves to be remarkably resilient, having survived mass extinctions that wiped-out millions of other species over nearly half a billion years. But will they survive us!? Sharks are being killed by the millions each year on a scale that is hard to fathom. 75-100 million sharks are killed each year, many for their fins alone. After millions and millions of years of exquisite adaptation, several species of shark may not make it to the next century. Over the past four hundred million years, sharks have become uniquely adapted to their ocean environment with seven highly refined senses: smell, hearing, touch, taste, sight, electroreception, and hydrodynamic reception. Many sharks, such as the Great white (Carcharodon carcharias), are apex predators that play a key role in the ecosystems they inhabit. In many ways, sharks help create their own habitat. Scientists have discovered that the healthiest reef ecosystems have large numbers of fish and high biodiversity. AND these healthy ecosystems have surprisingly high numbers of sharks! The sharks keep ecosystems healthy by controlling mid-level predatory fish which prey on the smaller reef fish. This takes the pressure off colorful reef fish so they can become more abundant! I hope you can help celebrate these awesome creatures by telling others about the “magic” of shark senses and how they work! References Fields, R. Douglas (2007) The Shark's Electric Sense: An astonishingly sensitive detector of electric fields helps sharks zero in on prey. Scientific American. 297. 74-80. https://doi.org/10.1038/scientificamerican0807-74 Gardiner, Jayne & Atema, Jelle. (2014). Flow Sensing in Sharks: Lateral Line Contributions to Navigation and Prey Capture. https://doi.org/10.1007/978-3-642-41446-6_5 Kalmijn, A. J. (1971) The Electric Sense of Sharks and Rays. Journal of Experimental Biology, Vol. 55, pages 371–383. King, B., Hu, Y. and Long, J.A. (2018), Electroreception in early vertebrates: survey, evidence and new information. Palaeontology, 61: 325-358. https://doi.org/10.1111/pala.12346 Martin, R. Aidan (2003). Elasmo research article on electroreception. http://www.elasmo-research.org/education/white_shark/electroreception.htm Meredith, T.L., Kajiura, S.M. and Hansen, A. (2013), The somatotopic organization of the olfactory bulb in elasmobranchs. J. Morphol., 274: 447-455. https://doi.org/10.1002/jmor.20106 Mogdans J., Engelmann J., Hanke W., Kröther S. (2003) The Fish Lateral Line: How to Detect Hydrodynamic Stimuli. In: Barth F.G., Humphrey J.A.C., Secomb T.W. (eds) Sensors and Sensing in Biology and Engineering. Springer, Vienna. https://doi.org/10.1007/978-3-7091-6025-1_12 Ristroph, L., Liao J. C. and Zhang, J. (2015) Lateral Line Layout Correlates with the Differential Hydrodynamic Pressure on Swimming Fish, Physical Review Letters, 114, 018102 https://doi.org/10.1103/PhysRevLett.114.018102 Check Out Our TOP Articles for Even More Fascinating Creatures How do Octopus Reproduce? (Cannibalistic Sex, Detachable Penis)Do Jellyfish have Brains? 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