Making Sense of Senses: How Pelagic Sharks Find Prey - by former intern Shane White

When asked to name senses, most people would describe the five we can relate to: sight, hearing, smell, touch, and taste. These senses help us interpret our surroundings and react accordingly. However, if you live in the open ocean, your senses won’t be the exact same due to differences in environmental obstacles you need to overcome in order to pinpoint where food is, a process called prey localization. This article will discuss the processes of how a shark would close in on a distant food source using its array of senses. Although the use and efficiency of these senses vary from species to species, the characteristics and sensory distances depicted in this article are most representative of pelagic sharks. Pelagic species are those that live in the open ocean and tend to travel longer distances for reproduction or feeding purposes, including oceanic whitetips (Carcharhinus longimanus), shortfin mako sharks (Isurus oxyrhincus), and great hammerheads (Sphyrna mokarran) which are found in the coastal waters of Bimini from January to April (Figure 1).

Figure 1: A great hammerhead shark (Sphyrna mokarran) named Queen. (Image: Chelle Blais, Bimini Biological Field Station)

Over 1,000 m from food source

Typically, a pelagic shark’s hearing has the furthest distance related to its other senses. They’ve evolved to hear low frequencies in the range of 10-1,100 Hz, suitable for low-pitch sounds like the erratic swimming of struggling fish. The typical human hearing range is between 20-20,000 Hz, so humans are able to hear much higher pitches, but aren’t able to hear the low frequencies a shark would. Sound also travels much faster and further in water than air due to water being incompressible and much denser, meaning the molecules of water are closer together which allows sound waves to travel much more effectively. Sound waves dissipate quicker in deeper waters and with softer seafloor sediments.

~100 - 1,000 m from food source

The sense of smell, or olfactory system, of a shark is incredibly sensitive to amino acids, which are the chemical building blocks of proteins found in all shark prey. This is useful for detecting struggling or dying animals, as blood and other bodily fluids have a variety of amino acids. Some sharks are able to detect one part of blood in up to 20 million parts of water. For comparison, this would be like a human trying to smell where a single pinch of salt is in a 2,000-pound pile of potato chips. Depending on the purity of the scent, as well as water conditions like currents and tides, sharks have been known to detect strong scents, such as dead whales, over several kilometers. (Figure 2).

Figure 2: The nostrils (called nares) of a large lemon shark. Sharks use one nostril at a time to follow a scent and navigate to a prey source. They navigate in the direction of the nostril that caught the scent first, even if the other nostril detected higher concentrations of the smell. (Image: Chelle Blais, Bimini Biological Field Station)

~10 - 50 m from food source

As sharks approach potential prey, they begin to look for visual cues, dependent on water clarity and light. Many pelagic sharks have large eyes suitable for chasing agile prey, like barracuda and tuna. Many pelagic sharks have body colorations suitable for seeing prey before they’re seen. Many sharks, pelagic and otherwise, posses countershading along their body where the underside is lighter in color than the topside. This provides a shading camouflage where the shark is harder to see by prey from above, but also harder to see by predators from below (Figure 3).

Figure 3: The coloration of Caribbean reef sharks (Carcharhinus perezi). They possess a countershading body plan, a form of camouflage from prey and predators. From below, this allows the shark to blend into the sunlight above. From above, this allows the shark to blend into the sediment below. (Image: Chelle Blais, Bimini Biological Field Station)

~1 - 10 m from food source

Using tiny mechanosensors located in canals in the skin, a shark can detect struggling prey from a meter or more away. The most notable source of this sense is the lateral line you’d notice on many fish species, including lemon sharks (Negaprion brevirostris) (Figure 4). These pores along the skin, known as hair cells, allow sharks to feel pressure and directional changes in the water around them, like currents or the movement of nearby swimming animals. This can be especially helpful to finding camouflaged prey or when hunting at night.

Figure 4: The lateral line of a juvenile lemon shark (Negaprion brevirostris). The lateral line is found on both sides of the shark’s body, extending from head to tail. (Image: Chelle Blais, Bimini Biological Field Station)

Less than 1 m from food source

An odd sense sharks use to find prey is their electroreception, where sharks can detect weak electric fields generated by muscle contractions of potential prey. Scattered across the head and faces of sharks are tiny organs called the ampullae of Lorenzini, which detect voltage differences that are translated by the brain into a three-dimensional map of electric fields around the body. Great hammerheads are known to use this strategy to predate on stingrays hiding beneath the sand (Figure 5). There have even been cases of shark bites found on underwater electrical power lines, likely from them mistaking the electricity of the cables for hiding fish.

Figure 5: The great hammerhead (Sphyrna mokarran). Notice the magnification of some ampullae of Lorenzini, small black pores used in electroreception, which are scattered across the underside of the shark’s head. The head of hammerheads, termed the cephalofoil, is a useful shape for both electroreception of the sea-floor as well as pinning prey for food handling. (Image: Chelle Blais, Bimini Biological Field Station)

On contact with food source

We can handle food with our hands or silverware to see if it’s edible. Although sharks don’t have hands, they use their snout to bump objects to feel how soft or hard potential prey are, allowing them to decide if it’s worth a test bite. With the dangers of hunting prey that can sometimes fight back, such as other sharks or stingrays, sharks often need extra protection for their vision when feeding. Some species, like tiger sharks (Galeocerdo cuvier) use nictitating membranes to cover the eye when they go in for a bite, whereas some other species like the white shark (Carcharadon carcharias) roll their eyes back into their head, a behavior known as ocular rotation (Figure 6). Lastly, on contact with a piece of food, sharks will bite and taste whether the prey is worth eating or not before finally ingesting it. This helps prevent the sharks from eating vegetation or venomous animals.

Figure 6: The white nictitating membrane of the tiger shark (Galeocerdo cuvier) covering the entire eye (left), compared to the ocular rotation of a white shark (Carcharadon carcharias) when surfacing (right). Although these mechanisms are biologically different, they serve similar purposes. (Images: Chelle Blais - Tiger Beach, Bahamas | Mossel Bay, South Africa)

Finding suitable prey in the ocean is a demanding task, especially when your food is so well-adapted to hiding or evading. The pelagic sharks’ plethora of senses allow them to find food over vast distances and in a variety of conditions (Figure 7). However, a lot of human activity may intercept a shark’s attention from food or other essential activities. The smells of oil spills and other pollution may repel sharks from their usual feed-ing spots, whereas the smell of fishing and baiting activities are known to attract sharks. The sounds of oceanic drilling and commercial boat traffic may deter sharks from their usual migration patterns or reproductive areas. All of these sensory interruptions, and many more, may detrimentally affect shark behavior over time, at the cost of the sharks’ and aquatic environment’s overall health. With sharks being essential predators for a variety of marine ecosystems, further conservation efforts must take into account where, how, and to what magnitude human activity affects shark sensory. The further study of shark sensory mechanisms among different species and their corresponding behaviors may allow us to more effectively plan environmental stewardship practices and protective regulations relative to shark conservation. The lab of the Bimini Biological Field Station Foundation is located adjacent to a variety of aquatic environments, such as shallow mangrove lagoons and the deep Gulf Stream. And so, the BBFSF is invaluable for understanding the sensory mechanisms & behaviors of subtropical sharks in disparate environments.

Figure 7: Illustrated interpretation of a pelagic shark’s sensory and range in the context of prey localization of a struggling fish. Variable by species and environmental factors. Touch and taste not shown. (Source: Made by Shane White, inspired by lecture content from BIOEE101x:Sharks! of Cornell University).

Making Sense of Senses: How Pelagic Sharks Find Prey - by former intern Shane White

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