Smell and Taste

Sharks are famous for their remarkably acute sense of smell. In contrast, sharks are widely regarded as having little or no sense of taste. The first of these perceptions is well documented and thus warranted, the second is less clearly so. The scent organs of the White Shark are paired structures located in capsules about mid-way along the undersurface of the snout, each covered by a relatively simple flap of skin. The taste organs of the Great White are finger-like buds scattered over the lining of the mouth and pharynx, with the greatest density occurring on the soft tissue just behind the teeth. Although most people think of smell and taste as separate senses, they are actually different gradations of the same sensory experience.

Both olfaction (smell) and gustation (taste) depend upon a dissolved sample of chemical compound fitting into a receptor cell, rather like a key fits into a lock. When a chemical fits into a receptor, an electrical change is induced in the cell that is transmitted via the nervous system to the brain, where the stimulus is interpreted. As different chemicals have differently shaped molecules, a variety of shape-specific receptors is required. The 'tightness' of the fit between chemical and receptor dictates the intensity of sensation. The major difference between the 'two' sensations is quantity of chemical sampled: smell is an imprecise sampling of small quantities of chemical carried by a transporting medium some distance from its source; taste is a detailed sampling of a large quantity of chemical in direct contact with chemical receptors. But since both smell and taste require actual contact between a shape-specific receptor cell and a dissolved chemical sample, they are merely different degrees of the same sense, termed chemoreception.

Because a shark's olfactory organs are blind sacs not in any way connected to the respiratory passageways, their external openings are termed nares rather than nostrils. While many bottom-dwelling elasmobranchs have nares adorned with elaborate flaps (often with grooves connecting the nares to the mouth) that can control water-movement over the olfactory organs, those of the White Shark have small, relatively simple flaps. As a Great White swims, scent-bearing water flows into and out of each olfactory capsule in an S-shaped pattern. The olfactory organ itself is roughly spherical and composed of a series of closely spaced, parallel lamellae (plates) studded with chemoreceptors. This arrangement maximizes the number of receptors that can be packed into the smallest possible space, greatly increasing sensitivity. The olfactory sensitivity of sharks in general is nearly legendary, fostered by countless wide-eyed stories of these predators following a trail of blood a quarter-mile (four-tenths of a kilometre) or more to its source. Laboratory tests of shark olfactory acuity have revealed that even these anecdotal tales pale in comparison to carefully measured reality. Experiments on isolated olfactory lamellae of certain skates (family Rajidae) have revealed astonishingly low threshholds to chemical stimuli responding to concentrations as low as 10-14 moles per litre of water for the amino acid serine (or about 1 molecule of serine in 1015 molecules of water). In terms of relative volume, this is comparable to detecting a golf ball in Loch Ness.

No one has yet measured the olfactory acuity of the White Shark, but there is good neurological and behavioral evidence to suggest that this species' scent tracking ability is exceptional. A 1996 paper by neurophysiologists Leo Demski and R. Glen Northcutt examined the brain and cranial nerves of the Great White. Demski and Northcutt found that 14 percent of the White Shark's total brain mass is composed of the olfactory bulbs. This is over 4.5 times the proportion Northcutt had previously found in a typical skate and twice that in the Scalloped Hammerhead (which has huge, sausage-shaped olfactory bulbs). In fact, Demski and Northcutt point out that the White Shark has the largest olfactory bulbs relative to brain mass of any cartilaginous fish examined to date. Since so much of the Great White's brain is dedicated to olfaction, it seems reasonable to conclude that scent is highly important to this species. Shark behaviorist Rocky Strong has speculated that the strong odors that characterize seal and sea lion colonies may offer White Sharks a wealth of olfactory stimuli, enabling these predators to locate concentrations of pinnipeds. A number of field and aquarium observations indicate that pheromones (hormonal secretion used to communicate within species) may play an important role in shark reproduction by signalling a female's readiness to mate. In a 1983 paper, Demski and Northcutt reported their discovery of a cranial nerve in the common Goldfish (Carassius auratus) that is apparently dedicated to the detection of pheromones; in their 1996 paper, Demski and Northcutt confirmed that this nerve is both present and well-developed in the White Shark. A dedicated pheromone-detection system may enable sexually receptive but widely separated White Sharks to find one another in the vastness of the World Ocean.

How, exactly, sharks track odors in the open sea has long been a mystery. Given their remarkable olfactory acuity and apparent zig-zag hunting behavior, it had traditionally been assumed that sharks could actually compare the relative intensity of scent received by each nare. It was believed that, by continually adjusting their course according to whichever nare received the strongest whiff, a hunting shark could quickly locate the source of any attractive odor. Some workers pointed out that a major problem with this rather complicated scenario is that seawater dissipates chemicals very rapidly. It was therefore supposed that, at any significant distance from the source of a water-borne chemical, there would be only a few molecules of attractant for every ten or hundred billion molecules of water. Under such conditions and despite its astonishing olfactory acuity it seemed extremely unlikely that a shark could detect any concentration gradient that might exist in the relatively short distance between its two nares. Yet in a 1985 paper, Peter Johnsen and John Teeter demonstrated that Bonnethead Sharks (Sphyrna tiburo) actually could detect and respond to concentration gradients between the left and right nares. The Bonnethead is a small species of hammerhead, with a 'hammer' that is only slightly expanded laterally. But it has not yet been demonstrated that non-hammerheaded sharks such as the Great White can detect which nare receives the stronger whiff.

It is more likely that the directional mechanism of scent tracking in most sharks is refreshingly simple. When a point source releases chemical compounds into the ocean, the prevailing currents establish a rapidly dissipating odor corridor. A shark's lateral line system enables it to detect subtle water movements. Therefore, when a shark's acute olfactory system detects an attractive chemical, all it needs to do is turn into the current. Sooner or later, this will bring a shark to the source of the odor. But the marine environment is huge and concentrated food sources are often few and far between, especially in the open ocean. Volatile chemicals such as the gases that might be liberated from a decaying whale carcass are dispersed through air much more quickly than through water. An intriguing 1994 paper by Russian sensory biologists S.V. Savel'ev and V.P. Cherinkov suggests that at least one pelagic shark, the Oceanic Whitetip (Carcharhinus longimanus) is able to employ aerial olfaction in the search for food. They found that the close-packed, collagen-strengthened olfactory lamellae of the Oceanic Whitetip enable this species to trap and detect surface bubbles that may carry airborne scents. Savel'ev and Cherinkov found that the loosely-packed, floppy lamellae of the Spiny Dogfish could not trap bubbles. The authors proposed that, by holding its snout tip above the surface, an Oceanic Whitetip may be able to detect airborne scents and locate distant food sources more quickly than many potential competitors.

One of the most curious behaviors of the White Shark is a habit of raising its head above the water surface by up to 3 feet (1 metre). As visually-biased creatures, people had long assumed that a Great White performing this un-fishy act was raising its eyes above the surface to visually inspect potential meals, such as hauled out pinnipeds (seals and sea lions) or humans on boats. This is a chilling thought. But, given the profound refractive shift between water and air, I doubt that the White Shark's lens-moving muscles could accommodate strongly enough to enable the animal to see clearly. Yet Demski and Northcutt's study of brain organization in the White Shark revealed that this species has particularly well developed olfactory bulbs. Perhaps like the Oceanic Whitetip the Great White can detect airborne scents, enabling it to locate large concentrations of odoriferous food, such as a floating whale carcass or pinniped rookery. Since wind is slowed by friction with the ocean surface, air movement is slower at the air-water interface than above it. By raising its head several feet out of the water, a White Shark may be better able to detect airborne scents that could indicate a rich source of food in the otherwise featureless expanse of the open ocean.

To what extent the Great White relies on its sense of taste is also a matter of conjecture. There are a few, scattered reports of other sharks apparently rejecting food on the basis of taste, but this has not been tested. In a 1980 paper, biologists Jack Ames and G. Victor Morejohn reported a perplexing mystery: at least 59 (9%) of all Sea Otter (Enhydra lutris) carcasses washed up on California beaches between 1968 and 1979 had clear evidence of having been bitten by Great Whites: fragments of White Shark teeth embedded in the open wounds. The phenomenon of shark bitten Sea Otter carcasses had been known for decades. Yet no Sea Otter had ever turned up among the stomach contents of White Sharks. Why not? In his excellent 1976 book, artist-author Richard Ellis speculated that, since Sea Otters are members of the weasel family (Mustelidae) a group which includes skunks and many other strong-smelling relatives perhaps the Otters emitted an odor that is distasteful to White Sharks. This certainly seemed reasonable at the time. But new evidence suggests a more intriguing solution to this mystery.

 

ReefQuest Centre for Shark Research
Text and illustrations R. Aidan Martin
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