Hearing and Vibration Detection
In sharks, hearing and vibration detection are fundamentally linked. In the Great White and other sharks, the inner ears are nestled inside the posterior part of the braincase on either side. The only external manifestation of a shark's ears are two small openings located near the long axis on top of the head, just behind the eyes. A shark's main vibration sensing mechanism is the lateral line, which is visible externally by a row of tiny pores along each flank. Anteriorly, this system of pores branches out over the shark's head in complex patterns. Despite their apparent differences, the shark inner ear and lateral line system are based on the same basic mechanism.
The functional unit of both the shark inner ear and lateral line is the hair cell. Each hair cell consists of a more-or-less globular basal body from one end of which project a series of cilia (hair-like structures). One of these cilia - called a klinocilium - is much longer than the others. The klinocilium extends into a gelatinous dome called a cupola, which is partially exposed to the external environment. On the opposite pole of the basal body is a bundle of five or so sensory nerves. Since water conducts vibrations quite efficiently, any oscillation in the surrounding liquid medium causes the cupola to jiggle, jelly-like. This jiggling causes the klinocilium to bend, which, in turn, provokes the lesser cilia surrounding it to bend in response - reminiscent of cascading dominos. Bending of a hair cell's cilia induces an electrical change in the basal body, which is transmitted - via chemical messengers called neurotransmitters - to the sensory nerve and on to the brain, where the stimulus is interpreted as sensation. All this complex jiggling and chemical choreography is highly sensitive to even the tiniest vibration in the surrounding water.
The shark inner ear is a fluid-filled structure consisting of a cartilaginous sac to which are attached three D-shaped cartilage tubes. These fluid-filled tubes are set at right angles to one another and are lined with hair cells. Each D-shaped tube responds only to accelerations within the plane parallel to its orientation. Thus, collectively, the three D-shaped tubes are sensitive to accelerations in all three geometric planes and grant the shark a simultaneous sense of its movements in all three-dimensions of its liquid environment. Parts of the cartilaginous sac have modified hair cells in which the klinocilium attaches to a calcareous (calcium carbonate) mass called an otolith (or 'ear stone'). These otoliths respond to gravity, providing the shark with information about its orientation in the water, be it head up, head down, on its side, right-side-up or upside-down. In a fascinating 1981 paper, otolaryngyologist Jeffrey Corwin reported that in some sharks one of these otolith-equipped parts of the inner ear - called the macula neglecta (because it had long been ignored by sensory physiologists) - responds particularly strongly to vibrations through the top of the skull. Based on his functional morphology studies of many shark species, he proposed that the macula neglecta may provide actively predatory sharks with an enhanced ability to hear sounds originating from above and in front. If true, this would grant sharks directional hearing, despite the close-set arrangement of their inner ear mechanisms. The whole inner ear structure is connected to the outside of the shark's body by yet another fluid filled cartilaginous tube. Thus the shark inner ear is unique among vertebrates in that the fluid inside this organ is in direct contact with the watery medium outside the animal's body.
The shark lateral line consists of a fluid-filled, hair cell-lined tube extending along each flank, just beneath the skin. This tube connects to the external environment via secondary fluid-filled tubules that branch off from the main tube and penetrate the skin at regular intervals. Vibrations ocean are transmitted by successive fluid compressions and rarefactions from the secondary tubules to the main tube. These vibrations then jiggle the gelatinous domes of hair cells lining the main tube and alert the shark. As the lateral line system extends along most of a shark's body, it grants the animal a highly directional sense of movements of potential predators and prey in its immediate vicinity. Sharks that have been temporarily blinded in experiments have been able to avoid colliding with the wall of the tank which contained them, apparently by sensing water waves reflected from the tank wall. Thus, even in highly turbid water — where vision is all-but useless — a shark can tell exactly where obstacles and other creatures are, even if it cannot see them.
Sound is a multi-stage event that requires four components to occur: a source of vibration, a transmitting medium, a receiving detector, and an interpreting nervous system. Sound energy is carried by the oscillation of particles composing a transmitting medium. In the case of sharks, the transmitting medium is the water through which they swim. Thus, distinguishing what a shark hears with its inner ears from what it senses as vibrations via the lateral line is a kind of Gordian knot comparable to separating singer and song. As a result, many shark sensory biologists refer to the combination of inner ears and lateral lines as the acoustico-lateralis system. Experiments with various species by Arthur Myrberg, Donald Nelson, and their co-workers have revealed that sharks are most attracted to irregular, pulsed sounds of relatively low frequencies. Field and laboratory experiments have demonstrated that sharks can hear sounds with frequencies ranging from about 10 Hertz (cycles per second) to about 800 Hertz, but are most responsive to sounds less than 375 Hertz. In contrast, most adult humans can hear sounds ranging from about 25 Hertz to roughly 16,000 Hertz (young children can hear sounds up to 25,000 Hertz, but much high-frequency sensitivity is lost by late adolescence.) Although sharks and humans detect some low frequency sounds in common, sharks can hear sounds that are inaudible to us. A shark's hearing is adapted to detecting very low-frequency vibrations such as those made by a struggling fish.
Recently de-classified U.S. Navy studies have revealed that the ocean is criss-crossed by meandering ribbons of very cold, dense water surrounded by warmer, less dense water. Since sound travels more efficiently in dense materials than in rare ones, these liquid ribbons act as 'sound tunnels'. Sound inside these tunnels bounces along like light in a fiberoptic cable, with very little loss of energy to outside water masses. During the height of the Cold War, the Navy used a $16 billion system of underwater microphones placed within these networks of sound tunnels to keep tabs on the positions and activities of enemy submarines (the system is known by the acronym SOSUS, for SOund SUrvaillance System). Some cetologists believe that whales may use these sound tunnels to communicate across entire ocean basins. Due to its physiological heat-retaining mechanisms, the White Shark may be able to penetrate these sound tunnels, listening for the low-frequency sounds of potential prey inside the cold, dense ribbons of seawater.
To the best of my knowledge, no one has yet measured the hearing or vibration-detecting capabilities of the White Shark. But we do have some intriguing qualitative observations that suggest these functionally linked senses may be important to this species. In the earliest days (1970's and early 80's) of organized cage diving with White Sharks off South Australia, luring these animals with chum (ground bait) required many days or even weeks of sustained effort. By the mid-1980's and early '90's - despite increased landings of White Sharks by local sport anglers - luring these animals required substantially less time, often only a matter of three to six days. From sonic tracking studies conducted on White Sharks, we know that this species cruises at an average speed of 2 miles (3.2 kilometres) per hour. If the average time required to lure a Great White to a cage is three days (not an uncommon waiting period), the shark may have traveled from a distance as great as 144 miles (232 kilometres). This is farther than has ever been demonstrated — or suggested — to be within any shark's scent-tracking ability. However, field experiments suggest that hearing may be a shark's longest-range sense, some species apparently able to hear artificially produced sounds at least several miles (kilometres) away from their source. It is feasible that the White Sharks of South Australia had learned to associate the metallic clanging of shark cages with the handout of a free meal, gathering round to be fed like overgrown puppies.