Kauai Sea Tours


Basic Shark Biology - Part One



Is a shark a fish? Yes, it is a type of fish in which the skeleton is made out of cartilage, instead of bone. Cartilage is a flexible, semi-transparent elastic material full of cell spaces. Cartilage is formed from a complex protein which encloses a network of connective tissue fibers. Sharks and their relatives the skates, the rays, and chimaeras, are separated from the 26,000 species of bony fishes, because they are cartilaginous fishes, known collectively as the Chondrichthyes. There are between 344-368 different species of sharks and some have fossils dating back 400 millions years (dramatic compared to man which is only 2 million years).


How well do sharks see? Most sharks have a well-developed sense of vision. Their eyes can adapt to low light environments because they have a special structure in the eye called a tapetum. This reflective layer beneath the retina amplifies the light signal. Sharks have the ability to distinguish individual flashes of flickering light down to 45 impulses per second (less than a dragonfly which is 220/second, but similar to cockroach at 50/second). This enables sharks to discern the brief, flickering movements of prospective prey in the dimly lit undersea world. Some species, such as the great white shark, have been shown to have color vision. The corneas of sharks have been successfully transplanted in human eyes.


How well do sharks hear? Sound travels 4-5 times faster in water than air and is an important medium for all sea life - the shark is no exception. Sharks have very sensitive hearing, primarily in the low frequency range. Many sharks can "hear" potential prey more than a mile away. Sharks have very sensitive ears and dolphins will actually use loud "sound bursts" as weapons of defense, successfully discouraging shark attacks.


Do sharks taste what they eat? Yes, although they don't really chew their food much. Once, the taste has been identified as acceptable, they will swallow the prey practically whole.


How well do sharks smell? It depends upon what the smell is. If it is tuna, some species of shark (i.e. gray and black-tip reef sharks) can detect a dilution of 1 part in 10 billion (ppb)!, though other sharks detect a "mere" 1 part in 25 million. Needless to say, sharks have an excellent sense of smell. They are not normally attracted to the smell of human blood, but more likely they are investigating sound or electrical signals. With an ability to detect 1 in 10 ppb, sharks can probably smell a partially digested seafood meal. If you ever meet a shark bite victim you might ask them, "Did you eat seafood recently?"


Besides the shark having an excellent sense of smell, sight and hearing, how else do they hunt for prey? Sharks have an ability to detect electrical signals. In fact, they are considered to have the highest known electrical sensitivity of any living animal - 5 billionth of a volt per cm. So if they can't hear, see or smell their prey, they can still detect the electrical currents. These are given off by contracting muscles of all living prey - from shrimps to seals.


Do sharks make any vocalizations similar to the dolphin or humpback whale? I guess you can't have everything. Sharks are not capable of making any acoustic sounds, and have absolutely no sound -making apparatus. However, that does not mean they can't communicate with each other, it just means, they have evolved to use other mediums.


Are sharks intelligent? It would seem that they are more intelligent than previously thought. The larger, faster moving sharks generally have larger, more complex brains compared to some mammals and birds (specifically ratio of brain weight to body weight). In captivity, some sharks have been trained to hit specific targets for food depending on the shape and color of the target. Also, "play" has been observed with some species of sharks.


How often does a shark have to eat? It depends upon the species of shark but it may be that they can store ingested prey in their stomach for limited periods. Also they can regurgitate only unwanted items (i.e. the shell of a turtle). The Great White sharks can go 3 months without a meal.

What determines the size of a shark and how large can they grow?
The liver is the key organ which determines the overall length of a shark, and it is filled with oil that is lighter than sea water. Livers from some of the largest whales weigh 200 pounds and contain more than 18 gallons of oil! Because sharks are the muscle machines (unlike the 'blubbery' humpbacks who use their blubber for buoyancy) - how is it that they can float? While most other types of fishes have a swim bladder which they fill with air for buoyancy; the shark has adapted its liver and specifically the liver oil. But there are limitations to how much room the liver can have within the body cavity of the shark which has other important organs to house as well. Thus, the liver can only be so large, and it limits the overall size of the shark. 


Cool Facts (summarized from MARMAM, an internet e-mail group):


"Sea Lions Trained to Film & Tag Whales", Researchers are training sea lions to become their spies of the deep. A 400-pound, 17-year-old male named Beaver and his 190-pound, eight-year-old female, sake, will take on the mission of videotaping and tagging whales off Monterey Bay. With six years training, Beaver will be the first to scale the ocean depths on hand and acoustic command - perhaps as early as this winter - aiming his video camera as his fellow sea creatures, gathering footage of the secretive giants at work, play, love or war, as never before seen by man. "We've got a biased view of what whales do because most observations have been done from shipboard," said Jennifer Hurley of the Moss Landing Lab, who leads a team of 25-plus behavioral trainers. "In reality, whales spend 90-95% of their time under the sea. We need to know what's going on down there. The whales are used to seeing sea lions, so by getting sea lions to be our filmmakers, we should be able to record what the whales are doing without any undue outside influence. Sea lions respond well to training and, we think, can learn the complex skills involved in swimming beside whales while wearing a harness with a video camera or carrying a tag in their mouths and affixing the tag to the side of a swimming whale." The equipment the sea lions carry will allow instantaneous recording of depth, time, sounds and speed of the whales along with the video image. Underwater recordings will be used to analyze diving patterns, speed and underwater behaviors of gray whales, and depth, speed and feeding behaviors of humpbacks.



Upcoming, Part Two on Shark Biology will cover respiration, mating and reproduction, as well as, the fastest, oldest and deepest diving sharks. The latest 1996 Statistics on shark attacks around Kauai will also be included.


Reservations Toll Free: 800-733-7997
Tel: 808-826-PALI | Fax: 808 335 3422