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Introduction:
Many paleontologists are very interested in living groups, because the
study of the living organisms can both unlock their evolutionary
history and provide important keys towards interpreting their fossil
record. Some living groups have ancient histories. Today, sharks and
rays (elasmobranchs) are represented by over 600 species that show a
remarkable range of ecological and morphological diversity. Although
many people are told that sharks are primitive in comparison to other
groups, this is not true. Many sharks are efficient and specialized
hunters that have thrived for millions of years. This small exhibit
shows an amazing predator: the great white shark (Carcharodon
carcharias). The white shark is found in temperate waters throughout
the world's oceans, and it is an important, though not common, predator
in California's coastal habitats.
Food And Feeding Behavior:
White sharks are predatory animals that begin life by feeding on fish,
rays, and other sharks, and as they grow, switch to feeding on marine
mammals and scavenging on large animal carcasses. Their first mammalian
prey are usually the small harbor seal, but as the sharks increase in
size, they become large enough to eat sea lions, elephant seals, and
small toothed whales. Attack strategy consists of a swift, surprise
attack from below, inflicting a large, potentially fatal bite. The
pinniped often dies from massive trauma or blood loss, but the bites may
be superficial or misplaced on the body, allowing the seal to escape and
survive the attacks with their scars as witness. Large white sharks will
also scavenge on the carcasses of whale sharks, and on the fat-rich
blubber layer of dead whales. They will occasionally feed on sea turtles
and sea otters, and are known to attack, but not eat, humans.
Habitat And Distribution:
The waters off central California offer a rich bounty of food for white
sharks, and every summer and fall they actively feed in nearshore areas.
The Farallon Islands, a national wildlife refuge about 27 miles off San
Francisco, is a common feeding ground for the sharks. Four species of
pinnipeds, the northern elephant seal, the California sea lion, the
Steller sea lion, and the harbor seal, live around and breed on these
islands, making a plentiful food resource for the white sharks. The
coastal waters along central California, especially around the Ano Nuevo
State Reserve and along the Marin Headlands, is another common feeding
area for the sharks. In the summer the sharks feed on seals and sea lions
along the coast as far north as Oregon and occasionally the Gulf of
Alaska, and in the Fall, they turn south and feed along the offshore
islands. It is believed that female white sharks migrate to southern
California to give live birth to their offspring. In abnormally
warm-water (El Niño) years, white sharks are more plentiful off
central California because both they, and their prey, are shifted north.
1984-85 and 1991-92 show high trends in white shark predatory activity,
and were the warmest oceanic seasons in recent history.
Sharks And Media:
Media sensationalism and widespread ignorance has given the white shark a
bad rap. Although the species is responsible for an average of 2-3
non-fatal attacks on swimmers, surfers, and divers each year, its role as
a menace is exaggerated; more people are killed in the U.S. each year by
dogs, than have been killed by white sharks in the last 100 years.
Additionally, scientific studies show that population of white sharks is
low, with perhaps fewer than 100 adult animals in the state's waters.
White sharks are important predators in the marine ecosystems of the
California coast, and the people of California recognize that. In 1992,
the white shark was placed on the protected species list for the state of
California, and is legally protected from unlawful killing or
exploitation. The original bill was supported by both scientists and
fishing organizations, surfing clubs and diving groups, private citizens
and an array of government organizations.
Studying the white shark requires the use of media equipment. Dr. Douglas
Long has been doing research on the ecological interactions, in
particular the feeding habits and predatory behavior of great white
sharks on marine mammals along the coast of central California for several
years now. His research has included autopsying dead marine mammals,
studying the feeding behavior of sharks, and trying to estimate shark
population size. These images are directly related to the latter two
aspects of his research. From a post on the Farallon Islands, researchers
watch the waters for injured marine mammals and signs of sharks feeding.
When they spot a feeding shark, they travel in a small boat to the site,
and when the shark comes close they stick a small hand-held waterproof
video recorder under the water and film the shark as it passes by . The
sharks are identified by characteristic marks like scars and skin
blemishes. The ability to visually identify sharks allows researchers to
tell more about migration, ranges, and the number of individual sharks in
the area. Douglas believes that there are a suprisingly small number of
individual sharks off the California coast, and that they have larger
ranges than previously thought.
Cold Hard Facts About The Great White Shark:
• Greek Name - Carcharodon carcharias:
This is derived from carcharos meaning "ragged" and odon
meaning "tooth".
• Skeleton:
Like all other sharks, the GW's skeleton is cartilaginous (composed of
cartilage) instead of bony. This is chiefly why we don't have any
fossilized shark bodies - just their teeth.
• Size:
There isn't an agreed upon size range for the GW but most experts agree that
the length of the shark is usually about 12-16 feet with the maximum figure
being about 19-21 feet (although the 21-footer may be apocryphal... it's a
record from 1948!)
• Teeth:
The teeth of the GW are serrated like a knife. In fact, the serration's make
the tooth sharp enough to saw wood!.
• Attacks:
Most attacks aren't fatal. In fact, a current hypothesis theorizes that the
GW will attack it's prey with a massive first bite that causes huge blood
loss - and thus, loss of life - and then returns to feed on the carcass.
Therefore, the GW is a manbiter, not a man-eater.
Predatory Behavior Of The Great White Shark:
To give an idea of what a GW usually preys upon, think of those cute seals
and sea lions (pinnipeds) you often see performing at Sea World. They are the
preferred food of the GW - over other fish, other sharks, or even your Aunt
Sally.
Seals have highly developed hindflippers and smaller, underdeveloped
foreflippers. Sea lions are just the opposite, with highly developed
foreflippers and small, underdeveloped hind flippers.
Seals are more likely to be attacked and more often end up lunch for the GW
as sea lions tend to escape. This may have to do with their different
anatomical structure.
• Time Of Attacks:
Timing of attacks, based on research done off the South Farallon Islands, a
location about 30 miles west of San Francisco, indicates that most attacks
occurred during the day. This is also supported by the fact that GW sharks
have eyes that seem to be more sensitive to daylight viewing. Also, it
appeared that the attacks were occurring at the same approximate time each
day, possibly due to fact that the seals are forced each day to go into the
water because of the tides.
• Differences In Attack Strategies:
Again, based on evidence from over 100 attacks viewed by researchers at the
South Farallon location, interesting differences in attack strategies were
noted based on the species of the prey. In the case of the seal, the prey is
attacked just beneath the surface by a GW rising from below. A large,
elongating blood stain at the surface indicates that the shark carries the
seal underwater (or sometimes at the surface) for a distance before removing
a bite and releasing the carcass which floats to the surface. Since the
attack usually occurs in the area of the head and the seal has a large
network of blood vessels in that area, death by exsanguination (loss of
blood) or decapitation is often both the result of the first bite and the
cause of death. The GW later returns to feed on the carcass.
With the sea lion, a different method of attack is employed. The primary
strike is a brutal strike on the animal as it swims at the surface. This
strike often propels the shark out of the water with the sea lion captive in
its jaws. The sea lion, lacking the network of blood vessels found in the
seal, isn't killed by the bite. It instead flounders at the surface until the
shark returns for the final kill and feeding.
This evidence complements the widely held "bite and spit"
hypothesis (that the shark bites and then releases its prey, a fact observed
in a great number of human attacks) but refutes the idea that sharks wait for
the prey to die before feeding. In a the great majority of attacks witnessed
by the Farallon researchers, the shark would begin feeding after the initial
strike, whether the animal was dead or not.
• Attacks On Humans:
Finally, why do sharks not follow this pattern with an attack on humans? In a
human attack, the primary strike is the only contact, as though the shark
finds us to be unpalatable. There is a theory on this as well, involving the
differences in our anatomy and the pinnipeds. We are mostly muscle where the
pinniped body has a great deal of fat. It is theorized that the shark somehow
senses this and abandons us as a potential meal because are bodies are not as
energy-rich as the pinnipeds. Of course, this is just a theory and although
humans are rarely consumed after the initial strike, the strike itself is
often enough to kill us - or at least, really screw up our day!
Answers To Faq's About Great Whites:
• What Do Great Whites Eat?
The GW's diet consists mostly of fish (lingcod, salmon, and tuna), squid,
other sharks, cetaceans (dolphins and whales), and pinnipeds (seals and sea
lions). They also show a preference for carcasses, especially large whales.
• About How Long Does The Average GW Live?
No one has accurate answers to this question. Since the GW population is so
migratory and also so isolated from one fish to the next, it is impossible to
pick an animal and follow it. However, scientists studying GW's off the
Farallon Islands (currently one of the hotbeds in the world for GW research)
have identified by dorsal markings several individual animals and one, Stumpy,
so named because she is missing the top portion of her caudal fin, has been
observed returning to feed for 18 years. Is she an old shark? A young shark? No
one can say for sure.
How Big Do They Grow?
The GW's maximum size is unknown, mostly because if today we see a 25-footer,
we'll say 25. If tomorrow we see a 30-footer, it'll jump to 30 feet. To stick
with the latest numbers, the average GW is between 12-16 feet in length. Some
of the monsters at the Farallons are 18 feet but this is the exception, rather
than the rule. (why bigger at the Farallons? The GW population hasn't been
fished out like in other GW hotspots, Australia and South Africa).
• Where Are Most GW's Found?
My first inclination is to say 'water' and I would only be half-joking. GW's
have been found in the coastal waters of every continent except Antarctica.
Most GW excursions happen in Australia and South Africa, due to the denser
populations found there.
• How Fast Can They Swim?
Estimates of 15 miles an hour in burst mode are common.
• How Many People Are Killed Each Year By GW Attacks?
There are about 100 shark attacks each year, 30 of which are fatal. Of these 30,
the GW can be reasonably assumed to be responsible for perhaps half to a third.
Just 10 to 15 people each year out of millions swimming on the beach.
How Powerful Is Their Sense Of Smell Or How Do They sense Their Prey?
It would be easiest to start with the GW's nostrils. Unlike us humans, who
require the nostrils to assist in breathing, the GW (who has gills for this!)
uses his sniffer for just that.... sniffing (or olfaction, as it is
scientifically known)! Each nostril is divided by a skin flap that separates the
water it is swimming through into an inflowing current and an outflowing current.
As the shark moves it's head from side to side, water enters the nostril and
passes through an area that contains a large number of tiny sensory organs known
as lamellae. These lamellae are shaped like tiny flower petals and are covered
with millions of olfactory cells. Since the water travels over so many of these
lamellae, it in turn passes over most of the olfactory cells, giving the GW its
primary sensitivity to odors in the water. These cells in turn are directly
connected to the olfactory bulb in the brain and turn the shark into a
"swimming nose".
The second sensory weapon the GW employs against its prey is seen on the shark as
hundreds, perhaps thousands of dark holes that cover the top and underside of the
snout. These holes are the ampullae of Lorenzini, which makes them sound more
like a dish at the Olive Garden if you ask me... but Dr. Lorenzini didn't so
that's what they're called. These holes are very small capsules that are filled
with jelly excreted by the shark. Each is sensitive to electrical discharges as
small as .005 microvolt, this gives the GW the ability to sense the electrical
field distributed by a 1,000 mile-long copper wire hooked to a D-size battery.
The pores achieve this astonishing feat by first their great number (the GW,
being a large shark, has a great deal more than say a medium-sized blue) and
also because, like the nasal lamellae, each pore in turn uses several sensory
cells to "pick up" the signal. The sensory cells lies inside alveoli
(little sacs) and these alveoli are connected directly to the brain, no muss, no
fuss... just a straight signal to Eating Central.
What Good Does Having A Built-In Electrical Field Detection Have?
Quite a bit. Every creature in the water generates a small electrical field from
where their skin meets the water. the mucous membranes that coat the mouth and
gills of fish also create steady direct current fields and these fields are
directly effected by their breathing patterns. Furthermore, if the animal is
bleeding, this puts more ions into the water and further increases the signal
which the shark can hone in on like a cruise missile in Iraq. In summation, these
two sensory packages, the nasal lamellae and the ampullae of Lorenzini, have
evolved over millions of years into detection systems for the very things that
make prey more attractive - blood, which indicates injured prey and thus easier
to catch prey; and movement, generated by an animal in the water. The GW, which
is arguably the most efficient super-predator in nature, has evolved as well,
using these packages to their fullest extent. Keep in mind that a study showed
that the chum and bait used to bring sharks closer to scientists in research
vessels could attract sharks from as far away as 5 kilometers. Pretty
sensitive...
How Much Food Does The Average GW Eat, And How Often?
Again, no way to be sure. Studies at the Farallons show the animals feasting on a
large elephant seal and then the animal doesn't eat for days. This feast may
consist of one large 800-pound seal, so it is safe to assume (by scientific means
but not fact) that the shark eats at least twice a week and when it does eat, it
eats a lot.
Myths, Legends, And General Fallacies:
• GW's Like Human Blood!:
Duh. Like I have to correct you on this generally accepted theory on the
occurrence of GW shark attacks these days is that the shark attacks because it
mistakes us for its favorite food - the seal or sea lion. (Not that I'm saying you
look like a seal or sea lion but hey, to a GW, we're all the same.)
• GW's Will Attack Instantly If They Spot A Human:
Again, an easy one to debunk. Just as several people have been attacked in the
water when a GW happened by, several divers have reported sightings in which the
shark quickly turned tail and left. As in the real world, the only constant in the
world of the shark is that nothing is constant!
• GW's Grow To Be 40 Feet Long!:
Man, this would be a fun one. Imagine being in the water with a forty foot Great
White. We'd be like popcorn shrimp! Luckily though, the average size (as reported
above) isn't nearly as long - only 12-15 feet. The GW did have an ancient relative,
the Carcharodon Megalodon who grew to be between 45-50 feet long. Of course, there
are scientists who theorize that the Megalodon could still be down there... down
deep enough where the bodies would never wash up on shore. Kind of creepy to think
about, huh?
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