Underwater Adventures

Southern & cow-nosed stingray, moon jellyfish, lionfish, coral reef inhabitants, blind cave characian fish
All photos taken at the Toronto Zoo.

In the last 50 years we have lost 90% of all the large fish throughout the world.
Big groupers were the most sensitive to changing climate, increasing water
temperature, pollution, and over-fishing. The plight of our oceans is a hidden,
but severe one. We at Naked Mole Rat Productions wish to help preserve all
species of animal and plant, from the smallest plankton to the largest
elephant and everything in between

Sharks

Sharks (super-order Selachimorpha) are a type of fish that tend to strike fear into the hearts of those
who merely hear the word. A highly misunderstood animal, sharks are only now beginning to be seen not
as the bloodthirsty, brainless killers of the sea, but in actual fact are highly intelligent and curious animals
who interact with their surroundings. In 1987, near Smitswinkle Bay, South Africa, a group of up to seven
great white sharks worked together to relocate the partially beached body of a dead whale to deeper waters
to feed. Sharks have even been known to engage in playful activities (a trait also observed in cetaceans and
primates). Porbeagle sharks have been seen repeatedly rolling in kelp and have even been observed chasing an
individual trailing a piece behind them.

Shark bodys are highly refined and a natural marvel of streamlining with sophisticated sensory organs. Their
bodies are made up entirely of cartilage, the only true bones in a shark's body are its teeth. The teeth of
carnivorous sharks are not attached to the jaw, but are embedded in the flesh, and in many species are
constantly replaced throughout the shark's life; some sharks can lose 30,000 teeth in a lifetime! All sharks
have multiple rows of teeth along the edges of their upper and lower jaws. They stick out of their mouth at
angles of up to thirty degrees. New teeth grow continuously in a groove just inside the mouth and move forward
from inside the mouth on a "conveyor belt" formed by the skin in which they are anchored. In some sharks rows
of teeth are replaced every 8?10 days, while in other species they could last several months. The lower teeth are
primarily used for holding prey, while the upper ones are used for cutting into it. The teeth range from thin,
needle-like teeth for gripping fish to large, flat teeth adapted for crushing shellfish. Their teeth must be
constantly replaced to ensure that they are sharp and ready for action.

They respire with the use of five to seven gill slits, and often must move constantly or locate themselves in an
area of high water flow to ensure enough oxygenated water flows though them to prevent suffocation. Sharks
have a covering of dermal denticles that protect their skin from damage and parasites and improve fluid dynamics.
Unlike bony fish, sharks have a complex skin made of flexible collagenous fibers. This works as an outer skeleton,
providing attachment for their swimming muscles and thus saving energy. The skin is smooth-feeling in one direction,
while rough enough to cut human flesh if rubbed in the opposite direction. This allows for better streamlining and
hydrodynamic flow over the body of the animal. In the past, sharks' skin has been used as sandpaper, and more
recently, the shark's skin has been used as a template for creating better swim apparel for those who race and want
to create less drag while competing professionally.

Sharks range in size from the dwarf lanternshark, a deep sea species of only 17 centimetres (7 in) in length, to
the whale shark, the world's largest known species of fish, which grows to a length of approximately 12 metres (39 ft)
and which feeds only on plankton, squid, and small fish through filter feeding. Amazing an animal of such size could
subsist on some of the tiniest animals of the sea! The bull shark, Carcharhinus leucas, is the best known of
s everal species that swim in both seawater and freshwater, as well as in deltas.

A few of the larger species, such as the shortfin mako and the great white, are mildly homeothermic; that is: they
are able to maintain their body temperature above the surrounding water temperature. This is possible because of
the presence of the suprahepatic rate, a counter current exchange mechanism that reduces the loss of body heat.
Muscular contraction also generates a mild amount of body heat. However, this differs significantly from true
homeothermy, as found in mammals and birds, in which heat is generated, maintained, and regulated by metabolic activity.

The maximum lifespan of a shark varies from species to species. Most sharks live for 20 to 30 years, while the spiny
dogfish lives a record lifespan of more than 100 years. Whale sharks have been hypothesized to also live over 100
years! Evidence for the existence of sharks extends back over 450-420 million years, making them some of the oldest
living animals today, and have been virtually unchanged in that entire period. As the old saying goes, "If it ain't broke,
don't fix it." and this saying is true of the ultimate underwater predator. Modern sharks began to appear about
100 million years ago. One of the most recent families of sharks that evolved is the hammerhead sharks. The oldest
white shark teeth date from 60 to 65 million years ago, around the time of the extinction of the dinosaurs. In early
white shark evolution there are at least two lineages: one with coarsely serrated teeth that probably gave rise to the
modern great white shark, and another with finely serrated teeth and a tendency to attain gigantic proportions. This
group includes the extinct Megalodon, which like most extinct sharks is only known from its teeth and a few vertebrae.
This shark could grow to more than 16 metres (52 ft) long and is recognized as the biggest known carnivorous fish to
have ever existed. Fossil records reveal that this shark preyed upon whales and other large marine mammals. It is
believed that the immense size of predatory sharks such as the great white may have arisen from the extinction of
giant marine reptiles, such as the mosasaurs and the diversification of mammals. It is known that at the same time
these sharks were evolving some early mammalian groups evolved into aquatic forms. Certainly, wherever the teeth
of large sharks have been found, there has also been an abundance of marine mammal bones, including seals, porpoises
and whales. These bones frequently show signs of shark attack. There are hypotheses that suggest that large
sharks evolved to better take advantage of larger prey.

The sex of a shark can be easily determined. The males have modified pelvic fins which have become a pair of claspers.
The name is somewhat misleading as they are not used to hold on to the female, but fulfill the role of the mammalian
penis. Mating has rarely been observed in sharks. The smaller catsharks often mate with the male curling around the
female. In less flexible species the two sharks swim parallel to each other while the male inserts a clasper into the
female's oviduct. Females in many of the larger species have bite marks that appear to be a result of a male
grasping them to maintain position during mating. The bite marks may also come from courtship behavior: the male
may bite the female to show his interest. In some species, females have evolved thicker skin to withstand these
bites. Sharks have a different reproductive strategy from most fish.

There are two documented cases in which a female shark who has not been in contact with a male has conceived a
p up on her own through a process known as parthenogenesis. The details of this process are not well understood
but genetic fingerprinting has shown that the pups in these cases had no paternal contribution to their genome and
were clones of their mothers, ruling out sperm-storage as an alternative hypothesis. It is unknown as to the extent
of this behavior in the wild, and how many species of shark are capable of parthenogenesis. This observation in
sharks made mammals the only remaining major vertebrate group in which the phenomenon of asexual reproduction
has not been observed. Parthenogenesis has also been observed in sharks, though scientists warned that this type
of behavior in the wild is rare, and probably a last ditch effort of a species to reproduce when a mate isn't present.
This leads to a lack of genetic diversity, required to build defenses against natural threats, and if a species of shark
were to rely solely on asexual reproduction, it would probably be a road to extinction, and may have contributed to
the decline of blue sharks off the Irish coast.

Instead of producing huge numbers of eggs and fry (a strategy which can result in a survival rate of less than 0.01%),
sharks normally produce around a dozen pups. Blue sharks have been recorded as producing 135 and some species
produce as few as two. These pups are either protected by egg cases or are born live.

Sharks have keen olfactory senses, meaning that they have a highly developed sense of smell. Located in the short
duct (which is not fused, unlike bony fish) between the anterior and posterior nasal openings, with some species able
to detect as little as one part per million of blood in seawater. They are attracted to the chemicals found in the
guts of many species, and as a result often linger near or in sewage outfalls. Some species, such as nurse sharks,
have external barbels that greatly increase their ability to sense prey. Sharks generally rely on their superior sense
of smell to find prey, but at closer range they also use the lateral lines running along their sides to sense movement
in the water, and also employ special sensory pores on their heads (Ampullae of Lorenzini) to detect electrical fields
created by prey and the ambient electric fields of the ocean. Shark eyes are similar to the eyes of other vertebrates,
including similar lenses, corneas and retinas, though their eyesight is well adapted to the marine environment with
the help of a tissue called tapetum lucidum. This tissue is behind the retina and reflects light back to the retina, thereby
increasing visibility in the dark waters. The effectiveness of the tissue varies, with some sharks having stronger
nocturnal adaptations. Sharks have eyelids, but they do not blink because the surrounding water cleans their eyes. To
protect their eyes some have nictitating membranes. This membrane covers the eyes during predation, and when the
shark is being attacked. However, some species, including the great whit shark do not have this membrane, but instead
roll their eyes backwards to protect them when striking prey. The degree to which sight is used probably varies with
species and water conditions. At any rate, the shark's field of vision can swap between monnocular and
stereoscopic at any time.

The Ampullae of Lorenzini are the electroreceptor organs of the shark, and they vary in number from a couple
of hundred to thousands in an individual. Sharks use the Ampullae of Lorenzini to detect the electromagnetic fields
that all living things produce. This helps sharks (mostly the hammer head) find its prey. The shark has the greatest
electrical sensitivity known in all animals. This sense is used to find prey hidden in sand by detecting the
electric fields inadvertently produced by all fish. It is this sense that sometimes confuses a shark into attacking
a boat: when the metal interacts with salt water, the electrochemical potentials generated by the rusting metal
are similar to the weak fields of prey, or in some cases, much stronger than the prey's electrical fields: strong
enough to attract sharks from miles away. The oceanic currents moving in the magnetic field of the Earth also
generate electric fields that can be used by the sharks for orientation and may be used in navigation. Sharks
also utilize their lateral lines, which are found in most fish. It is used to detect motion or vibrations in the water.
The shark uses this to detect the movements of other organisms, especially wounded fish. The shark can sense
frequencies in the range of 25 to 50 Hz.

Some sharks can be highly social, remaining in large schools, sometimes up to over 100 individuals of scalloped
hammerheads congregating around seamounts and islands e.g. in the Gulf of California. Cross-species social
hierarchies exist with oceanic whitetip sharks dominating silky sharks of comparable size when feeding.

When approached too closely some sharks will perform a threat display to warn off the prospective predators.
This usually consists of exaggerated swimming movements, and can vary in intensity according to the level of threat.

Until recently only a few benthic species of shark, such as hornsharks, leopar sharks and catsharks could survive
in aquarium conditions for up to a year or more. This gave rise to the belief that sharks, as well as being difficult
to capture and transport, were difficult to care for. A better knowledge of sharks has led to more species (including
the large pelagic sharks) being able to be kept for far longer. At the same time, transportation techniques have
improved and now provide a way for the long distance movement of sharks. The only species of shark to have
never been successfully held in captivity was the great white, until September 2004 when the Monterey Bay
Aquarium successfully kept a young female great white shark for 198 days before releasing her back into the wild.