Sharks are not necessarily afraid of dolphins.  Nor are dolphins often found killing or attacking sharks.  There have been a few instances where a dolphin has taken on a shark that is relatively small in order to defend itself. Rumor has it that the myth that sharks are afraid of dolphins may have come from an incident that took place at a seaquarium in the 1950’s when a sandbar shark showed too much interest in the birth of a Bottlenose Dolphin calf, and as a result three of the adult male dolphins ganged up on the shark, butting it in the gills and stomach until it finally died.

During the 1960’s the United States Navy also trained Bottlenose Dolphins to incapacitate large sharks by butting them in their gill pouches.  The dolphins quickly learned to attack certain species of sharks such as the Sandbar shark or the Lemon shark but still seemed to be afraid to try and take on a Bull shark.  It should be noted that bull sharks are about the same size and shape as the dolphins and that in the wild the Sandbar and Lemon sharks are not typically found to attack dolphins, whereas the Bull shark has been found to attack the dolphins in the wild.  This suggests that dolphins may be able to sense which types of sharks pose a threat and which do not.

For the most part sharks and dolphins leave each other alone.  However, Some 75% of wild dolphins show some degree of shark scarring, and we usually only see the ones who were fortunate or strong enough to get away.

For most shark species, spending a day in fresh water would be similar to placing us humans on the moon without a spacesuit.  They most likely would not be able to survive due to the inhospitable surrounding environment.  One of the main problems that would pose a serious threat to sharks in this case is the process known as osmosis.  Osmosis refers to when a fluid moves through a semi-permeable membrane from a solution with a low solute concentration to a solution with a higher solute concentration until there is an equal concentration of liquid on both sides of the membrane.  In this case, the dissolved substances involve sodium and chloride.

Because sharks evolved in salt water, they are equipped with salty bodies.  Even sharks in fresh water contain more than twice the amount of salt and chloride as other fish that are more common among freshwater.  In theory, sharks that are placed here should burst like a balloon when it is overfilled with air, given the osmosis effect; however since they urinate a lot they are able to avoid this problem.

The sharks take in a lot of extra water however they excrete much of it as urine that is diluted and has a rate of over 20 times that of typical saltwater sharks.  What this means is that their kidneys are required to work harder than normal, thus utilizing additional energy.  Much like humans that have become accustomed to life in low oxygen regions, sharks in fresh water appear to adapt to what would seem to be formidable conditions.

Although there have been several studies over the years that have determined that there are in fact some species of sharks residing in freshwater environments, relatively few sharks spend a substantial amount of time here.  In fact, river shark populations are now at dangerous lows.  Bull shark’s population numbers are higher since they often move between fresh water and salt water environments.  Other species of sharks however, that are more adapted to life in lakes and rivers are faced with having to withstand both natural and human induced problems within their habitats.  Problems that these sharks face include changes in temperature, oxygen, mineral content, and other climate changes.  Human activities such as damn building, water modifications such as irrigation and the introduction of pollutants to the water all pose a serious threat to these particular species of shark.

Sharks are not mammals.  Rather, they are a species of fish because they have a skeleton that is entirely composed of cartilage and is equipped with tough, leathery, scale less skin.  These skeletons differ greatly from those of the bony fish or other terrestrial vertebrates.  Cartilage is flexible, durable and has about half the amount of density as bones.  As a result this helps to reduce the skeleton’s weight and conserves energy.  Sharks also lack a rib cage which would result in its own weight crushing it if it were placed on land.  Sharks do not have lungs that breathe air; they breathe oxygen by passing water over their gills.  These creatures have highly streamlined bodies.

The earliest known shark dates as far back as 420 million years ago.  Since that time, sharks have diversified into 440 different species ranging in size and length.  A shark’s teeth are embedded in the gums rather than directly affixed to the jaw and are constantly replaced throughout its life.  The jaw’s surface and its gill arches need extra support due to its heavy exposure to physical stress and its need for strength.

In sharks, the respiration and circulation process takes place when deoxygenated blood travels to the creature’s two chambered heart and then pumps blood to the gills via the ventral aorta artery where it branches into afferent brachial arteries. Reoxygenation takes place in the gills and the reoxygenated blood flows into the efferent brachial arteries which come together to form the dorsal aorta.  The blow flows from this aorta throughout the body.  The deoxygenated blood then flows through the posterior cardinal veins to enter the posterior cardinal sinuses.  It is here that the blood then enters the heart ventricle then causing the cycle to repeat.

Unlike mammals which are warm blooded creatures, most sharks are cold blooded meaning that their internal body temperature matches that of their ambient environment.  Another thing that makes sharks differ from other mammalian species is the way in which they digest their food.  A shark has an extremely short intestine.  This short length of the intestine is achieved by the spiral valve with multiple turns within a single short section instead of a long tube like intestine that many mammals possess.  The valves provide a long surface area that requires food to circulate inside the shark’s short gut until it has been fully digested, when remaining waste products pass into the cloaca.

Most sharks are ovoviviparous, meaning that their eggs hatch in the oviduct within the mother’s body and that the egg’s yolk and fluids secreted by glands in the walls of the oviduct help nourish the embryos.  The young continue to be nourished this way until they hatch.  Differing from sharks, mammals give birth to live young and feed them with milk from their mammary glands. There have also been two cases where sharks have reproduced asexually; another difference compared to mammals as mammals are now the only major group of vertebrates in which asexual reproduction has not yet been observed.