In the summer months especially, most people think of sharks as terrors of the beach. Steven Spielberg's influence aside, these ancient fish have an important role in the oceanic ecosystem and have many interesting and unique physical attributes.
Sharks are renowned for their teeth. What you may not know is that shark teeth are not permanent, but rather rest on a membrane in their mouths that is similar to a conveyor belt. Those teeth in the front are usually the oldest and as they become damaged or worn down, teeth from the back move forward to replace them. Additionally, many sharks' jaws don't attach in a traditional way. Sort of like snakes, many have jaws that can detach to allow for larger prey.
While sharks are generally known for their ferocity, there are three species that are relatively passive eaters: the whale shark, the megamouth shark, and the basking shark. All three species feed through filter-feeding. The first two are able to actively move water and particles over their gills to help facilitate feeding, but the basking shark must keep swimming forward with their mouths open to move the water. For all three, anything caught in their gills is then swallowed.
Perhaps the most interesting physical aspect of sharks are the ampullae of Lorenzini. Ampullae are canals or ducts in anatomy, and the ampullae of Lorenzini are very sensitive jelly-filled ducts on a shark's head that detect the slightest change in electrical current in the water. The tops of these ducts are visible to the naked eye on most sharks as tiny pores located all over the snout. While sharks are not the only animals that have these (rays, chimeras, and sturgeons also have them), all sharks have them.
Sharks also have enormous livers. Most smaller fish have a swim bladder, which is a pocket of air that helps keep the fish buoyant. Sharks do not have these. To make up for this, shark livers are rich in light-weight oils and hydrocarbons, which are less dense than water. This results in a need for very, very large livers to accommodate all the oils needed for buoyancy. According to ReefQuest Centre of Shark Research, a 13-foot tiger shark's liver would yield roughly 18 gallons of oil.
So this year for Shark Awareness Day, please be aware that these fish are more than the monsters of the sea that our favorite fiction (and, occasionally, the Discovery Channel) might lead us to believe.