Sharks, one of the great predators of the great blue deep, closely related to the Rays, and regarded as one of the most deadliest underwater animals known to man, funny since men are the reason they seem to be going extinct. Categorized as a fish, with its entire body composed of flexible cartilage, two eyes, each on a side of the head. They have several gills on each side of their body as well, as well as pectoral fins that are used to navigate, and move around the area. Sharks come in many different size, shape, species, and physical characteristics. With around 500 species of sharks all together, raging in different sizes, some as small as a persons arm, others ranging from over ten meters in size. Sharks have been spotted in all types of seas, most around 2000ft deep of sea level, other sharks have even been spotted in freshwater (though this is usually not the case). This further arguments Sharks adaptable nature of environment, knowing well they usually are kings no matter where they remain. Many famous sharks include, Whale shark, Tiger Shark, Great White Shark, Hammer-head Shark, and many more.
Let's talk about the anatomy of these Sharks a little bit shall we. Using their gills as the main component of breathing while under water, they have replaceable teeth as well, so whenever losing a tooth, they can easily replace it with their many layers of teeth. The sharks are attached to their gums, with the teeth being replaced throughout a lifespan of a shark. Shark teeth also vary in how they are shaped, usually dependent on their needs of survival and dieting. Some are viciously sharp, instantly able to pierce through flesh, some are jagged, able to grip and attach to anything it it devours. Others are even smaller, non-lethal, meant only to chew on smaller fish or plankton. Moving on from the teeth, shark's have extremely powerful, and large/heavy jaws. Their jaws easily able to crush anything it that comes between them, their sheer force is impressive alone, how they can latch onto a target and grip it, seemingly impossible to break free from their clutches. With this heavy jaw, they need much more support to stabilize and work with. This is why a sharks jaw has a "tesserae" (plates attached around the jaw). The size, and layers of these plates will be different amongst sharks, obviously larger sharks and jaws will require more layers of plates, a smaller shark will need probably just one layer.
Speaking now about their body composition, as mentioned before they are completely made of cartilage (similar to what a humans nose or ear is made of). This makes their body quite flexible, and extremely durable. A great way of noticing is juts how they move, they move in a swaying fashion, almost like a snake on the ground. Despite its durable nature, cartilage is light-weight compared to regular bone, making a shark much more light for travel, and consume less energy. Sharks are also covered in a special material called "Dermal Denticles " around their skin, used to protect it from parasites, the fluid substance also helps with their aerodynamic nature when traveling under water. Sharks use their eight fins to navigate and move around the ocean floors. These fins are quite soft, resembling feathers or small wings on the shark, what makes these quite interesting, is they only allow a shark to move forward when traveling underwater. The tails on a shark is its main limb used for acceleration and fast travel, sizes vary depending on shark of course, they also allow for a shark to twist and turn depending on how they move and manipulate their tail fins.
Enough talking about their bodies, lets talk about their senses. Sharks have the ability to determine the direction of a given scent based on the timing of scent detection in each nostril. This is why spilling blood in water is so dangerous, they can be smelt from far distances, and known exactly where to go at a moments notice to find their target. Their eyesight similar to many other vertebrae, they are adapt within water territory. They use the water of the sea to clean their eyes, hence why you hardly see them blink (if at all). This keeps them constantly on the search, vision wise. However, their main use of sensing is using electroceptors, where they can detect electromagnetic fields around the ocean floor. This helps them find prey in small areas, making hiding from sharks nearly impossible in their terrain. Lastly, their hearing has not been studied much, hard to pinpoint how it is exactly, they spectate that they have sharp hearing, and can hear animals and disturbances from miles away.
That should be enough for real life sharks that matter. For NB, not much has been shown. They are incredibly fast, excel in water usage, were seen used by Kisame mainly. They command to anything the user wants them to, this also means to even kill their summoner. They have great pure power, and large bodies of size. They work well with other water usage, meaning they have been seen to work with a users own techniques, and due to their unique body structure, they can even be stealthy just as much as destructive. Hiding in the water, and attack viciously when called upon. Of course their ability to notice electromagnetic fields underwater helps them become such stealthy killers. Tho they are not as fast underwater as most people would think (16 mph usually the fastest) in the real world, in the NW they are extremely quick. I'd venture to guess they came from Kirigakure/Land of water. I have not seen them outside of a body of water, so I doubt they would work well in regular, non-water source, environments, however making some nice customs would work to fix that problem. A big reason why I like sharks is its custom potential, a completely unexplored area to work with, a lot of potential. Knowing they are dominate with water, I would assume they likely can control most of waters alternative releases, that includes mist and syrup water.