Shark anatomy

Shark anatomy differs from that of bony fish in a variety of ways. Variation observed within shark anatomy is a potential result of speciation and habitat variation.

The five chordate synapomorphies are present in chondrichthyes as follows. The five synapomorphies are pharyngeal slits, a dorsal nerve cord, notochord, endostyle, and the post-anal-tail which is depicted and labeled well on the chordates page. This image is helpful to visualize the regions where the five synapomorphies existed in chordates and what they looked like. In cephalochordates, the pharyngeal slit, or pharynx, are lateral to the throat of the chordate and work as filters by letting water pass over this region in order to retain nutrients and oxygen from gas exchange occurring. The dorsal nerve cord serves as a hollow-like backbone where signals are sent throughout the body due to nervous tissue being located in this region. The notochord is also toward the tail of the chordate but closer toward the middle of the body than the dorsal nerve cord and is a water-filled structure that allows the chordate to move in water. The endostyle is underneath the pharyngeal gill slits where proteins are trapped to eventually provide the chordate energy and sustenance. Lastly, the post-anal-tail is muscular and allows the chordate to move in water.

These evolved synapomorphies are crucial for the current shark's lifestyle, for example, the pharyngeal slit changed to become the jaw and gills. The dorsal nerve cord sends signals to the body like it has done before but now the dorsal nerve cord becomes the central nervous system (CNS). The notochord changed from allowing movement in water to discs being formed in between vertebrae allowing for protection and acting as a buffer when movement occurs. The endostyle is the homolog when compared to the thyroid gland and it pre-established itself before sharks; this adaptation was beneficial for the sharks' metabolism to become faster. The post-anal-tail helps the shark move in water but also helps with balance too.

Sharks are cartilaginous fish. The skeleton of a shark is mainly made of cartilage. They belong to the class of Chondrichthyes. In particular, the endoskeletons are made of unmineralized hyaline cartilage which is more flexible and less dense than bone, thus making them expel less energy at high speeds. Each piece of skeleton is formed by an outer connective tissue called the perichondrium and then covered underneath by a layer of hexagonal, mineralized blocks called tesserae.

Fins allow the sharks to be able to guide and lift themselves. Most sharks have eight fins: a pair of pectoral fins, a pair of pelvic fins, two dorsal fins, an anal fin, and a caudal fin. Pectoral fins are stiff, which enables downward movement, lift, and guidance. The members of the order Hexanchiformes have only a single dorsal fin. The anal fin is absent in the orders Squaliformes, Squatiniformes, and Pristiophoriformes. Shark fins are supported by internal rays called ceratotrichia.

The tail of a shark consists of the caudal peduncle and the caudal fin, which provide the main source of thrust for the shark. Most sharks have heterocercal caudal fins, meaning that the backbone extends into the (usually longer) upper lobe. The shape of the caudal fin reflects the shark's lifestyle, and can be broadly divided into five categories:

Shark teeth are strong and made of enamel. Many sharks have 3 rows of teeth. These teeth are embedded in the gums, not the jaw. Sharks are born with teeth that are constantly being replaced. Teeth are replaced every two weeks, approximately. The shape of the teeth determine the diet of the shark. For instance, a shark with flat teeth are used for crushing shellfish, pointed teeth are used for gripping fish, while the notoriously sharp teeth with jagged edges are used for large prey.

The liver is a large and oily organ that comprises 25% of the total body weight of the shark. The two purposes of this organ in the shark are to store energy and oil. The liver is a hydrostatic organ. This organ helps with buoyancy since the liver stores oils, decreasing the density of the shark's body. The shark liver is also full of an oily-like substance called shark liver oil that helps the sharks be more buoyant and acts as an energy storer, where it can be utilized when needed. The shark's liver also helps with filtrating the blood and waste while also acting as a storage region for vitamins which is incredibly important; especially if the shark goes a long time without eating or if the shark has extreme amounts of urea within the system, the liver helps with both of these scenarios. Sharks also have osmoregulation which permits the shark to have high concentrations and amounts of urea which allows them to not become dehydrated from living in seawater as opposed to freshwater. The shark kidney excretes urea that is needed for the shark to have in its system so the shark does not become dehydrated from living in seawater. Sharks hearts have two chambers. The shark heart's main importance is providing oxygenated blood to the entire body while filtering out the deoxygenated blood. A shark's spleen is also incredibly important because it is where red blood cells (RBC's) are derived and is also where the immune system functions to fight off pathogens.