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.Identifying the five synapomorphies in sharks
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. 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.Skeleton
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
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.Tail
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 upper lobe. The shape of the caudal fin reflects the shark's lifestyle, and can be broadly divided into five categories:
- Fast-swimming sharks of open waters, such as the mackerel sharks, have crescent-shaped tails with upper and lower lobes of almost equal size. The high aspect ratio of the tail serves to enhance swimming power and efficiency. In these species, there are usually also lateral keels on the caudal peduncle. The whale shark and basking shark also have this type of tail, although they are generally more sedate animals than the other examples.
- "Typical sharks", such as requiem sharks, have tails with the upper lobe longer than the lower. The upper lobe is turned upwards at a moderate angle relative to the body, which balances cruising efficiency with turning ability. The thresher sharks have an extreme example of this tail in which the upper lobe has evolved into a weapon for stunning prey.
- Bottom-dwelling sharks such as catsharks and carpet sharks have tails with long upper lobes and virtually no lower lobe. The upper lobe is held at a very low angle, which sacrifices speed for maneuverability. These sharks generally swim with eel-like undulations.
- Dogfish sharks also have tails with longer upper than lower lobes. However, the backbone runs through the upper lobe at a lower angle than the lobe itself, reducing the amount of downward thrust produced. Their tails cannot sustain high speeds, but combine the capability for bursts of speed with maneuverability.
- Angel sharks have unique tails among sharks. Their caudal fins are reverse heterocercal, with the lower lobe larger than the upper.
Teeth
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.Internal organs
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 are derived and is also where the immune system functions to fight off pathogens.Digestive system
The stomach terminates at the pylorus, which leads to the duodenum, and then to the spiral valve. The spiral valve is a coiled organ, it increases surface area so that nutrients can be absorbed. The spiral valve then empties into the rectum and anus, then into the cloaca. Within the shark stomach, buoyancy is established from air taking up space and providing sharks the ability to float. The shark stomach also has shorter intestines than most animals, which causes food to take greater amounts of time to fully digest before being excreted from the body. This digestive gland passes secretions through the vental lobe and into the duodenum. The pancreas of the shark helps with digestion by producing the enzymes needed to break down large chunks of food, and the pancreas serves to help keep the metabolism at a fast pace to accommodate for the large amounts of food taken in. At the very end of the short intestine lies the rectal gland which is important for the excretion waste from the animal.Reproductive system
s' reproductive organs serve to reproduce cr7 sexually, where the male delivers sperm to the female using claspers that insert into the female's oviduct. This then allows the female to give birth to live young, although some do lay eggs. This image depicts a Squalus acanthias shark dissection where this female happened to be pregnant with multiple shark pups. This image is important as it shows how sharks can give birth to multiple live young.Temperature
such as great white sharks, shortfin mako, longfin mako, salmon shark, and porbeagle are endothermic, which helps them move quickly in cold water. These mackerel sharks warm their blood by using a heat exchange system called rete mirabile. The body temperature of mackerel sharks can be up to 10o higher than the surrounding water.Integument
Unlike bony fish, the sharks have a complex dermal corset made of flexible collagenous fibers and arranged as a helical network surrounding their body. This works as an outer skeleton, providing attachment for their swimming muscles and thus saving energy. A similar arrangement of collagen fibers has been discovered in dolphins and squid. Their dermal teeth give them hydrodynamic advantages as they reduce turbulence while swimming.Skin
Unlike the scales of bony fish, sharks have placoid scales, known as denticles. Denticles are V-shaped and are made of layers of dentine and a surface of enamel. Riblets are sockets in the shark's skin which hold the denticles. These denticles on the skin allow for the shark to move quietly, swiftly, and almost effortlessly. The skin of sharks is similar to the feeling of sandpaper, rough and abrasive. During swimming, the flexible bias of the skin that is positioned 45 degrees to the body length allows for lateral bending. This ensures that the skin stays tight to the surface, but is also flexible, preventing wrinkling and possible turbulence in streamlines passing over the body. Skin is composed of a dermis and an epidermis. In vertebrates, the epidermis produces a mucus coating to help moisten the surface of the skin and can also be used as a defense mechanism from bacterial infections. This can also help with smooth, swift, laminar flow while swimming.