Shark Anatomy
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Shark anatomy differs from that of
bony fish Osteichthyes ( ; ), also known as osteichthyans or commonly referred to as the bony fish, is a Biodiversity, diverse clade of vertebrate animals that have endoskeletons primarily composed of bone tissue. They can be contrasted with the Chondricht ...
in a variety of ways. Variation observed within
shark Sharks are a group of elasmobranch cartilaginous fish characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the head. Modern sharks are classified within the ...
anatomy is a potential result of speciation and habitat variation.


The five chordate synapomorphies

The five chordate
synapomorphies In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to ...
are present in
chondrichthyes Chondrichthyes (; ) is a class of jawed fish that contains the cartilaginous fish or chondrichthyans, which all have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fish'', which have skeleto ...
as follows. The five synapomorphies are
pharyngeal slit Pharyngeal slits are Filter feeder, filter-feeding organs found among deuterostomes. Pharyngeal slits are repeated openings that appear along the pharynx caudal to the mouth. With this position, they allow for the movement of water in the mouth a ...
s, a
dorsal nerve cord The dorsal nerve cord is an anatomical feature found in chordate animals, mainly in the subphyla Vertebrata and Cephalochordata, as well as in some hemichordates. It is one of the five embryonic features unique to all chordates, the other fo ...
,
notochord The notochord is an elastic, rod-like structure found in chordates. In vertebrates the notochord is an embryonic structure that disintegrates, as the vertebrae develop, to become the nucleus pulposus in the intervertebral discs of the verteb ...
,
endostyle The endostyle is an organ found in invertebrate chordate species of tunicates, lancelets, and in the larval stage of vertebrate lampreys. It assists in filter-feeding. It has evolved into the thyroid in vertebrate chordates. Since the endo ...
, and the post-anal-tail which is depicted and labeled well on the
chordate A chordate ( ) is a bilaterian animal belonging to the phylum Chordata ( ). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics ( synapomorphies) that distinguish them from ot ...
s 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 Pharyngeal slits are Filter feeder, filter-feeding organs found among deuterostomes. Pharyngeal slits are repeated openings that appear along the pharynx caudal to the mouth. With this position, they allow for the movement of water in the mouth a ...
, or pharynx, are
lateral Lateral is a geometric term of location which may also refer to: Biology and healthcare * Lateral (anatomy), a term of location meaning "towards the side" * Lateral cricoarytenoid muscle, an intrinsic muscle of the larynx * Lateral release ( ...
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 The dorsal nerve cord is an anatomical feature found in chordate animals, mainly in the subphyla Vertebrata and Cephalochordata, as well as in some hemichordates. It is one of the five embryonic features unique to all chordates, the other fo ...
serves as a hollow-like backbone where signals are sent throughout the body due to nervous tissue being located in this region. The
notochord The notochord is an elastic, rod-like structure found in chordates. In vertebrates the notochord is an embryonic structure that disintegrates, as the vertebrae develop, to become the nucleus pulposus in the intervertebral discs of the verteb ...
is also toward the tail of the chordate but closer toward the middle of the body than the
dorsal nerve cord The dorsal nerve cord is an anatomical feature found in chordate animals, mainly in the subphyla Vertebrata and Cephalochordata, as well as in some hemichordates. It is one of the five embryonic features unique to all chordates, the other fo ...
and is a water-filled structure that allows the chordate to move in water. The
endostyle The endostyle is an organ found in invertebrate chordate species of tunicates, lancelets, and in the larval stage of vertebrate lampreys. It assists in filter-feeding. It has evolved into the thyroid in vertebrate chordates. Since the endo ...
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 Pharyngeal slits are Filter feeder, filter-feeding organs found among deuterostomes. Pharyngeal slits are repeated openings that appear along the pharynx caudal to the mouth. With this position, they allow for the movement of water in the mouth a ...
changed to become the jaw and
gill A gill () is a respiration organ, respiratory organ that many aquatic ecosystem, aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow r ...
s. The
dorsal nerve cord The dorsal nerve cord is an anatomical feature found in chordate animals, mainly in the subphyla Vertebrata and Cephalochordata, as well as in some hemichordates. It is one of the five embryonic features unique to all chordates, the other fo ...
sends signals to the body like it has done before but now the dorsal nerve cord becomes the central nervous system (CNS). The
notochord The notochord is an elastic, rod-like structure found in chordates. In vertebrates the notochord is an embryonic structure that disintegrates, as the vertebrae develop, to become the nucleus pulposus in the intervertebral discs of the verteb ...
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 The endostyle is an organ found in invertebrate chordate species of tunicates, lancelets, and in the larval stage of vertebrate lampreys. It assists in filter-feeding. It has evolved into the thyroid in vertebrate chordates. Since the endo ...
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 Hyaline cartilage is the glass-like (hyaline) and translucent cartilage found on many joint surfaces. It is also most commonly found in the ribs, nose, larynx, and trachea. Hyaline cartilage is pearl-gray in color, with a firm consistency and has ...
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 The perichondrium (from Greek and ) is a layer of dense irregular connective tissue that surrounds the cartilage of developing bone. It consists of two separate layers: an outer fibrous layer and inner chondrogenic layer. The fibrous layer conta ...
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 fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish aquatic locomotion, swim. Apart from the tail or caudal fin, fish fins have no direct connection with the vertebral column ...
s, a pair of
pelvic fin Pelvic fins or ventral fins are paired fins located on the ventral (belly) surface of fish, and are the lower of the only two sets of paired fins (the other being the laterally positioned pectoral fins). The pelvic fins are homologous to the hi ...
s, two
dorsal fin A dorsal fin is a fin on the back of most marine and freshwater vertebrates. Dorsal fins have evolved independently several times through convergent evolution adapting to marine environments, so the fins are not all homologous. They are found ...
s, an
anal fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish swim. Apart from the tail or caudal fin, fish fins have no direct connection with the back bone and are supported o ...
, and a
caudal fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish swim. Apart from the tail or caudal fin, fish fins have no direct connection with the back bone and are supported only ...
. Pectoral fins are stiff, which enables downward movement, lift, and guidance. The members of the order
Hexanchiformes The Hexanchiformes Help:IPA/English, /hɛkˈsæŋkɪfɔːrmiːz/ are a primitive Order (biology), order of sharks, numbering just five extant species in two Family (biology), families, Chlamydoselachidae and Cow shark, Hexanchidae. Chlamydoselach ...
have only a single dorsal fin. The anal fin is absent in the orders
Squaliformes The Squaliformes are an order of sharks that includes about 126 species in seven families. Members of the order have two dorsal fins, which usually possess spines, they usually have a sharp head, no anal fin or nictitating membrane, and fi ...
,
Squatiniformes Squatiniformes is an order of sharks belonging to Squalomorphi. It contains only a single living genus '' Squatina'', commonly known as angelsharks. The oldest genus of the order, '' Pseudorhina'' is known from the Late Jurassic of Europe. Three o ...
, and
Pristiophoriformes A sawshark or saw shark is a member of a shark order (Pristiophoriformes ) bearing a unique long, saw-like rostrum (snout or bill) edged with sharp teeth, which they use to slash and disable their prey. There are eight species within the Pristio ...
. Shark fins are supported by internal rays called ceratotrichia.


Tail

The tail of a shark consists of the
caudal peduncle Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish swim. Apart from the tail or caudal fin, fish fins have no direct connection with the back bone and are supported only ...
and the caudal fin, which provide the main source of thrust for the shark. Most sharks have
heterocercal Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish swim. Apart from the tail or caudal fin, fish fins have no direct connection with the back bone and are supported only ...
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: * Fast-swimming sharks of open waters, such as the
mackerel shark The Lamniformes (, from Greek ''lamna'' "fish of prey") are an order of sharks commonly known as mackerel sharks (which may also refer specifically to the family Lamnidae). It includes some of the most familiar species of sharks, such as the g ...
s, 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 The whale shark (''Rhincodon typus'') is a slow-moving, filter feeder, filter-feeding carpet shark and the largest known Extant taxon, extant fish species. The largest confirmed individual had a length of . The whale shark holds many records for ...
and
basking shark The basking shark (''Cetorhinus maximus'') is the second-largest living shark and fish, after the whale shark. It is one of three Planktivore, plankton-eating shark species, along with the whale shark and megamouth shark. Typically, basking sh ...
also have this type of tail, although they are generally more sedate animals than the other examples. * "Typical sharks", such as
requiem shark Requiem sharks are sharks of the family Carcharhinidae in the order Carcharhiniformes. They are migratory, live-bearing sharks of warm seas (sometimes of brackish or fresh water) and include such species as the bull shark, lemon shark, blac ...
s, 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 shark Thresher sharks are large mackerel sharks of the family Alopiidae found in all temperate and tropical oceans of the world; the family contains three extant species, all within the genus ''Alopias''. All three thresher shark species have been ...
s have an extreme example of this tail in which the upper lobe has evolved into a weapon for stunning prey. *
Bottom-dwelling The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from the Ancient Greek word (), meaning "the depths". ...
sharks such as
catshark Catshark may refer to: * Scyliorhinidae, a family of ground sharks, many species of which are commonly referred to as "catshark". * Pentanchidae, a family of ground sharks with the overall name deepwater catsharks, but many species are referred ...
s and
carpet shark Carpet sharks are sharks classified in the order (biology), order Orectolobiformes . Sometimes the common name "carpet shark" (given because many species resemble ornately patterned carpets) is used interchangeably with "wobbegong", which is the ...
s 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 shark Squalidae, more commonly known as dogfish, dog sharks, or spiny dogfish, are one of several families of sharks categorized under Squaliformes, making it the second largest order of sharks, numbering 119 species across 7 families. Having earned ...
s 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 shark Angel sharks are sharks belonging to the genus ''Squatina''. They are the only living members of the family Squatinidae and order Squatiniformes. They commonly inhabit sandy seabeds close to in depth. ''Squatina'' and other Squatiniformes d ...
s 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 Buoyancy (), or upthrust, is the force exerted by a fluid opposing the weight of a partially or fully immersed object (which may be also be a parcel of fluid). In a column of fluid, pressure increases with depth as a result of the weight of t ...
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 Shark liver oil is an oil obtained from the livers of sharks. It has been used for centuries as a folk remedy to promote the healing of wounds and as a remedy for respiratory tract and digestive system problems.
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 Osmoregulation is the active regulation of the osmotic pressure of an organism's body fluids, detected by osmoreceptors, to maintain the homeostasis of the organism's water content; that is, it maintains the fluid balance and the concentration ...
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.
Shark Sharks are a group of elasmobranch cartilaginous fish characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the head. Modern sharks are classified within the ...
s 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.


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 Sharks are a group of elasmobranch cartilaginous fish characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the head. Modern sharks are classified within the ...
stomach, buoyancy is established from air taking up space and providing sharks the ability to float. The
shark Sharks are a group of elasmobranch cartilaginous fish characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the head. Modern sharks are classified within the ...
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

Shark Sharks are a group of elasmobranch cartilaginous fish characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the head. Modern sharks are classified within the ...
s' reproductive organs serve to reproduce sexually where the male delivers sperm to the female using
clasper In biology, a clasper is a male anatomical structure found in some groups of animals, used in mating. Male cartilaginous fish have claspers formed from the posterior portion of their pelvic fin which serve to channel semen into the female's ...
s that insert into the female's
oviduct The oviduct in vertebrates is the passageway from an ovary. In human females, this is more usually known as the fallopian tube. The eggs travel along the oviduct. These eggs will either be fertilized by spermatozoa to become a zygote, or will dege ...
. 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

Another way that helps sharks to move through the water effortlessly is partially due to the regulation of their body temperature.
Great white Great White is an American hard rock band formed in Los Angeles in 1977. The band is named after both the shark with the same name, and guitarist Mark Kendall's former stage nickname. In August 2008, Great White estimated they had sold aroun ...
sharks, shortfin mako, longfin mako,
salmon shark The salmon shark (''Lamna ditropis'') is a species of mackerel shark found in the northern Pacific ocean. Similar to other mackerel sharks, salmon sharks have a thunniform body type. As an apex predator, the salmon shark feeds on salmon, squid, s ...
, and
porbeagle The porbeagle or porbeagle shark (''Lamna nasus'') is a species of mackerel shark in the family Lamnidae, distributed widely in the cold and temperate marine waters of the North Atlantic and Southern Hemisphere. In the North Pacific, its ecolo ...
are
endothermic An endothermic process is a chemical or physical process that absorbs heat from its surroundings. In terms of thermodynamics, it is a thermodynamic process with an increase in the enthalpy (or internal energy ) of the system.Oxtoby, D. W; Gillis, ...
, which helps them move quickly in water. They are able to regulate their body temperature depending on the temperature of the water they are in, in order to contract their muscles and swim faster. White sharks are often referred to as "cold-blooded killers," but they actually have the ability to warm their blood. Having the ability to keep their warmth helps them as predators as well. Another group of sharks, known as the
mackerel sharks The Lamniformes (, from Greek ''lamna'' "fish of prey") are an order of sharks commonly known as mackerel sharks (which may also refer specifically to the family Lamnidae). It includes some of the most familiar species of sharks, such as the grea ...
are able to warm their blood. These mackerel sharks retain their blood by using a heat exchange system called
rete mirabile A rete mirabile (Latin for "wonderful net"; : retia mirabilia) is a complex of arteries and veins lying very close to each other, found in some vertebrates, mainly warm-blooded ones. The rete mirabile utilizes countercurrent blood flow within the ...
. 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
dolphin A dolphin is an aquatic mammal in the cetacean clade Odontoceti (toothed whale). Dolphins belong to the families Delphinidae (the oceanic dolphins), Platanistidae (the Indian river dolphins), Iniidae (the New World river dolphins), Pontopori ...
s and
squid A squid (: squid) is a mollusc with an elongated soft body, large eyes, eight cephalopod limb, arms, and two tentacles in the orders Myopsida, Oegopsida, and Bathyteuthida (though many other molluscs within the broader Neocoleoidea are also ...
. 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 fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to laminar flow, which occurs when a fluid flows in parallel layers with no disruption between ...
in streamlines passing over the body. Skin is composed of a
dermis The dermis or corium is a layer of skin between the epidermis (skin), epidermis (with which it makes up the cutis (anatomy), cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from s ...
and an
epidermis The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermal layer provides a barrier to infection from environmental pathogens and regulates the ...
. 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.


Placoid scales

Super Smooth scales (dermal denticles) coat the skin of sharks, rays, and cartilaginous fishes due to the absence of dermal bone. These scales are present in the dermis, which has fibrous connective tissue components, and project through the epidermis, which contains secretory cells and stratified epidermal cells, to the surface. Homologous in structure to the teeth of vertebrates, these extremely strong scales serve the function of reducing turbulence and drag in water as they reduce high-velocity flow. The larger the fish, the more placoid scales they are likely to have. These projections are extremely teeth-like. The scale projection consists of enamel and a pulp cavity surrounded by
dentin Dentin ( ) (American English) or dentine ( or ) (British English) () is a calcified tissue (biology), tissue of the body and, along with tooth enamel, enamel, cementum, and pulp (tooth), pulp, is one of the four major components of teeth. It i ...
.


Ampullae of Lorenzini

Being most prevalent in
cartilaginous fish Chondrichthyes (; ) is a class of jawed fish that contains the cartilaginous fish or chondrichthyans, which all have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fish'', which have skeleto ...
, fish have a series of sensory organs that are arranged as a network of hundreds to thousands of pores filled with jelly near their eyes, ears, mouth, and nose. These electroreceptors are called
ampullae of Lorenzini Ampullae of Lorenzini (: ''ampulla'') are electroreceptors, sense organs able to detect electric fields. They form a network of mucus-filled pores in the skin of Chondrichthyes, cartilaginous fish (sharks, Ray (fish), rays, and chimaeras) and of ...
, and in 1678 they were first discovered by an Italian physician and
ichthyologist Ichthyology is the branch of zoology devoted to the study of fish, including bony fish (Osteichthyes), cartilaginous fish (Chondrichthyes), and jawless fish (Agnatha). According to FishBase, 35,800 species of fish had been described as of March 2 ...
,
Stefano Lorenzini Stefano Lorenzini (born around 1652, Florence, Italy — date of death unknown) was an Italian physician and noted ichthyologist. He studied medicine in Pisa and surgery at the Hospital of St. Florence Maria Nuova, with teachers including Fra ...
. These pores are used to sense and detect
electromagnetic field An electromagnetic field (also EM field) is a physical field, varying in space and time, that represents the electric and magnetic influences generated by and acting upon electric charges. The field at any point in space and time can be regarde ...
s, and oftentimes these aid in navigational skills and hunting down prey. This can be particularly important at night because sharks cannot just depend on their vision in dark settings, they need another mechanism to help them navigate. Specifically, they are able to detect prey that is buried beneath the sand. There are two different forms of electrolocation, passive electrolocation, and active electrolocation, and sharks rely heavily on these for navigation.


Muscles

Viewed as pelagic predators, sharks have a constantly elevated body temperature through their continuity in swimming, ultimately posing as a physiological advantage for sharks. A large reason they possess this advantage is due to the fact that they possess a red, aerobic, locomotor muscle (RM) and a white locomotor muscle (WM). Temperature largely affects the ability of muscles to contract, and this is with respect to both the environment and internal organismal temperature.


Red locomotor muscle

Producing approximately 25-50% of a shark's power, the RM is what powers the continuous swimming of sharks. This muscle thrives in elevated temperatures and is seen as almost mammal-like. The optimal temperature range for function is 20 to 30 degrees Celsius, and the muscles are deemed ineffective if exposed to cooler temperatures. Overall, the temperature of the RM is retained metabolically and is greatly above that of the external water temperature. This muscle also receives a sufficient blood supply which is why sharks can swim for extended periods of time, which helps break down fat. Red muscle fibers are concentrated in the ventral region of the shark and are next to the vertebral column ultimately making the spinal column stronger. In other words, the first dorsal fin is posterior to the RM. In other fishes, the RM is more lateral. This muscle is increasingly thermally sensitive in both
salmon shark The salmon shark (''Lamna ditropis'') is a species of mackerel shark found in the northern Pacific ocean. Similar to other mackerel sharks, salmon sharks have a thunniform body type. As an apex predator, the salmon shark feeds on salmon, squid, s ...
and
tuna A tuna (: tunas or tuna) is a saltwater fish that belongs to the tribe Thunnini, a subgrouping of the Scombridae ( mackerel) family. The Thunnini comprise 15 species across five genera, the sizes of which vary greatly, ranging from the bul ...
.


White locomotor muscle

The WM in sharks is not as thermally dependent, therefore it is more optimal in functioning across various temperatures. This helps power short bursts in a shark's swimming. This muscle is close to the RM, ultimately allowing for heat transfer from the RM to the WM. Although more suitable for cold temperatures, there has been considerable benefit from its proximal location the RM, only increasing its function. This muscle is really important in tail locomotion and is responsible for the pulsating of a sharks tail and propelling the shark forward. The muscle contracts, and then stiffens to allow the shark to coast through the water.


Camouflage

Sharks may have a combination of colors on the surface of their body that results in the camouflage technique called
countershading Countershading, or Thayer's law, is a method of camouflage in which animal coloration, an animal's coloration is darker on the top or upper side and lighter on the underside of the body. This pattern is found in many species of mammals, reptile ...
. A darker color on the upper side and lighter color on the underside of the body helps prevent visual detection from predators. (For example, the white on the bottom of the shark blends in with the sunlight from the surface when viewed from below.) Countershading can also be accomplished through
bioluminescence Bioluminescence is the emission of light during a chemiluminescence reaction by living organisms. Bioluminescence occurs in multifarious organisms ranging from marine vertebrates and invertebrates, as well as in some Fungus, fungi, microorgani ...
in the few shark species that produce and emit light, such as the kitefin shark, a species of dogfish shark. The species migrates vertically and the arrangement of light-producing organs called
photophore A photophore is a specialized anatomical structure found in a variety of organisms that emits light through the process of boluminescence. This light may be produced endogenously by the organism itself (symbiotic) or generated through a mut ...
s provides ventral countershading. Some species have more elaborate physical camouflage that assists them with blending into their surroundings.
Wobbegong Wobbegong is the common name given to the 12 species of carpet sharks in the family Orectolobidae. They are found in shallow temperate and tropical waters of the western Pacific Ocean and eastern Indian Ocean, chiefly around Australia and Indone ...
s and
angelshark Angel sharks are sharks belonging to the genus ''Squatina''. They are the only living members of the family Squatinidae and order Squatiniformes. They commonly inhabit sandy seabeds close to in depth. ''Squatina'' and other Squatiniformes d ...
s use camouflage to perform
ambush predation Ambush predators or sit-and-wait predators are carnivorous animals that capture their prey via stealth, luring or by (typically instinctive) strategies utilizing an element of surprise. Unlike pursuit predators, who chase to capture prey u ...
.


Circulatory system

Sharks possess a single-circuit circulatory system centered around a two-chambered heart. Blood flows from the heart to the gills where it is oxygenated. This oxygen-rich blood is then carried throughout the body and to the tissues before returning to the heart. As the heart beats, deoxygenated blood enters the sinus venosus. The blood then flows through the atrium to the ventricle, before emptying into the conus arteriosus and leaving the heart.


Respiratory system

In the shark anatomy image, it depicts the beginning half of the shark, including the gills. The shark gills are especially important and were evolved from the
chordate A chordate ( ) is a bilaterian animal belonging to the phylum Chordata ( ). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics ( synapomorphies) that distinguish them from ot ...
pharyngeal gill slits
synapomorphy In phylogenetics, an apomorphy (or derived trait) is a novel Phenotypic trait, character or character state that has evolution, evolved from its ancestral form (or Plesiomorphy and symplesiomorphy, plesiomorphy). A synapomorphy is an apomorphy sh ...
. Like lungs in other animals,
gills A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are ...
are essential for sharks to breathe underwater by extracting oxygen from water. The water enters through the mouth, passes into the pharynx, and exits through the
gill slits Gill slits are individual openings to gills, i.e., multiple gill arches, which lack a single outer cover. Such gills are characteristic of cartilaginous fish such as sharks and rays, as well as deep-branching vertebrates such as lampreys. In cont ...
. Most shark species have five gill slits on each side, however, some species can have up to six or seven like the sixgill sawshark and sharks in the order
Hexanchiformes The Hexanchiformes Help:IPA/English, /hɛkˈsæŋkɪfɔːrmiːz/ are a primitive Order (biology), order of sharks, numbering just five extant species in two Family (biology), families, Chlamydoselachidae and Cow shark, Hexanchidae. Chlamydoselach ...
. As part of their respiratory system, sharks also have an accessory respiratory opening called a spiracle behind their eyes. Spiracles are cartilaginous structures located on the top of a shark's head to draw oxygenated water from above in addition to it passing over the gills.


Gill structure and components

Like most fishes, sharks
gill slits Gill slits are individual openings to gills, i.e., multiple gill arches, which lack a single outer cover. Such gills are characteristic of cartilaginous fish such as sharks and rays, as well as deep-branching vertebrates such as lampreys. In cont ...
are located on its external surface on both lateral sides near the head. Inside the gill slits, are long projection-like structures called gill filaments. Gill filaments are lateral to the gill arches and have a high surface area, where they form folds (
lamellae Lamella (: lamellae) means a small plate or flake in Latin, and in English may refer to: Biology * Lamella (mycology), a papery rib beneath a mushroom cap * Lamella (botany) * Lamella (surface anatomy), a plate-like structure in an animal * Lame ...
) inside the gill slits. Lamellae in the gill slits are thin, membrane folds that have access to blood supplies via arteries and are the site of gas exchange. When oxygen-rich water enters the gills, the blood takes up the oxygen through diffusion at the site of lamellae and expels carbon dioxide. To support the gills in ventilation, spiracles take in more water and ventilate the gill, even when sharks are feeding.
Gill rakers Gill rakers in fish are bony or cartilaginous processes that project from the branchial arch (gill arch) and are involved with suspension feeding tiny prey. They are not to be confused with the gill filaments that compose the fleshy part of the ...
are cartilaginous structures inside gill arches that act in the filtration of food particles in feeding as water moves in through the gills.


Mechanisms of breathing

There are two mechanisms that sharks can use to move water over their gills: in buccal pumping, the shark actively pulls in water using its buccal muscles, while in ram ventilation, the shark swims forward, forcing water into its mouth and through its gills. Buccal pumping is more energy intensive than ram ventilation. Sedentary, bottom-dwelling sharks generally use buccal pumping to move water over to their gills compared to more active sharks, who will use ram ventilation and swim to force water to their mouth and gills. Most sharks can switch between these mechanisms as the situation requires depending on the abundance of oxygen in the water. A few species, such as the
great white shark The great white shark (''Carcharodon carcharias''), also known as the white shark, white pointer, or simply great white, is a species of large Lamniformes, mackerel shark which can be found in the coastal surface waters of all the major ocea ...
, have lost the ability to perform buccal pumping and will suffocate if they stop moving forward due to insufficient oxygen passing over their gills.


See also

*
Fish anatomy Fish anatomy is the study of the form or Morphology (biology), morphology of fish. It can be contrasted with fish physiology, which is the study of how the component parts of fish function together in the living fish. In practice, fish anatomy ...


References

{{anatomy Sharks