Shark Anatomy: Labeled Diagrams and Interactive 3D Model

A shark is a cartilaginous fish (class Chondrichthyes): its skeleton is cartilage, not bone. Its body has three regions: the head, from the snout to the last gill slit; the trunk, which holds the body cavity; and the tail, behind the cloaca. It breathes with gills that open through separate gill slits, has no swim bladder, stays buoyant with an oily liver, and digests food in a short intestine with a spiral valve.

This page uses the spiny dogfish, Squalus acanthias, as the reference shark. It is a small shark, usually 70–100 cm long, found in temperate seas worldwide, and it is the standard laboratory shark. Its anatomy is typical of sharks, so what you learn here applies to great whites, hammerheads and other species, with the differences listed further down.

Directions: anterior is towards the snout, posterior towards the tail, dorsal the back and ventral the belly. Left and right are always the shark's own left and right.

The external anatomy of a shark includes the snout, two nostrils, two eyes, a spiracle behind each eye, five gill slits on each side, the mouth, two dorsal fins, paired pectoral and pelvic fins, a heterocercal caudal fin, the lateral line and the cloaca. Males also have two claspers.

Three features identify a spiny dogfish: a sharp spine in front of each dorsal fin, no anal fin, and small white spots on its grey back. The skin is covered with tiny tooth-like placoid scales (dermal denticles), which make it feel like sandpaper when stroked from tail to head.

Inside a shark, the body cavity is filled mostly by a huge two-lobed liver. Between its lobes lie the J-shaped stomach, the spleen, the pancreas and the valvular intestine with its spiral valve, which ends at the rectum, rectal gland and cloaca. The kidneys lie flat against the roof of the cavity, the gonads sit at its front, and the heart sits in a separate pericardial cavity under the gills.

For a step-by-step guide to finding these organs in a real specimen, see our dogfish shark dissection guide.

This is the text version of the 3D model: every labeled structure of the spiny dogfish, grouped by body system, with what it does.

No. A shark's skeleton is made entirely of cartilage. Some parts, such as the vertebrae and jaws, are strengthened by a layer of calcified tiles called tesserae, so they feel hard, but sharks have no true bone. Cartilage is lighter than bone, which helps a fish without a swim bladder stay afloat.

• Chondrocranium: Protects the brain and sense organs and supports the snout. The jaws hang from it by ligaments and the hyomandibular cartilage.
• Palatoquadrate cartilage: Forms the upper jaw. It is not fused to the skull, so the jaws can protrude when the shark bites.
• Meckel's cartilage: Forms the lower jaw. It is closed by the powerful adductor mandibulae muscle.
• Hyomandibular cartilage: Suspends the jaws from the skull and lets them swing forward during a bite.
• Branchial arches: Support the gill septa and gill rakers, and open and close the pharynx during breathing.
• Vertebral column: Supports the body, protects the spinal cord (in the neural arches) and the caudal vessels (in the haemal arches of the tail).
• Pectoral girdle: Supports the pectoral fins and forms the back wall of the pericardial cavity.
• Pelvic girdle: Supports the pelvic fins and, in males, the claspers.
• Ceratotrichia: Support the thin outer fin while keeping it flexible.

Because cartilage rarely fossilises, most fossil sharks are known only from their teeth, spines and vertebrae.

Shark teeth are modified placoid scales: a core of dentine covered by a hard enamel-like layer, with a pulp cavity, set in rows on the jaw cartilages rather than in sockets. New teeth form on the inside of the jaw and move outward to replace worn ones throughout life.

Spiny dogfish teeth are small and similar in both jaws, each with one oblique cusp, so together they form a continuous cutting edge for slicing prey. Other sharks have teeth shaped for their diet: serrated triangles in great white sharks, hooked cockscomb-shaped teeth in tiger sharks, and hundreds of tiny, non-functional teeth in the filter-feeding whale shark.

Most of a shark's body is swimming muscle arranged in zig-zag blocks called myomeres, separated by sheets of connective tissue (myosepta). A horizontal septum divides the epaxial muscles above from the hypaxial muscles below. Waves of contraction along the myomeres bend the body and drive the tail.

• Epaxial muscles: Bends the body and tail from side to side and lifts the head and tail.
• Hypaxial muscles: Bends the body sideways and supports the body cavity.
• Myomeres & myosepta: Each myomere pulls on the myosepta and vertebral column. Contracting them in a wave from head to tail produces swimming.
• Adductor mandibulae: Snaps the lower jaw up against the upper jaw during a bite.

A shark has one heart with four chambers in a row: the sinus venosus, atrium, ventricle and conus arteriosus. It sits in the pericardial cavity just in front of the pectoral girdle and pumps only oxygen-poor blood, forward to the gills.

Blood leaves the conus through the ventral aorta, passes up five pairs of afferent branchial arteries into the gills, picks up oxygen, and is collected by efferent branchial arteries into the dorsal aorta, which supplies the body. Veins and large thin-walled sinuses return it to the sinus venosus. This is a single circuit, unlike the double circulation of mammals.

Sharks breathe with gills inside five pairs of gill pouches. Water enters the mouth (and spiracles), flows over the gill filaments, and leaves through the gill slits. In the gill lamellae, blood flows opposite to the water (countercurrent exchange), which extracts oxygen efficiently.

In spiny dogfish, a half-gill (hemibranch) lines the hyoid arch and full gills (holobranchs) sit on gill arches 1–4, giving nine demibranchs per side. Gill rakers on the inner edges of the arches keep food out of the gill pouches. Dogfish can pump water with their mouth and throat, so, unlike some open-ocean sharks, they do not have to swim to breathe.

Food passes from the mouth and pharynx down the short esophagus into the J-shaped stomach, through the pyloric sphincter and duodenum into the valvular intestine, then out through the rectum and cloaca. The liver and pancreas add bile and enzymes in the duodenum.

• Liver: Produces bile, stores energy as oil and gives buoyancy. Sharks have no swim bladder; low-density liver oil offsets their weight.
• Cardiac stomach: Stores food and begins digestion with acid and enzymes. Internal folds (rugae) let it stretch.
• Pyloric stomach: Mixes food and passes it to the intestine in small amounts.
• Pancreas: Makes digestive enzymes (released into the intestine) and the hormones insulin and glucagon.
• Valvular intestine: The main site of digestion and absorption.
• Spiral valve: Greatly increases the surface for absorption and slows food so it stays longer in contact with the gut lining.

The spiral valve lets a short intestine absorb food efficiently. CT scans of 22 shark families (Leigh et al., 2021) showed that its spiral shape also makes fluid flow mostly in one direction, like the valve Nikola Tesla designed in 1920.

Sharks keep their body fluids about as concentrated as seawater by retaining urea and TMAO in the blood, rather than by drinking and pumping out salt like bony fish. The long, ribbon-like kidneys conserve urea, and the rectal gland secretes excess sodium chloride into the rectum. Burger and Hess discovered this gland's salt-secreting role in the spiny dogfish in 1960.

Sharks combine smell, hearing, vision, the lateral line and electroreception to find prey. The ampullae of Lorenzini detect weak electric fields from prey, the lateral line senses water movement, and large olfactory sacs give an acute sense of smell.

• Olfactory bulbs & tracts: Receive the olfactory nerve fibres and process smell before relaying it to the forebrain.
• Telencephalon: Integrates smell with other senses and helps control behaviour.
• Diencephalon: Relays sensory signals and controls hormones: the epiphysis (pineal) senses light and the hypophysis (pituitary) hangs from its floor.
• Optic lobes: Process vision and coordinate eye and body movements with what the shark sees.
• Cerebellum: Coordinates swimming and balance.
• Medulla oblongata: Controls breathing and heart rate, and is the origin of most cranial nerves (V–X).

Spiny dogfish eyes have no movable eyelids and no nictitating membrane (third eyelid). Requiem and other ground sharks, such as tiger, bull and lemon sharks, close a nictitating membrane over the eye when they bite.

You can tell male and female sharks apart from the outside: males have two claspers on the inner edges of the pelvic fins, and females do not. Fertilisation is internal. Inside, both sexes have paired gonads at the front of the body cavity, but their ducts differ.

Spiny dogfish are aplacental viviparous (older texts say ovoviviparous): the pups develop inside the uteri, living on the yolk in their yolk sacs, and are born alive. Gestation lasts about 18–24 months, one of the longest known in any vertebrate, and litters range from 1 to about 20 pups, each 18–30 cm long at birth (FishBase). Switch the 3D model to Female to see the uteri and pups.

The great white shark (Carcharodon carcharias) has the same basic organs as a dogfish, but it is built for fast, open-water hunting of large prey. It is a regional endotherm: networks of blood vessels (retia mirabilia) keep its swimming muscles, eyes and brain warmer than the water, and its red swimming muscle commonly runs several degrees above water temperature.

• A nearly symmetrical, crescent-shaped (lunate) tail with keels on the caudal peduncle, instead of the dogfish's heterocercal tail.
• Large, serrated, triangular teeth for cutting flesh; no dorsal fin spines; an anal fin is present.
• No nictitating membrane: it rolls its eyes backward when it strikes.
• Spiracles are tiny or absent, and it relies on swimming to ventilate its gills (ram ventilation).

A hammerhead shark's wide, flattened head is called the cephalofoil. It spreads the eyes, nostrils and ampullae of Lorenzini across a wide surface. Juvenile scalloped hammerheads search a wider strip of seabed for buried prey than sandbar sharks with the same electrosensitivity (Kajiura & Holland, 2002), and the widely spaced eyes give hammerheads larger binocular overlap than requiem sharks: 48° in the winghead and 34° in the scalloped hammerhead, versus about 10° in the lemon shark (McComb et al., 2009).

Inside, hammerheads have the same organ systems as the dogfish. Unlike the dogfish, they nourish their young through a yolk-sac placenta.

The whale shark (Rhincodon typus), the largest living fish, feeds on plankton. Its mouth is at the front of the head (terminal), not underneath, and its teeth are tiny and play no part in feeding. Instead, water passes through filter pads that block the pharynx in front of the gills: Motta et al. (2010) described 20 pads covered with a mesh of openings averaging about 1.2 mm, which they suggested work by cross-flow filtration so they do not clog.

The basking shark (Cetorhinus maximus), the second-largest fish, is also a filter feeder. It swims slowly with its huge mouth open, and water flows out through enormous gill slits that almost encircle the head. Long, bristle-like gill rakers on the gill arches strain out zooplankton. These are the same structures that, in the dogfish, are short knobs that simply keep food out of the gill pouches.

Every shark shares the cartilaginous skeleton, gills, spiral valve, rectal gland and ampullae of Lorenzini. This table lists the anatomical features that set common species apart from the spiny dogfish.

Use the 3D model or the diagrams above to answer these. Answers are on the dissection page.

1. Which external feature tells you a shark is male?
2. How many gill slits does a spiny dogfish have on each side?
3. Name the four chambers of the shark heart in order.
4. Which organ gives a shark buoyancy instead of a swim bladder?
5. What structure inside the intestine increases its surface area?
6. Which gland removes excess salt?
7. Name two sense organs that sharks have and humans do not.

Full lab questions with an answer key are in the shark dissection guide.