Size
The largest of the sharks is the whale shark (Rhiniodon typus). Although the whale shark averages 4 to 12 m (13-39 ft.) in length, one specimen was found to be 18 m (59 ft.) long. The smallest free-swimming whale shark observed was 56 cm (22 in.) which may be close to its birth size.
Whale sharks (Rhiniodon typus) can be as big as two school buses or more.
Only 39 shark species are known to grow more than 3 m (10 ft.) while 176 species stay under 1 m (39 in.) in length. The spined pygmy shark (Squaliolus laticaudus) is probably the smallest of all sharks. Females reach about 18 cm (7.1 in.) in length while mature males may only be 15 cm (5.9 in.) long.
For most shark species, females grow around 25% larger than males.
The largest batoid is the manta ray (Manta birostris) which reaches widths more than 6.7 m (22 ft.).
Body Shape
Sharks typically have a fusiform body (rounded and tapered at both ends). This shape reduces drag and requires a minimum amount of energy to swim.
Wobbegong sharks (Family Orectolobidae) and angelsharks (Family Squatinidae) have a flatter appearance than most sharks. Batoids are flattened, with a ventral mouth and gill openings.
Coloration
Sharks and batoids are generally drably countershaded. Countershading is a type of camouflage in which the dorsal side is darker than the ventral side. The dark top of a countershaded animal blends in with the dark ocean depths when viewed from above. The light ventral side blends in with the lighter surface of the sea when viewed from below. The result is that predators or prey do not see a contrast between the countershaded animal and the environment.
The spined pygmy shark has dense photophores covering its ventral surface but little or none on its sides or top of the body. This bioluminescent pattern has been described as "photophore countershading". On a moonlit night, fish swimming through the water would normally produce a shadow that predators would see. The glowing underside of the spined pygmy shark reduces or eliminates this shadow, making it less conspicuous to predators.
Some sharks and batoids are camouflaged to blend in with the ocean bottom.
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Most stripes and other markings are juvenile colors that fade or disappear with age, as in the case of the tiger shark (Galeocerdo cuvier).
- Some sharks, such as wobbegongs and the whale sharks, keep their markings throughout their life.
The markings of some species change as the shark ages. For instance, young zebra sharks (Stegostoma fasciatum) have dark bands and saddles that fade to rather uniformly distributed spots on adult sharks. Adult Stegostoma fasciatum are called Australian leopard sharks.
Fins
Fins are rigid, supported by cartilaginous rods.
Most sharks have five different types of fins.
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Unlike most bony fishes, the upper lobe of a shark's caudal fin is larger than the lower lobe. As the caudal fin moves back and forth to propel the shark forward, it also moves upward. As the caudal fin continues to lift, the shark's head points down. The overall effect of the motion of the caudal fin results in a forward and downward motion.
- The paired pectoral fins compensate for this downward motion. One function of the rigid pectoral fins is to provide lift in the forward region of the shark's body. This counteracts the downward force caused by the caudal fin.
- Paired pelvic fins stabilize the shark.
- One or two dorsal fins stabilize the shark. Some species have dorsal fin spines.
- A single anal fin provides stability in species where it is present; not all sharks have an anal fin.
The different families of batoids show various amounts of fin fusion and reduction. The greatly expanded pectoral fins are fused to the sides of the head, and the anal fin is absent. Stingrays have a barbed, venomous spine on a whip-like tail.
Head
Eyes
Eyes are lateral on sharks, dorsal on batoids.
Some species have an eyelid-like structure called a nictitating membrane. The nictitating membrane protects the eye from being injured by thrashing prey while the shark is feeding.
Eye size and position vary, depending on the particular habitat or behavior of the species. In general, deep-water sharks have bigger eyes than shallow-water sharks.
Nostrils
Sharks and batoids have ventral external nostrils.
Some species have barbels near the nostrils.
Mouth
On both sharks and batoids, the mouth is usually ventral. It is located at the tip of the snout in the whale shark, megamouth shark (Megachasma pelagios), frilled sharks (Chlamydoselachus spp.), and in the carpet sharks (Family Parascylliidae).
The mouth may have labial folds or furrows.
Teeth are modified, enlarged placoid scales. Sharks have numerous rows of teeth attached at their bases by connective tissue. Several rows of replacement teeth continually develop behind the outer row(s) of functional teeth. As the functional teeth fall out, replacement teeth take their place.
Some species of sharks may shed as many as 30,000 teeth in a lifetime.
Gill slits
Sharks have five to seven pairs of lateral gill slits.
Batoids have five or six pairs of ventral gill slits.
Spiracles
Some species of elasmobranchs have small openings called spiracles behind the eyes at the top of the head. These openings bring oxygen-carrying water into the gill chamber. Spiracles originate from rudimentary first gill slits and are reduced or absent in active, fast-swimming sharks.
Scales
Sharks and batoids have placoid scales, also called dermal denticles (dermal=skin, denticles=teeth). Placoid scales have the same structure as a tooth, consisting of three layers: an outer layer of vitro-dentine (an enamel), dentine, and a pulp cavity. Placoid scales are arranged in a regular pattern in sharks and an irregular pattern in batoids.
Unlike other types of scales, placoid scales do not get larger as the fish grows. Instead, the fish grows more scales.
Like teeth, the shape of the scales is variable among species and can be used to identify the species.
Placoid scales gave rise to teeth, stingrays' spines, and the dorsal spines on horn sharks (Family Heterodontidae) and dogfishes (Family Squalidae).
As a shark or batoid swims, placoid scales may create a series of vortices or whirlpools behind each scale. This enables a shark to swim efficiently.
Placoid scales may also create drag when compared to other animals. The drag on a brown shark (Carcharhinus plumbeus) is up to ten times greater than the smooth and sleek skin of a dolphin.
Shark skin is still used as sandpaper (called shagreen). If the denticles are removed, shark skin is used to make leather products.
Spines
Most rays (Order Myliobatiformes) have one or more venomous spines on the tail. Spines are an adaptation for defending the animal against predators and are not used aggressively. Shallow-water species do, however, pose a risk to unwary bathers because if a ray is disturbed, its natural reaction is to lift its spine.
Skates (Family Rajidae) and guitarfish (Family Rhinobatidae) may have rows of short spines or prickles on the back.
Some shark species, such as horn sharks and dogfishes, have spines associated with their dorsal fins. These spines are an adaptation for defense against predators.
Piked dogfish (Squalus acanthias) have dorsal spines equipped with an irritating toxin. When threatened, piked dogfish curl up and whip their longer second dorsal spine towards an enemy. The toxin can cause allergic reactions in humans that may require hospitalization.
