FaunaHub

Shark

Cartilaginous FishMarine Predator

Blacktip reef shark (Carcharhinus melanopterus) swimming in shallow coral reef water.

Blacktip reef shark (Carcharhinus melanopterus) — used here to represent the broader shark group.

Image: Luc Viatour, CC BY-SA 3.0, via Wikimedia Commons.

Overview

Sharks are a diverse group of cartilaginous fish (class Chondrichthyes, superorder Selachimorpha) found in oceans worldwide and, in a few cases, in freshwater systems. More than 500 living species are currently described, ranging from small reef-dwelling species to the whale shark (Rhincodon typus), the largest living fish.

Unlike bony fish, sharks have skeletons made of cartilage and possess several distinctive sensory adaptations, including electroreception via specialised pore systems known as the ampullae of Lorenzini.

Habitat & Range

Sharks occupy a very wide range of marine habitats: coastal seas, coral reefs, the open pelagic ocean, polar waters, and the deep sea. A few species — notably the bull shark (Carcharhinus leucas) — tolerate freshwater and have been recorded far up some river systems.

Diet

Diet varies enormously across the group. Many species are active predators of fish, cephalopods, and marine mammals or seabirds. Others — including the whale shark and basking shark — are filter-feeders that strain plankton and small fish from the water column. Several species are bottom feeders specialising in molluscs, crustaceans, and other benthic prey.

Behavior

Shark behaviour is similarly varied. Some species are largely solitary, while others form aggregations, particularly around feeding or breeding sites. Migratory behaviour is well documented in a number of species, with some individuals covering very long ocean distances each year.

Reproductive strategies include egg-laying (oviparity), live birth (viviparity), and intermediate forms. Many sharks are slow to mature and produce relatively few offspring, which has direct implications for population recovery from exploitation.

Human Interaction & Conservation

Many shark species are currently of significant conservation concern. Drivers include targeted fisheries (including for fins), bycatch, habitat degradation, and slow life-history traits that make populations vulnerable to over-exploitation. Shark attacks on humans are very rare in global terms; from a population perspective the more significant interaction is human impact on sharks rather than the reverse.

Appearance & Recognition

Across the group, sharks share a recognisable body plan: a streamlined fusiform body, paired pectoral and pelvic fins, one or two dorsal fins, an anal fin in many (but not all) groups, and a heterocercal tail in which the upper lobe is typically longer than the lower. Five to seven gill slits on each side of the head distinguish sharks from bony fish, which have a single operculum-covered gill opening. The skin is covered in tooth-like dermal denticles that reduce drag and give shark skin a characteristic rough texture.

Beyond this shared plan, body shape varies dramatically by ecology: torpedo-shaped pelagic species such as the great white and mako are built for speed; the hammerhead has a laterally expanded head (cephalofoil) housing widely spaced sensory pores; the whale shark and basking shark are very large, slow-moving filter-feeders with wide gaping mouths; and benthic species such as wobbegongs are flattened and camouflaged for life on the seabed. Field identification relies on body shape, fin profile, head shape, colouration, and tooth shape.

Similar Animals

Sharks are most closely related to other cartilaginous fish, including rays and skates (Batoidea) and chimaeras (Chimaeriformes). Dolphins, although superficially similar in shape, are mammals and not closely related to sharks — see the dolphin profile for the practical mammal-vs-fish differences.

More photos of the shark

Great white shark (Carcharodon carcharias) launching out of the water at a decoy off South Africa.

Great white shark (Carcharodon carcharias) breaching at a decoy.

Image: Bernard DUPONT from FRANCE, CC BY-SA 2.0, via Wikimedia Commons.

Great white shark (Carcharodon carcharias) breaching at the surface in pursuit of prey.

Great white shark (Carcharodon carcharias) — a contrasting species to the blacktip reef shark in the hero image.

Image: Bernard DUPONT from FRANCE, CC BY-SA 2.0, via Wikimedia Commons.

Frequently Asked Questions — Shark

Are sharks fish?
Yes. Sharks are cartilaginous fish, distinguished from bony fish by skeletons made of cartilage rather than bone, and by a number of other anatomical features. They are not mammals — unlike dolphins, sharks have gills, are cold-blooded in most species, and do not breathe air or nurse young with milk.
How many shark species exist?
More than 500 living species are described, and new species are still being recognised. The exact count varies with taxonomic revisions; the IUCN Red List and major taxonomic databases are the best sources for current figures.
Are sharks dangerous to humans?
The number of recorded unprovoked shark attacks on humans worldwide is small in any given year, and fatalities are rarer still. Most shark species pose no realistic threat to humans. A small number of species — for example the great white, tiger and bull shark — are responsible for most documented serious incidents.
Can sharks live in freshwater?
Most cannot, but a few species — most notably the bull shark — are physiologically able to move between salt and freshwater and have been recorded in some river systems. River sharks (genus Glyphis) also occupy freshwater and estuarine habitats.

Sources and further reading

Authoritative wildlife references used for general educational context. Conservation status should always be verified against current IUCN Red List data. External links open in a new tab.