Sharks are cartilaginous fish with tooth-like skin, five to seven gill slits on each side and replacement teeth stacked behind the working row, and their behaviour follows directly from that build: they close on prey through a sequence of senses, most take fish and invertebrates, and only three of the more than five hundred described species filter plankton.
The behavioural range is wider than the reputation allows. Some species drown if they stop swimming while others lie on the bottom for hours; most hunt alone, a few gather in the hundreds; and in an incident record of 7,145 entries going back five centuries, a small number of species account for almost every serious bite.
- Described shark speciesMore than 500
- Species that filter feedThree
- Incidents in our database7,145 entries, 1,427 fatal
What sharks eat and how they find it
Diet across the group runs from copepods to seals. Reef species take crabs, octopus and small fish; houndsharks and bullhead sharks crush shellfish with flattened back teeth; the large coastal predators take rays, other sharks and, in a few species, marine mammals; and three species filter plankton rather than biting anything. Scavenging matters more than most accounts admit, because a dead whale is a far cheaper meal than a live seal.
The hunt itself runs as a sequence, and each sense hands over to the next. Smell works only along the path a current has already carried the scent, which is why the claim about detecting blood from miles away does not survive contact with how water moves — the real range is a few hundred metres in a decent flow and effectively nothing in still water. Sound and the lateral line pick up the irregular movement of something injured. Vision takes over in the last stretch, and in the final moment the ampullae of Lorenzini read the electric field of the target's own muscles, at a range of centimetres. A shark that bites something unfamiliar is usually finishing that sequence rather than deciding anything.
Movement, breathing and rest
Water has to keep crossing the gills, and species solve that in two ways. Fast open-water sharks such as the mako and the white shark rely on ram ventilation: they swim with the mouth slightly open and let forward motion do the work, which is why stopping is not an option for them. Bottom-dwelling species pump water across the gills with the throat muscles, or draw it in through a pair of spiracles behind the eyes, and can lie still indefinitely. Nurse sharks stacked in a cave and whitetip reef sharks under a ledge are doing exactly that.
Whether any of this counts as sleep was an open question until work on the draughtsboard shark measured a drop in metabolic rate during long still periods, along with a posture change — the physiological signature of sleep rather than simple inactivity. At the other end of the scale, movement can be enormous. Blue sharks cross ocean basins, white sharks in the north-east Pacific run out to a patch of open water between Baja California and Hawaii and back each year, and depth range within a single species can span the surface to well over a kilometre. Where sharks live covers the habitats in more detail.
Social behaviour, and behaviour around people
The solitary hunter is the default but not the rule. Scalloped hammerheads form daytime aggregations over seamounts that can run into the hundreds, blue sharks travel in groups sorted by size and sex, and lemon sharks have been shown in captivity to learn faster when they can watch another shark do the task first. Many species also segregate by size, which keeps juveniles out of the areas the adults use — a straightforward way of not being eaten by your own kind. The collective nouns people use for those gatherings are a separate matter, and mostly modern inventions.
Behaviour around swimmers is the part most often described backwards. Our own incident file holds 7,145 records from the sixteenth century to this year, 1,427 of them fatal, and 2,873 of those records fall after 2000 with 243 deaths — roughly nine a year worldwide against hundreds of millions of hours spent in the water. Most entries describe a single bite followed by the animal leaving. For the definition and the taxonomy behind all of this, see what a shark is; the species themselves are listed under types of sharks, and the record is broken down by country under shark attacks.
Frequently asked questions
How many species of shark are there?
More than 500 species have been described, and the total climbs most years as deep-water surveys turn up new ones. The figure of 440 that circulates on older pages is out of date. They fall into eight orders, two of which, the ground sharks and the dogfish sharks, hold roughly four species in every five.
Can sharks smell blood from miles away?
No. Laboratory work measures sensitivity, not range, and scent only reaches a shark if a current has carried it there. In a reasonable tidal flow that means a few hundred metres downstream; in still water it means almost nothing, and a shark sitting upcurrent of a bleeding swimmer detects nothing at all.
Do all sharks have to keep swimming?
No. Fast open-water species such as makos and white sharks depend on forward motion to push water across the gills and cannot stop for long. Bottom-dwelling sharks pump water with their throat muscles or draw it through spiracles behind the eyes, and rest on the seabed for hours at a time.
Do sharks hunt in groups?
Most hunt alone, but group behaviour is real. Scalloped hammerheads gather over seamounts in large daytime aggregations, blue sharks travel in groups sorted by size and sex, and several species feed together opportunistically at a carcass. Coordinated hunting of the kind seen in dolphins has not been demonstrated in sharks.
How dangerous are sharks to swimmers?
Our database holds 7,145 incidents across five centuries, 1,427 of them fatal. Since 2000 it records 2,873 incidents and 243 deaths, which works out at roughly nine fatalities a year worldwide. Most entries describe one bite and then departure, and a handful of large species account for almost all of the serious cases.
References
A small team focused on researching sharks and making marine biology easy to understand. Every page is reviewed against current species records and the SharkSider attack database, and updated when new incidents or research appear.
Published November 30, 2016 · Last updated September 23, 2026 · Part of the Shark Facts section.
