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School of colorful reef fish, including Pennantfish, Pyramid and Milletseed butterflyfish, at Rapture Reef, French Frigate Shoals.

The term swarm (schooling, swarming or flocking) is applied to fish, insects, birds and microorganisms, such as bacteria, and describes a behavior of an aggregation (school) of animals of similar size and body orientation, generally cruising in the same direction.

Swarming of honey bees is a more specific term, referring to the reproductive action of an entire colony of bees (as opposed to the reproduction of single bees); see Queen bee and Honey bee life cycle.

Underwater video (looping) of a school of Atlantic herring Clupea harengus on its migration to their spawning grounds in the Baltic Sea. With such high speed they can migrate over thousands of kilometers. Some scientists are of the opinion that cruising in a close group has advantages in the energy consumption, one fish utilizing the pressure field created by the next fish. In the North Atlantic herring cruise between Norway and Greenland every year.

Shoal can describe any group of fish, including mixed-species groups, reserving "school" for more closely knit groups of the same species swimming in a highly synchronized and polarized manner.

Fish derive many benefits from shoaling behaviour including defense against predators (by enhanced predator detection and diluting the chance of capture), enhanced foraging success, and higher success of finding a mate. It is also likely that fish benefit from shoal membership through increased hydrodynamic efficiency.

Juvenile herring hunt for the very alert and evasive copepods in schools: The copepods can sense with their antennae the pressure-wave of the approaching herring and react with a fast escape jump. The length of the jump is quite consistent. The fish arrange in a grid of this characteristic jumplength. The copepods can dart about 80 times before they tire out. It takes 60 milliseconds to spread out the antennae again, and this timeslot is utilized by the herring to finally snap a copepod. A single juvenile herring would never be able to catch a large copepod ("Synchropredation" — results from in situ videos taken from the ATOLL laboratory).

One feature of a shoal is the strong resemblance between member fish. Fish use many traits to choose shoal mates including size of shoal, species type, body size, health of shoal members, and kinship.

Fish often choose to be in a shoal that consists of individuals similar in appearance to themselves; the "oddity effect" posits that any shoal member that stands out in appearance will be preferentially targeted by predators. The oddity effect would therefore tend to homogenize shoals.

One puzzling aspect of shoal selection is how a fish can choose to join a shoal of animals of similar appearance, given that it cannot know its own colour. Experiments with zebrafish have shown that shoal preference is a learned ability, not innate. A zebrafish tends to associate with shoals that resemble shoals that it was reared in (that is, a form of imprinting).

Other open questions of shoaling behaviour include determining the direction of shoal movement. In the case of Migratory movement, most members of a shoal seem to know where they are going, but foraging behaviour is more problematic. Animal behaviourist Stephan G. Reebs, writing in the journal Animal Behaviour, argues that shoals of golden shiner (a kind of minnow) were led by a small number of more experienced individuals.

school of fish at Madagascar

• "One of the most striking behaviours of a school is its synchronization. Hundreds of small fish glide in unison, more like a single organism than a collection of individuals" Hiro-Sato Niwa, Journal of Theoretical Biology, 1996: 181,p 47

This is a partial list of animals that swarm.

• Ants
• Birds
• Eels
• Grasshoppers (Locusts)
• Honey bees
• Termites

• Herd
• Swarm Intelligence
• Flocking (behavior)

New York times article on investigations into swarming