Until recently, many biologists believed that invertebrate "schools" were actually transient assemblages, brought together by wind, currents, waves, or common food sources. Jellyfish groupings, for example, cannot be described as schools—cohesive social units whose members are evenly spaced and face the same way. However, recent research has found numerous cases in which crustaceans and other invertebrates form schools as fish do. Schooling crustaceans such as krill regularly collect in such massive numbers that they provide abundant food for fish, seabirds, and whales.

Like schooling fish, invertebrates with sufficient mobility to school will swim in positions that are consistent relative to fellow school members, and are neither directly above nor directly below a neighbor. The internal structure of such a school changes little with external physical disruption but dramatically with the advent of a predator.

Since schooling is an active behavior, researchers assume that it must bring important benefits. True, schooling would appear to make animals more visible and attractive to predators. However, schooling leaves vast tracts of empty water, thereby reducing a predator's chances of picking up the school's trail. A large group maintains surveillance better than an individual can, and may discourage predation by appearing to be one massive animal. And although an attacking predator may eat some of the invertebrates, any individual school member has a good probability of escaping.

In addition to conferring passive advantages, schooling permits the use of more active defense mechanisms. When a predator is sighted, the school compacts, so that a predator's senses may be unable to resolve individuals, or so that the school can execute escape maneuvers, such as freezing to foil predators that hunt by detecting turbulence. If the predator attacks, the school may split, or may employ "flash expansion"—an explosive acceleration of animals away from the school's center. When large predators threaten the entire school, the school may attempt to avoid detection altogether or to reduce the density of the school at the point of attack; when small predators threaten the margins, school members may put on dazzling and confusing displays of synchronized swimming.

Schooling may also enable invertebrates to locate food—when one group member finds food, other members observe its behavior and flock to the food source. On the other hand, competition within the school for food may be intense: some mysids circle around to the back of the school in order to eat food particles surreptitiously. Schooling can facilitate the search for mates, but as a school's numbers rise, food may become locally scarce and females may produce smaller clutches of eggs, or adults may start to feed on the young. Thus, circumstances apparently dictate the optimal size of a school; if that size is exceeded, some of the animals will join another school.