School and/or Shoal of Fish

To the surprise of many, there are actually two types of fish aggregations: shoals and schools. A shoal is a looser group, sometimes consisting of different species that hang out together (often temporarily) but are not organized. On the other hand, schools are notoriously highly structured with coordinated movements and a common direction. A group of fish can switch from shoaling to schooling and back again. Schooling seems to have evolved as a defense against predators and in some cases as a way to increase feeding efficiency. Every fish in a school must execute perfectly to get the best results.