Many fish aggregations contain members of more than one species, forming heterospecific shoals. In monospecific aggregations, most fishes are of similar sizes (Pitcher 1983). This level of conformity is necessary because unusual appearing individuals contrast with the background of the more common fish and are preferentially selected by attacking predators. In a school, a perfectly healthy and normally swimming individual will be conspicuous to a predator if it is a different size or coloration from the majority of its schoolmates. Additionally, most schools cruise at efficient speeds and escape predators at high speeds that are both dependent on body length. A relatively large or small fish is likely to have different optimal swimming speeds than different-sized schoolmates; a relatively small fish will find itself trailing the school after a fast acceleration. Stragglers, like odd fish, are preferentially attacked by predators. When several abundant, morphologically similar species school together, they tend to segregate by species, either associating with conspecifics more closely or even creating horizontal layers that are relatively monospecific. Hence each fish gains the added benefit of being in a large school but avoids the risk of being the odd individual among another species (Allan 1986; Parrish 1988, 1989a).
Hydrodynamics and predator avoidance dictate uniformity across a school. However, fishes that aggregate for foraging reasons are not as constrained by the need to be similar. For example, foraging schools of parrotfishes and surgeonfishes in the Caribbean frequently include other parrotfishes and surgeonfishes, as well as trumpetfishes, hamlets, butterfl yfishes, goatfishes, and wrasses. Surgeonfishes and parrotfishes both feed on algae and thus benefit from the large numbers that overwhelm territorial herbivorous damselfishes. Carnivorous species may consume invertebrates flushed by the activities of the herbivores or may also capitalize on territorial swamping and feed on invertebrates that live in algal mats of the territory or on the eggs of the damselfish. Larger predators, such as trumpetfish, may use the school or its members as moving blinds that conceal the predator and allow it to feed on the damselfish itself (trumpetfish will change color to match that of large or abundant school members). The presumed costs that small carnivores might suffer due to increased conspicuousness in a heterospecific shoal are apparently outweighed by gaining access to otherwise defended resources. The trade-off is underscored by the evasive maneuvers that minority fish take when a mixed species shoal is threatened. Rather than flee with the school, odd fish abandon the school and seek nearby shelter (Robertson et al. 1976; Aronson 1983; Wolf 1985).
Finally, fishes also form shoals with non-fish species. For reasons that remain puzzling, many tunas school with or below various dolphin species in the tropical Pacific. Fishing boats seek out the dolphin schools and surround them with large purse seine nets as the tuna remain below the mammals; the dolphins unfortunately become bycatch and frequently drown. On a less grand scale, postlarval French grunts school with dense clouds of mysid shrimps shortly after the grunts settle from the plankton and onto coral reefs. Both species are similar in size (8–13 mm) and appearance, but the mysids greatly outnumber the grunts. Grunts benefit from the antipredation function of the schools, affording them a degree of protection probably related to the number of mysids in a school. As the grunts grow, they school more on the periphery of the mysid aggregation and feed on the mysids. What began as a commensal or mutualistic relationship turns into a predator–prey interaction (McFarland & Kotchian 1982).