The overwhelming majority of species of Copepoda and Ostracoda show a 

 tendency to aggregation (Fasham et al . , 1974). Aggregations* may be 

 differentiated depending on sex, age and physiologic condition of individuals. 

 In the copepod E pilabidocera a mphitrites , accumulations of the crustaceans 

 near the surface of the water consist only of males, which differ from the 

 females in that they exhibit phototaxis (Park, 1965). In Metridia l onga , the 

 microdistribution of the older copepodites differs from the distribution of 

 sexually mature individuals, which inhabit the same layer of water 

 (Romanovskiy, Chusova, 1974). Doubtless, this sort of heterogeneity is 

 adaptive. An example is the Monstril loida , semiparasites of sublittoral 

 invertebrates. Their hosts live in clusters, and it is not surprising 

 that the crustaceans themselves are also clusterers. The grouping of 

 hosts helps the nauplii to find hosts, while the grouping of the 

 monstrilloidae themselves facilitates simultaneous emission of nauplii 

 and their settlement (Isaac, 1974). This increases the level of inbreeding 

 in the population, which in turn facilitates sympatric speciation, i.e., 

 the development of heterogeneity at a higher level. 



Among the Decapoda, schooling is widespread, the characteristics 

 of the school being frequently quite homogeneous. Schooling is most common 

 among surface-dwelling species, for example, the shrimp Sergestes s imilis , 

 which forms accumulations with densities of up to 4500 indiv./m^ (Omori et 

 al., 1972). 



Mixed aggregations are also found, consisting of individuals of various 

 species, e.g., the shrimp A cetes kochinensis and various genera of 

 amphipods (Achuthankutty et al . , 1973). The" reasons for formation of this 

 type of aggregation are not clear. In the pelagic crabs Polybius henslowi 

 and Charybdis edwardsi , surface schools contain individuals of both sexes, 

 oriented against the current and precisely observing their distance (J. A. 

 Allen, 1968; Delia Croce, 1961; Delia Croce, Holthuis, 1964, 1965; Rice, 1969). 

 Another method of observation of distance is the movement of crustaceans in 

 chains during seasonal migrations of the Stomatopoda Oratosquil la i nvestigatoris 

 (Losse, Merrett, 1971) and the lobster Panulirus argus (Herrnkind et al . , 

 1973). The lobsters move one after another, with their antennae or 

 pereopods touching the abdomen or tail of the lobster in front; a single 

 column may include as many as 65 individuals. During brief stops, the 

 lobsters also stay in an organized cluster, e.g., a "rosette." 



3.5 A daptive Value of Schooling Reactions 



The collective reactions of plankters, during which the school reorganizes 

 or is restored, consist of a combination of behavioral events which take 

 consideration of the behavior of neighbors. 



*The word "schooling," in our terminology is used to stress the active 

 role of inner factors in forming an aggregation and the intensity of 

 interdependence between the individuals. 



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