observed in areas with abundant plankton. It is not clear whether true 

 schooling occurs in euphausiids with predatory and mixed feeding habits. 

 Large aggregations are occasionally formed by Thysanoessa longicaudata 

 (Forsyth, Jones, 1956) , Euphausia d iomedae and Stylocheiron abbreviatum 

 (Roger, 1974), but these data, based on extended catches, do not prove that 

 what was encountered was a true school, rather than a simple accumulation 

 with an unknown degree of integration. 



Schooling is seen in most massive species of mysids (Mauchline, 1971). 

 Schools of L eptomysis linguura may be quite permanent and remain almost every 

 day in the same place. Schooling is observed in _L. mediterranea and Siriella 

 armata only in the day, particularly if there is a gradient of illumination. 

 Schools of !L. 1 inguura simultaneously contain individuals with different 

 photoactivation (Macquart-Moul in, 1971, 1973; Macquart-Moul in , Patriti, 

 1966). Schools of Mysidium gracile find shelter among the spines of the 

 sea urchin diadema antillarum (Emery, 1968). Fish never capture mysids 

 from these schools. 



The characteristics of schooling and the behavior of mysids in a 

 school agree in basic features among the various species (Zelickman, 

 1974; Zelickman, 1975); the behavior of mysids and euphausiids is similar 

 to the behavior of fish in a school as described by Radakov (1972). The 

 orientation of individuals in a school and observation of spacing are 

 apparently maintained by the optometer and kinesthetic systems of orienta- 

 tion. The forms of integration of groups of mysids and other crustaceans 

 which have been observed permit their behavior to be interpreted as an 

 integral, complex reaction of the organisms, rather than simply an ensemble 

 of kineses and taxes. The mechanism of orientation of the mysids is probably 

 governed by the level of illumination, the position of the light source, its 

 polarity and, possibly, spectral characteristics; no less important is 

 distance perception--the location of fluctuating movements of the water, at 

 least for species which school in darkness. It is possible that various 

 reception systems function simultaneously, with an overall coordinator. 

 There is e^^ery reason to assume, for example, synergy of receptor perceptions 

 in hermit crabs (Hazlett, 1970). 



We must not consider the heterogeneity of physical characteristics of 

 the aquatic medium the primary cause of the high degree of organization of 

 groups of animals, particularly crustaceans, both in nature and under 

 experimental conditions. The adaptive capabilities of schooling behavior 

 among crustaceans can be compared to selective advantages which arise upon 

 schooling of fish. These advantages must entail evolutionary optimization 

 of the collective reactions of animals favorable to the population. These 

 collective reactions are most useful to massive species subject to predation, 

 and are also necessary in situations requiring movement of entire populations, 

 e.g., spawning migrations. What are the parameters which must be optimized 

 in order to preserve a population? In this case, one of the semiquantitative 

 abstract models (Vine, 1971) of differentiation of the hunting behavior of a 

 "visual predator" as a function of the type of distribution of prey seems 

 applicable. This model considers a situation in which the prey, of identical 

 size, is attacked by a predator, which is located either within or outside 



34 



