stimulus, allowing them to coexist (Blaker et al . , 1964). It has been 

 experimentally shown that in Balanus improvisus , information on the condition 

 of a neighbor influences the mechanoreceptors, the micropopulation "tuning" 

 itself to the rhythm of beating of the most active individual (Vilenkin, 

 Vilenkina, 1971; Vilenkin, Zaikin, 1975). 



3.4 Intraspecific Structures of Behavior of a Group of Individuals 



One means of avoiding competition where ecologic niches overlap is 

 heterogeneity of the spatial distribution of micropopulations, hemipopula- 

 tions or groups of other ranks with different types of behavior. Hetero- 

 geneity, in particular, may result from a group response to gradients in 

 biotic or abiotic environmental conditions. These responses are most 

 typical for nonpredatory animals. The most important methods of formation 

 of discreteness probably differ in organisms of different sizes, but they 

 all lead to an increase in the isolation of groups. 



Schooling may disappear or appear under certain conditions. For 

 example, the cladoceran P olyphemus pedi cuius , in an aquarium with sufficient 

 food, normally live and breed individually, but in plankton live only in 

 groups with "jery constant locations (Butorina, 1972). Butorina considers 

 the group to be a food-hunting association of voracious predators feeding 

 on prey which also live in groups. The group of predators includes 

 individuals with identical spectra of prey, similar nutritional demands 

 and compatible speed of movement. When the population density is low, these 

 groups do not form (Luferov, 1970; Luferov, Stetsenko, 1971). Moina swim 

 in fish-free areas in dense schools, simultaneously changing direction and 

 retaining their order. However, in the presence of predatory fish, schools 

 do not develop (D. S. Johnson, Chua, 1973). 



Underwater observations have established the existence of schooling 

 in the Copepoda Arcatia spinata , Oithona nana, 0. oculata , Farranula 

 gracilis and others (Emery, 1968). A school of Farranula g racilis acts like 

 a single individual and holds its position against the current. If alarmed, 

 the animals scatter, but then the school reforms. The density of Copepoda 

 in the school may be as high as 105 indiv./m3. 



The reasons and mechanisms of formations of clusters, aggregations and 

 schools differ for different forms of life, and also for large and small 

 organisms. For nonselective filter-feeders--Appendicularia--which seem to 

 be poor swimmers, the "collective force" may create local hydrodynamic 

 factors, simultaneously facilitating increases in phytoplankton concentration. 

 Clusters of Oikopleura dioica (concentration on the order of 10^ indiv./m^) 

 have been described several meters wide and several kilometers long, 

 developing in the region of a phytoplankton bloom (Seki, 1973). Clusters 

 of Oikopleura frequently form parallel strips up to 30 m long, with a 

 width and depth of a few centimeters. Owen (1967) considers small-scale 

 horizontal surface vortices which develop at lines of convergence, i.e., 

 Langmuir circulation, to be responsible for these clusters. It is possible 

 that the movement of these strips of Oikopleura helps to reduce the 

 resistance of the water, due to the mucus which the animals excrete. 



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