water from an aquarium containing these females causes only mild manifesta- 

 tion of searching sexual activity. When males contact females which have 

 not moulted, only aggressive behavior is manifested (Atema, Engstrom, 1971). 

 The males of many crabs , e.g. , Carcinus m aenas , Macropipus hoi satus and 

 others (Bales, 1974), assume the mating position only in the presence of 

 females which have recently moulted or are in the premoulting stage. 

 The pheromone is species-specific and is excreted with the urine. 



Thus, the contact of the sexes at the biologically most favorable time, 

 and reproductive isolation are assured in many cases by the chemical 

 communication channel. The functions of this channel, the end result of 

 which is aggregation of the animals, have many aspects, like all manifesta- 

 tions of topic connection. One means of aggregation is excretion by mature 

 animals of substances which stimulate settling of meroplanktonic larvae. 

 The larvae of barnacles, in which this phenomenon was demonstrated for 

 the first time (Crisp, Meadows, 1963), respond only to the "settlement 

 factor" if it is found on a surface suitable for settlement in all its 

 parameters . 



The bentbic predators freauently excrete substances which "mark" the 

 place where prey is found. Thus, hungry Acanthaster planci sea stars move 

 more rapidly toward coral (their only food) on which A. planci are already 

 feeding (Ormond et al . , 1973). 



Chemotactic communication as a means of recognizing individuals of 

 one's own species (not necessarily of the opposite sex) results in the 

 formation of aggregations and also acts as a mechanism for regulation of 

 numbers. This is well known in the planaria. Phagocata g racilis and 

 Cura foremani , mixed together in a single vessel to the point of random 

 distribution, regroup in single-species clusters quite rapidly. The 

 adaptive value of this form of behavior consists in that P_. gracil is lays 

 its egg capsules only in clusters of at least 200 individuals (Reynierse, 

 1957; Reynierse et al . , 1969). Dugesia t igrina spontaneously decapitates 

 when greatly crowded--one individual per 10 ml of culture fluid. It is 

 assumed that the metabolite causing this phenomenon suppresses asexual 

 reproduction of the planaria, replacing the neurohumoral stimulus for 

 reproduction (Vowinckel et al . , 1970). 



Frequently, the level of activity of an animal and its diurnal 

 rhythm depend on the number of individuals in the group, increasing with 

 increasing number of individuals up to a certain limit, then sometimes 

 decreasing. The manifestations of the group effect are various, although 

 the mechanism of the phenomenon itself is frequently unclear. For example, 

 in males of the gastropod mollusks C repidula f ornicata , joining of the 

 mollusks into chains of several individuals activates the growth of all 

 the individuals except for the uppermost male, the growth rate of which 

 is the same as that of an isolated male (Le Gall, 1973). In a mixed culture 

 of three species of Black Sea mollusks, the oxygen consumption and growth 

 rate were both lower than for single-species cultivation under the same 

 conditions (Kiseleva, 1974). In any case, the metabolites have a direct 

 relationship to the regulation of growth of the members of a community. 



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