main biological parameters of the Cladocera, including lifetime of female, 

 the number of litters during a lifetime, intervals between litters, juvenile 

 numbers per litter, duration of maturation period duration prior to the 

 first litter, with the value of potential population productivity (Pigaiko 

 1971). If potential population productivity is reduced from generation to 

 generation, then it is a visual indicator of its pathological state, and the 

 increase of potential productivity, or its maintenance at a stable state, 

 are indicative of well-being, i.e., of the relative norm (Braginsky, et al . 

 1979). 



Apparently, a number of biological productivity methods of assessment of 

 aquatic animals, established for general hydrobiology (Vinberg 1968) with 

 proper ecological and toxicological interpretation can be used in an analo- 

 gous way to demonstrate the pathological state of a population of aquatic 

 animals under toxic environment conditions. 



For parthenogenetic invertebrates, i.e., Cladocera, Rotatoria, a switch 

 to sexual reproduction and laying of subitan eggs (ephippia) indicate un- 

 favorable conditions. However, under the influence of toxicants, this re- 

 sponse is not always observed. Thus, the shift to sexual reproduction and 

 formation of ephippia in Daphnia is absent in those cases exposed to chronic 

 additions of low concentrations of phenyl urea derivatives, triazine, heavy 

 metals, and surfactants. However, other pathological phenomena such as the 

 appearance of dwarf males and parthenogenesis in specimens of half the size 

 of the controls are observed. 



The most frequent manifestation of pathological disturbances in Clado- 

 cera is egg abortion and the appearance of embryonic malformations. While 

 these disturbances may be considered as a change at the organism level, 

 their mass manifestation influences the fate of populations considerably. 



Fluctuations in the number of aquatic populations in nature are highly 

 diverse, and depend upon many factors for which it is difficult to account. 

 Thus, knowledge of causes and mechanisms of these fluctuations is still ex- 

 tremely scanty. For this reason it is better to confine present activities 

 to the concept of developing model laboratory investigations. 



In the conduct of aquatic toxicological experiments it is necessary to 

 resort to the study of laboratory "mini-populations" or "pseudo-popula- 

 tions". An elementary estimation of the median lethal concentration is 

 made on the population model. If an experimental group of warm-blooded 

 animals or fishes is impossible to consider as population, and the LC50 

 value obtained from invertebrates is interpreted as an individual mean, then 

 the analogous group of invertebrate offspring are derived from the same 

 parent and may be considered as an extract of a single population. As 

 experience shows, conclusions drawn from studying such test-culture are in 

 generally valid for aquatic ecosystems where the same species may be repre- 

 sented by a rather numerous population. 



It is useful to consider the significance to the population the crite- 

 rion LC50. A wide utilization of this toxicometric criterion means that 



36 



