IntroductionBranchiopods Artemia Leach, 1819 (Crustacea: Branchiopoda: Anostraca), or brine shrimp, are inhabitants of continental hyperhaline water bodies. Inhabiting water bodies with sometimes extreme living conditions, having an incompletely elucidated species composition of the genus, possessing a complex ontogenetic development cycle, amazing viability of dormant eggs (cysts), as well as alternating sexual and parthenogenetic ways of reproduction, this group of living creatures has always attracted the attention of biologists [1-18]. The increased interest in crustaceans Artemia is also emphasized by the fact that their cysts are of commercial importance, being a strategically im-portant starting (primary) fodder for juveniles of most fish species rearing in aquaculture. The ever-increasing economic need for the brine shrimp cysts is realized exclusively by their removal (catch) from natural populations [16-19]. Dynamically changing climatic conditions lead to sharp fluctuations in the number of brine shrimp in water bodies throughout the season, contributing to a certain unpredictability in forecasting and providing of its catch. At the same time, attempts to cultivate brine shrimp in artificial conditions in order to produce their cysts on an industrial scale have not yet overcome the scope of laboratory experiments. In this regard, the study of the nature of changes in population processes in natural water bodies, as well as their relationship with the main abiotic and biotic processes, the results of which make it possible to justify the rational use of Artemia sp. stocks, do not lose their relevance today.Spreading of crustaceans Artemia in the water bodies of Crimea is extremely uneven, both geographically in terms of the location of hyperhaline water bodies and in time. Often water bodies in which these crustaceans live mostly have extremely shallow depths due to the seasonal distribution of precipitation. They quickly desalinate during heavy rains or, conversely, dry up during long dry periods, which leads to the ces-sation of the development of brine shrimp and its disappearance [1, 2, 8-11].In the eastern part of the Crimean Peninsula Artemia sp., or brine shrimp, lives in almost all water bodies with permanent or seasonal high water salinity with values from 40 to 300‰. For some water bodies, with contrasting hydrological characteristics changing throughout the year, the development of Artemia sp. is characterized by short-term outbreaks of population development. For other water bodies with relatively constant conditions the long period of populations existence can take place.Lake Aktashskoe is located in the northern part of the Kerch Peninsula (Crimea). As a result of its economic use, it is divided into a number of separate reservoirs with contrastingly different morphological characteristics (area, depths, nature of the banks and bed, interconnection with other water bodies) and hydrological regime (provision of surface runoff, water level dynamics) [20]. So, having the complicated structure, this complex is a typical hyperhaline reservoir and is most suitable for performing model population studies.The object of our study was the population of the branchiopod crustacean Artemia sp. inhabiting the reservoirs of the hyperhaline aquatic complex – Lake Aktashskoe. And the subject of the study is the structure of the population of this crustacean and its relationship with abiotic environmental factors.The novelty of this research lies in the fact that, despite the seemingly high degree of study of the brine shrimp, there can be found very poor data on the state of the populations of branchiopod crustacean in the lakes of the Crimea in the autumn-winter period, and there is no data at all on the complex of hyperhaline reservoirs of the Kerch Peninsula. Studies of the brine shrimp in the framework of the rational use of its commercial stocks are limited only to the period of late spring – early autumn.The aim of this work is to study the population structure of the branchiopod crustacean Artemia sp. in non-drying water bodies of the hyperhaline aquatic complex of Lake Aktashskoe in the autumn-winter period of 2018/2019.To achieve this aim, the following tasks were formulated:1. To determine control stations (typical parts of the reservoir that are contrasting in terms of abiotic conditions) and carry out sampling of the brine shrimp in the autumn-winter period, as well as to collect data on the habitat environment (salinity, temperature) in the reservoir.2. To perform cameral processing of collected samples of the brine shrimp to determine their qualitative (population structure by age groups and sex) and quantitative (number and biomass of each of the groups) structure.3. To analyze the relationship of changes in the structure of the brine shrimp population (by age and sex) under the influence of the key abiotic environ-mental factors (salinity and water temperature) occurring in the autumn-winter period. To study the size structure of the Artemia sp. population and its changes in the second half of the vegetation season.4. To determine the composition of other species of invertebrates living in the control hypersaline reservoirs of Lake Aktashskoe complex.Materials and methodsThe material for the study was seven series of zooplankton samples from two water bodies of Lake Aktashskoe complex (Fig. 1), sampled in September-December 2018. Fig. 1. Sampling sites at Lake Aktashskoe: 1 – cooling pond; 2 – natural part The first pond (No. 1 – the cooler) is a hydrotechnical structure lined with an artificial dam, intended for cooling heat-exchange waters of the Crimean nuclear power plant designed at the end of the 20th century (construction was not finished). It is characterized by a significant water surface area (7-9 km2) and comparatively big depth (2.0 m). Its bottom is relatively hard and clayish. The second pond (No. 2 – the natural part) is a part of the lake practically untouched by human activity. It is of much smaller area (up to 1.5 km2) and average depth (0.7 m). Bottom of this water body is covered with thick layer of silt.The control water bodies practically do not have their own catchment area, their water supply occurs mainly with the groundwater and, to a much lesser extent, from atmospheric precipitation. Pond No. 2 has an intermittent connection with the waters of the Kazantip Bay (Sea of Azov) through an artificial channel. Despite significant fluctuations in the water level in both reservoirs (during the time of observations of the authors in No. 1-0.7 m; in No. 2-0.5 m), they do not dry out throughout the year, and therefore it is possible to study the entire annual cycle of development of the Artemia sp. population. Basic characteristics of water environment in the ponds when zooplankton was sampled are shown in Table 1. Table 1 Water temperature and salinity in the ponds when sampling zooplankton PondParametersDates of zooplankton sampling30.09.201815.10.201818.11.201815.12.2018No. 1Temperature, °С19.016.07.83.0Salinity, ‰204196-200208186No. 2Temperature, °С26.0–9.44.0Salinity, ‰140–11276 Sampling was carried out using the Plankton Net acc. to Apstein by filtering 100 liters of water from the water body, taken along a transect with a length of at least 100 m. The samples taken were fixed immediately with a 4% formaldehyde solution. The water temperature (with a mercury thermometer) and its salinity (with a Kelilong RHS-28ATC refractometer) were determined at the same time.Sample processing was carried out in the laboratory according to the generally used methods. Quantitative accounting of different sex and age groups of brine shrimp was carried out using a Bogorov camera, the determination of the individual weight of brine shrimp was carried out using reference tables of the Russian Federal Research Institute of Fisheries and Oceanigraphy [19].The size of the brine shrimp was determined by measurements of at least 20 specimens of each of its size and age groups in each of the samples in the divisions of the eyepiece micrometer. The actual dimensions in micrometers were calculated after calibrating the division value against the micrometer object.Statistical processing of the obtained data was carried out using the Microsoft Excel 2010 and IBM SPSS Statistics v. 22 software packages.ResultsStatus of the Artemia sp. population in the cool-ing pond (No. 1) of Lake Aktashskoe. The Artemia sp. population in the part of Lake Aktashskoe that is transformed into a cooling reservoir is of a complex structure, as it is proved by the data on number and biomass of its different sex and age groups (Table 2). Table 2 Change in the number and biomass of the Artemia sp. by age groups in the cooling pond (No. 1) during September-December 2018Ages groups, genderNumber, ind./m3Biomass, g/m330.09.201815.10.201818.11.201815.12.201830.09.201815.10.201818.11.201815.12.2018Cysts1 0536 6954 455205 9220.010.070.042.06Nauplii21325600.000.000.000.010.00Metanauplii808581 6430.000.010.130.250.00Juvenile210242470.000.110.130.030.00Pre-adult1 9411372 6995533.320.234.620.95Females without ovi-sacs312137590.800.350.010.02Females with ovisacs24444768120.901.650.250.04Females55658473211.702.000.260.06Males2402581 2511410.660.713.450.39Total4 1018 80110 732206 6375.813.278.663.46 In the samples, except the sample taken in December, all age stages are recorded. The number of individuals per 1 m3 in groups is different, according to the dates of sampling.In September-October under conditions of a gradual but insignificant temperature decrease and a relatively stable salinity within 200‰, the number of younger (except for metanauplii) and older age groups changed little (Fig. 2). In early November there was a rapid increase of brine shrimp number in general – the proportion of younger groups increases many times (Table 2, Fig. 2). Fig. 2. Change in age structure of the Artemia sp. population in the cooling reservoir (No. 1) In November, there is a sharp (more than 2 times) decrease in temperature, which affects the structure of the branchiopod population. In the November plankton a huge amount of dead Artemia sp. is recorded, especially in older age groups. Subsequently, juveniles also die in the mass, and the number of brine shrimp in December goes to zero.An analysis of the data on sex structure of the Artemia sp. population is shown at the Fig. 3. Fig. 3. Change in sex structure of the Artemia sp. population from the cooling pond (No. 1) Their ratio also changed when the main abiotic factors (temperature, salinity) changed during the study period. At the same time, the proportion of females, including those with ovisacs, as well as males, their ratio, may indicate a change (predominance) of parthenogenetic or sexual reproduction in the population.During September parthenogenetic reproduction prevails in the brine shrimp population at the water body No. 1. The presence of cysts in plankton indicates the existence of a sexual mode of reproduction in this period. The ratio of females and males in September and October is stable, at the level of 2 : 1 (Fig. 3). At the same time, females with ovisacs make up about half of their number in September, but in October the part of females with ovisacs is already more than 75% (Fig. 3).With abrupt decrease in temperature in the water body in November, the proportion of adults is significantly reduced by less than 10% of their earlier number. But at the same time, the ratio of females and males changes vice versa amounting to 1 : 17. Perhaps, this fact indicates an increasing role of sexual reproduction. Parthenogenetic females die, and their place is taken by growing females that reproduce bisexually. This is also confirmed by the increasing number of the brine shrimp cysts in plankton. It is important that females predominated in the mass of dead mature individuals in November, and proportion of males was disproportionately smaller. In December the proportion of females slightly increases.The number and biomass of the Artemia sp. cysts in the cooling pond (No. 1) during the autumn-winter period changes significantly (Fig. 4). Fig. 4. Change in the number and biomass of the Artemia sp. cysts in the cooling pond (No. 1) In September, the smallest number of cysts was recorded (1 053 cysts/m3), in October, as the water temperature decreases, the proportion of cysts increases (6 695 cysts/m3), and in November it again slightly decreases (4 455 cysts/m3). This may be a consequence of the death of most of the females, including those bred bisexually. The maximum number of cysts was recorded in December 2018 (205 922 cysts/m3). Thus, during the study, the biomass of brine shrimp cysts from September to December increased 204 times and amounted to 2.04 g/m3.According to the data obtained from the reservoir No. 1 (part of Lake Aktashskoe), it should be noted that the starting point (limiting condition) for the beginning of changes in the structure of the population (by age groups and sex), preceding the winter pause, is a decrease in water temperature up to 10°C and lower at relatively stable salinity values (at the level of 200‰). Under these conditions, Artemia begins an increased production of dormant eggs – cysts.Status of the Artemia sp. population at the natural part of Lake Aktashskoe (pond No. 2)The brine shrimp population at the natural, no transformed part of Lake Aktashskoe (pond No. 2) also is of a complex age structure represented by all groups. However, their one-time ratio and quantitative indicators are different (Table 3). Table 3 Change in the number and biomass of the Artemia sp. by age groups during September-December 2018 at the natural part of Lake Aktashskoe (pond No. 2)Ages groups, genderNumber, ind./m3Biomass, g/m330.09.201818.11.201815.12.201830.09.201818.11.201815.12.2018Cysts85 40494 8231 068 2630.850.9510.68Nauplii2220320.000.000.00Metanauplii4 73020640.710.000.01Juvenile19 00410912710.450.060.07Pre-adult3 860400.006.600.070.00Females without ovisacs1 4043851583.590.990.40Females with ovisacs4 9691 28360018.384.752.22Females6 3731 66875821.975.742.62Males183356630.500.980.17Total119 57697 0361 069 30741.087.8013.55 The largest number of all groups (except cysts) was registered in September at water temperature of 26°Cand salinity of 140‰ (Fig. 5). Fig. 5. Change in the age structure of the Artemia sp. population at the natural part of Lake Aktashskoe (pond No. 2) Subsequently, under lower temperatures and desalination (decrease in salinity) of water, the number of each of the groups decreased significantly (by 9 or more times).The structure of the brine shrimp population by sex in the natural part of the lake has significant imbalance towards females (Fig. 6). Fig. 6. Change in the sex structure of the Artemia sp. population at the natural part of Lake Aktashskoe (pond No. 2) The ratio of females and males over the control periods varies from 5 : 1 to 34 : 1 with an obvious tendency to increase the proportion of males in the second half of the observation period (November-December 2018). The part of females with ovisacs throughout the entire period is consistently high – 77-79%.According to the structure of the population by sex, it is possible to assume that Artemia sp. at the pond No. 2 during the entire period of observations reproduces mainly parthenogenetically. Although the number of cysts is quite high. That is, there are two groups of females in the water body, using different methods of reproduction (reproductive strategies).Cyst production in the brine shrimp population of the natural part of Lake Aktashskoe remains practically unchanged under conditions of salinity decrease from 140 to 112‰, and water temperature from 26 to 9°C (Fig. 7). At salinity values below 110‰ and temperatures of 8°C, the number of cysts in plankton rapidly increases. Fig. 7. Change in the number and biomass of the Artemia sp. cysts in the natural part (No. 2) of Lake Aktashskoe Analysis of the size composition of the population in various water bodies of the aquatic complex – Lake Aktashskoe In processing of samples there were measured thebody length of brine shrimp in each of the sex and age groups, as well as the sizes (Dmax, Dmin, S) of its cysts. The measurement results are shown in Table 4. When comparing the size of brine shrimp at certain stages of development in ponds 1 and 2 (Fig. 8), according to the dates of selection, at first glance, a chaotic dynamic is observed. a bFig. 8. Change in the average length of the Artemia sp. by groups: a – in the cooling pond; b – in the natural part of Lake Aktashskoe However, it is possible to identify a number of specific trends. The younger age groups (nauplius – metanauplius, and at pond 2, juveniles) become smaller. Artemia sp. during the pre-adult stage grows faster up to the end of the season. In the cooling pond (No. 1) adult females get smaller in size when temperature decreases, while in the natural part of the lake (pond No. 2), on the contrary, they become larger. The sizes of males in both ponds practically do not change throughout the entire observation period (without taking into account the values for November in pond No. 1).In general, the size of the Artemia sp. in terms of stages of development in both ponds is relatively equal. The only thing that requires explanation is the fact of a sharp difference in the size of males in reservoirs No. 1 (4.73-5.25 mm) and No. 2 (6.10-6.44 mm). The issue of changes in the size of females during the period of deterioration of the living conditions of the population remains debatable. What is its cause: accelerates (or slows down) their differentiation by sex and their participation in reproduction; slows down (or accelerates) their growth (food supply, etc.)?When comparing the average values for groups in September (the period of relative well-being of the population) and November (the beginning of mass production of cysts and depression of the population) for ponds (Fig. 9), the following was established. a bFig. 9. Change in the average size of the brine shrimp in groups as it grows in September and November 2018: a – at the cooling pond; b – at the natural part of Lake Aktashskoe In the cooling pond (No. 1) under conditions ofa sharp temperature drop, the length of the Artemia sp. specimens decreases in all groups. In the natural part (No. 2), on the contrary, there are no significant changes in the values of the average body length in any of the groups.Other species of invertebrates in hyperhaline ponds of the aquatic complex – Lake Aktashskoe In the samples of zooplankton from the cooling pond (No. 1) at a salinity of 186-200‰, in addition to brine shrimp, a few individuals of one species of copepods were found: October 15, 2018 – 5 ind./m3; December 15, 2018 – 3 ind./m3.In the natural part (No. 2) of Lake Aktashskoe the biological diversity of other invertebrate species was significantly higher. Seven species of invertebrates were recorded here, including 5 species of larvae of near-aquatic insects, one species of beetles (imago) and 1 species of copepods. The numbers in species are given in Table 5. Table 5 Composition and dynamics of other species of invertebrates in the plankton of the natural part (No. 2) of Lake Aktashskoe during the study periodAquatic arthropods,juveniles and adultsNumber, ind./m330.09.2018 (salinity 140‰)18.11.2018 (salinity 112‰)15.12.2018 (salinity 76‰)Chironomidae larvae61111995Larvae of other Diptera7032Coleoptera1400Copepoda1973752 874 In addition, in September-November, the presence of green filamentous algae was noted in the plankton of the pond (No. 2), while their number decreased from the beginning of the study period to its end (there were no algae in December).ConclusionSex and age structure of populations of the bran-chiopod crustacean Artemia sp. in the autumn-winter period, it has a complex structure, which is due both to environmental conditions and too much greater adaptive abilities of this species than is commonly believed. The nature of the impact on brine shrimp of the accepted triggering factors of the natural environment (threshold values of water temperature and salinity), which predetermine the decline in population size during the transition from parthenogenetic reproduction of offsprings to sexual reproduction or, on the contrary, rapid increase of brine shrimp caused by the release of nauplii from wintering cysts, does not can always be unambiguously used in practice. In both ponds (No. 1 – cooling pond, No. 2 – natural part of Lake Aktashskoe), at different salinity at water temperatures below 10-8°C, an increase in the number of cysts takes place. At the same time, the difference in salinity in water bodies is almost 100‰. Apparently, this indicator in brine shrimp has a much wider range of optimum.The Artemia sp. population in the cooling pond (No. 1), due to its larger volume of water and depth, develops gradually, without sharp changes in numbers and short-term disappearance of certain age groups. At the same time, the productivity of this population is somewhat lower than in the second pond, which is probably due to increased salinity and food shortage, etc. On the contrary, the population in the natural part (No. 2) of the lake, which has a smaller volume and depth, develops explosively, but the biomass of the Artemia sp. at the peak of its development is much higher. Under conditions of a sharp decrease in temperature, the nature of changes in the sex, age and size structure in water bodies with different salinity is different.The nature of regulation of the Artemia sp. population, its sex and size-age structure are probably much more complicated than it was previously thought. At the same time, it is necessary to take into account a larger list of environmental factors, as well as the nature of their joint influence, which requires additional research involving more analytical material in terms of chronology (duration of observation) and geography.