Combined Effect of Methylfarnezoate and Food Concentration on the Reproductive Parameters of Two Cladoceran Species of the Genus Moina

Cladocera are mass representatives of zooplankton in freshwater ecosystems, which are of great ecological importance. To be aware of the dynamics of cladoceran populations in nature and to control the abundance of animals in artificial conditions, for example, in aquaculture, it is important to understand the reaction of the reproductive system of animals to external stress factors. Adverse environmental conditions trigger a chain of neurohormonal changes that largely determine the rate and type of reproduction (parthenogenesis or sexual reproduction) in Cladocera. With the discovery and isolation of methyl farnezoate (MF), one of the key hormones involved in the regulation of growth and reproduction of crustaceans, it became possible to study the combined endogenous and exogenous effects of this hormone on crustacean reproduction. In this work, we assessed the effect of MF on the reproductive parameters of two species of cladocerans of the genus Moina at suboptimal (for these species) food concentrations. As a working hypothesis, it was assumed that the combined effect of the external stress factor and MF would have a more pronounced effect on the growth and reproduction of animals than each of these factors separately. We used females of two species of cladocerans Moina brachiata (Jurine, 1820) and Moina macrocopa (Straus, 1918). In all experiments, a non-sterile culture of the green alga Chlorella vulgaris was used as a food, and aged tap water - as a culture medium. Animals were grown individually under conditions favourable for parthenogenesis (the volume of cultivation medium was 20 ml per animal, temperature (26°C), and photoperiod (16 hours of light, 8 hours of darkness); the concentration of food was 2*105 cells/ml per day for M. macrocopa and 5*105 cells/ml per day for M. brachiata). To start the experiment, one-size juvenile females (body length 0.6-0.7 mm) from the first offspring of third-generation crustaceans were placed individually in 40-ml beakers containing 20 ml of the medium. The medium was updated daily. When studying the effect of MF on growth and reproduction, the following food concentrations were used: for M. brachiata - 0.1; 1; 2 and 4*105 cells/ml per day; for M. macrocopa - 0.5 and 1*105 cells/ml per day. The differences in food concentrations are due to the fact that these two species have a different food threshold, at which females begin to produce resting eggs. For each food concentration, the development of females in the medium with MF and in the control medium was compared. The concentration of MF (Echelon, USA) in the medium was 400 nM. For each exposure, at least 10 animals were tested. The experiments were carried out until the females gave birth to the second offspring. For each female, the day of birth of the first offspring, the type of reproduction (parthenogenetic offspring or resting eggs), the number and sex of the hatched parthenogenetic offspring were recorded. On Day 1 and 4 of the experiment, the linear size of the animals’ bodies was measured. Based on the body length, the specific rate of somatic growth of animals was calculated. Experiments showed that MF stimulated females of both species to sexually reproduce at all tested food concentrations (see the figures). The proportion of males in the parthenogenetic offspring of females of both species for each of the tested food concentrations in the medium with MF was higher than in the control. Both MF and food concentration affected the proportion of females of both species that produced resting eggs. The proportion of females with resting eggs increased with a decrease in food concentration and with the addition of MF to the culture medium. MF and food concentration had a significant effect on fecundity, the age of the first reproduction, and the specific somatic growth rate of females of both study species (see the figures). Both species in the medium with MF gave birth to fewer offspring than in the control, while fecundity increased with increasing food concentration in the medium. In the medium with MF, the females hatched the first clutch later than in the control medium. The age of the first reproduction also increased with a decrease in the concentration of food. Both species in the medium with MF had a lower specific somatic growth rate than in the control, while a significant increase in the specific somatic growth rate with increasing food concentration was recorded only for one of the species. Thus, the study showed that the concentration of the juvenile hormone MF 400 nM in the culture medium leads to a decrease in the specific rate of somatic growth, a delay in the hatching of the first clutch, a decrease in fecundity, an increase in the proportion of males in the offspring, and the production of resting eggs by females of Moina brachiata and Moina macrocopa. Changes in the studied parameters of growth and reproduction of Moina brachiata and Moina macrocopa females with a decrease in the food concentration below the optimum had a similar pattern but were less pronounced. The combined effect of MF and low food concentration leads to the strongest changes in the studied parameters of growth and reproduction of the studied females. Such changes will lead to a sharp decline in the population growth rate, which indicates the possibility of controlling the number of natural or laboratory populations of cladocerans by interfering with the signalling pathway of juvenile hormone receptors. The article contains 3 Figures, 24 References. The Authors declare no conflict of interest.

Keywords

Authors

References