The intra-population variability of a freshwater crustacean Polyphemus pediculus (Cladocera, Onychopoda)

A discriminant canonical analysis was employed to study the morphometric structure of a shallow-water local population during the open-water period, and of an autumn deep-water local population extending from the depths of 1.2 m to 23.6 m. Statistically treated data on 11 quantitative and 12 meristic features of 505 mature individuals from the shallow-water population and 294 specimens from the deep-water population were analyzed in the study. P. pediculus belongs to an ecologically polymorphic species. The morphometric structure of local populations of the species and its peculiarities are governed by the environmental conditions of the habitat and have an adaptive value. The population structure is unstable, subject to spatial and temporal variations together with the biotopic and seasonal variations of the environment. A continuous sympatric speciation proceeding in the populations leads to the formation of a number of phenotypes adapted to the existence in specific sub-niches. The morphometric structure of the shallow-water population is formed by three short-term seasonal races having statistically significant differences, and following in succession in correspondence with the change of season. Their morphometric diversity and variability reduce from spring to autumn; the highest statistically significant differences are peculiar to the autumn race. Crustaceans of the summer months do not display statistically significant differences between each other. Morphometric differences between seasonal races do not exceed the limits of the species variability range, and are conditioned mainly by variations in the total length of Ant II and in the number of setae in the medial rows of thoracic limbs of the I-III pairs. The morphometric structure of the deep-water population is more complex and stable, and is formed by three permanent morphoforms, which differ statistically significantly from each other and exist at different depths: the main (1.2–4.7 m), the intermediate (9.3 m) and the deep water (23.6 m). The degree and the accuracy of differences between the morphoforms increase with the depth of habitation. Permanent biotopic races are present at different depths within the morphoforms. The races of the intermediate and the deep-water morphoforms differ statistically significantly from the races of the main morphoform. The races of the main morphoform do not have any statistically significant differences between them. The number of crustaceans permanently present within the biotopic races increases with the habitation depth, and the number of individuals performing vertical migrations reduces. The deep-water morphoform is the most isolated and stable. A considerable divergence of quantitative features is observed in the specimens of this morphoform, resulting in the formation of a new species P. deepwater. Morphometric differences of the biotopic races are conditioned mainly by variations in the total length of Ant II, the length of the caudal stem and setae, Ant I free segments, the head diameter, and the number and length of setae on the third segments of endopodites of thoracic limbs of the I-III pairs.