Three crustacean species of postglacial origin are found in lakes of northern Poland. One of them is Pallaseopsis quadrispinosa which inhabits deep, oligotrophic and mesotrophic lakes. The deepest lake in Poland (area 3.11 km², max. depth 108.5 m) and on the entire Central European Plain is Lake Hańcza, an α-mesotrophic lake of unique character. A study of P. quadrispinosa in this lake was carried out between AprilOctober in 2000. Samples were collected at ten sites, to a depth of 16 m, using a tow net. In April, this amphipod crustacean was observed at depths of between 1–12 m with maximal density at 4–6 m (800 individuals per 100 m²). In May and July/ August, the density was reduced and this organism was found mainly between 6–12 m. In October, P. quadrispinosa was distributed between 1–6 m, with a maximal density of 1400 individuals per 100 m² at 2–4 m. Seasonal changes in density at various depths and the spatial heterogeneity of the occurrence of this amphipod indicate migration, probably related to changes in the water temperature and the reproductive cycle. Breeding of P. quadrispinosa was observed throughout the study period, with reproductive peaks in the early spring and autumn. The average number of eggs carried by ovigerous females ranged from 12 to 64, and was significantly correlated with the size of the individual. Newborn juveniles were spatially isolated from the adults as they tended to accumulate at the shallower bottom depth. The sampled material revealed the presence of two subpopulations of P. quadrispinosa. Environmental conditions, especially the consistently high level of dissolved oxygen, reproduction, apparently long life-span and occurrence in all parts of the lake indicate that the P. quadrispinosa population in Lake Hańcza is stable and in good condition.
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