On the reproduction of Pisidium C. Pfeiffer, 1821 (Bivalvia: Sphaeriidae) from Sąpolno (NW. Poland)

During a malacofaunistic survey in the environs of Sąpolno (Pomeranian Lakeland, NW. Poland) 16 species of Pisidium were found within an area of 2.5×2.0 km: P. amnicum (O. F. Müller), P. casertanum (Poli), P. crassum (Stelfox), P. globulare Clessin, P. henslowanum (Sheppard), P. hibernicum Westerlund, P. milium Held, P. moitessierianum Paladilhe, P. nitidum Jenyns, P. obtusale (Lamarck), P. personatum Malm, P. ponderosum (Stelfox), P. pseudosphaerium Schlesch, P. pulchellum Jenyns, P. subtruncatum Malm and P. supinum A. Schmidt. The number of offspring during the reproductive season depended on the species and on the parent’s size. The most fecund species was P. ponderosum, with up to 75 juveniles released at once. The offspring of P. amnicum were the largest (mean shell length 2.05 mm), but relatively small compared to the parent’s shell (mean 26.1%). The smallest juveniles were produced by P. moitessierianum (mean shell length 0.59 mm). The juvenile shells of the studied species were more or less elongated, with the height/length ratio ranging from 0.75 (P. amnicum, P. ponderosum, P. henslowanum) to 0.83 (P. subtruncatum, P. hibernicum). Distinct differences were observed in the reproduction of P. casertanum and P. ponderosum. However the differences between the reproductive parameters of P. ponderosum and the form described as P. casertanum var. humeriformis were rather small. key wordS: ovoviviparity, fecundity, infra-specific differences, shell of offspring