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Egg number-female body weight relationship in freshwater snail (Viviparus viviparus L.) population in a reservoir

Jakubik B.

Polish Journal of Ecology

2007

Vol. 55, nr 2

325-336

Studies on the fecundity of V. viviparus were carried out in the years 1990-1994 in selected littoral sites of the Zegrzyński Reservoir (Central Poland) (60 km long, mean depth ca. 3.5 m, maximum depth at the dam up to 9 m, mean retention time from 1 to 15 days) and in outflow stretches of its tributaries. All sites were dominated by fertile females (i.e. having embryos) that lived in aggregations. Mean numbers of females ranged between 30 and 250 ind. m[^-2]. Females of III (shells 12.1 - 20.0 mm wide and 12.1 - 25.0 m high) and IV (20.1 - 25.0 mm width and 25.1 - 35.0 mm height) size class dominated. The embryos were noticed in females of II (8.1 - 12.0 mm width and height of the shell), III and IV size class and mean number of embryos per female was from 0.9 to 6.7, from 1.1 to 9.6 and from 2.0 to 9.1 for II, III and IV class respectively. Analysis of embryonic development revealed the presence of three growth stages in oviducts of females: the oval, transparent egg capsules called the youngest embryos), egg capsules with visible contour of a shell (called medium embryos) and snails with a shell (the oldest embryos). Embryos in a form of oval, transparent egg capsules and snails with developed shell dominated in particular size classes in analysed sites. Analysis of the correlation coefficient showed significant positive relationship between the mean sum of embryos and shell height and width, and dry weight of female's body and shell weight. Most significant correlation was found between dry body weight and mean number of embryos. The number of embryos in particular growth stages was also related to biometric indices as shown by the analysis of correlation coefficients. The number of oldest embryos (fully developed snails with a shell) increased with the increase of shell height and width. Female.s dry body weight, on the other hand, was best correlated with the youngest growth stage of embryos (oval, transparent egg capsules). Described relationships indicate the importance of the size of V. viviparus females for reproductive success and for survival and "condition" of young snails.