BIOMETRICS AND LIFE CYCLE OF PHYSA ACUTA DRAPARNAUD, 1805 (GASTROPODA: BASOMMATOPHORA: PHYSIDAE) UNDER HUMAN IMPACT

aBStract: Alterations in the shell dimensions of Physa acuta Draparnaud, 1805 were studied from May to September in a sinkhole pond with very hard water in Siemianowice-Bytków (Upper Silesia, Southern Poland). This Mediterranean species turned out to be well adapted to the climate of Southern Poland and resistant to industrial water pollution. A biometrical analysis indicated the occurrence of two breeding periods in the studied population.


INTRODUCTION
Physa acuta Oraparnaud, 1805 is a freshwater basommatophoran pulmonate snail occurring in Upper Silesia in several water bodies.It is a Mediterranean speciesintroduced in Poland about 80 years ago.At present the species seems to occur in var i ous habitats, there being, however, still only a few localities it is known from (Feliksiak 1939, Dutkiewicz 1959, Wiktor 1959, Zi~~a and Zacwilichowska 1966, Piechocki and Potocki 1976, Strzelec and Serafirtski 1984, Rembecka, Serafirtski and Strzelec 1986).
The shell variability and biology of Ph .aguta are poorly known (Duncan 1959, Piechocki 1979) and only in 1986 a series of shells collected in recent sediments of the River Vistula (Southern Poland) was described by Alexandrowicz (1986), considering also some biometrical data on the spe-Cies the present paper deals with.The aim of our study was to describe a life cycle pattern of Ph. acuta.
Living snail specimens were collected every two weeks from May to September, the area along the ~orth-western water-side being sampled.The shell parameters measured were the same as in the paper of Alexandrowicz (1986)  x so:':    The studied population of Ph. acuta was quite variable with resp~ct to its biometrical characters which varied from month to month.This resulted in very high values of the variability coefficient, except fdr the series collected in August.In that series there were no specimens of the smallest and largest size classes, and the average height of the shells approached the mean value of this dimension for the entire material.
The above observations suggest that young individuals appeared in June and September, indicating two breeding seasons: April/May, and July/August.The population of Ph. acuta in the stunied pond consisted probab~y of two Qroups of individuals.One group included individuals born in late summer, overwintering, breeding in spring and dying in early summer, while the other one comprised individuals born in spring, breeding in late summer and dying before winter.Both growth and sexual maturation must have been faster in the second than in the first group.•Thelife span of the _individuals of the first group was about 12 months, whereas that of the second Qrou~ seemed to be only 7 -8 months.These, however, are only suppositions to verify which a whole year of field observations is necessary.So

DISCUSSION
The results presented above indicate that Ph. acuta can bear rather low temperatures characterizing the climate of Southern Poland.The adaptation to such conditions must have been a rather rapid process, because 50 years ago Ph. acuta in Poland occurred only in reservoirs with artificially heated water.Our observations confirm also the resistance of this snail to industrial water pollution.
The size of shells in the studied population was similar as that in populations from greenhouse tanks and heated industrial reservoirs, the maximum shell height being larger than that recorded by Alexandrowicz (1986' but smaller than the one observed within the natural distribution range of the species. It is noteworthy that the life cycle of Ph. acuta in France is also annual, but with a single breeding season (Duncan 1959), while our results point out a probable occurrence of two breeding periods.

Table 2
Shell height (H) in particular months

Table 6
Relative height of spire S ¥ x lOO measured dimensions was observed being reflected in higher mean values.On• ly in September a decrease in the mean values of the dimensions was observable resulting from small individuals having appeared in that month (Fig.2).