SHELL BIOMETRICAL VARIABILITY OF PLANORBIS PLANORBIS ( LINNAEUS , 1758 ) ( GASTROPODA , PULMONATA ) IN MAN-MADE WATER BODIES OF THE UPPER SILESIAN INDUSTRIAL REGION

aBStract: The author has studied the effect of some environmental factors on the biometrical variability of the shell of the freshwater pulmonate snail Planorbis planorbis from several man-made water bodies of three types in the Upper Silesian Industrial Region. Out of the studied factors the calcium content in water did not seem to have a strong effect on the shell of the mollusc. On the contrary, a high content of magnesium ions in water seemed to inhibit the growth of the shell.


INTRODUCTION
Planorbis planorbis (Linnaeus, 1758) occurs in abundance in a.ll kinds of man-made water bodies, on various bottom types, regardless of the total hardness of the water and ea++ and Mg+~ content (Strzelec and Sera-fi~ski 1984, Strzelec in press).It is a typical ubiquitous species and its occurrence is not limited by any habitat conditions (Boycott 1936, Hubendick 1947, ~adin 1952, Urba~ski 1957, Lo!ek 1964, Piechocki 1979).It ,appears as one of the first colonizers in industrial reservoirs and rapidly forms abundant populations.
The shell of P. planorbi~ is slightly variable.Its variability covers the shell size and the position of the keel on the body whorl, which is more or less shifted upwards, On the other hand, in industrial regions, the diversity of the habitats the gastropod occu~s in brings about bio-•etrical variability, both within and between populations inhabiting particular industrial reservoirs.
The aim of the present study1s to gives biometrical characteristics of several populations of P. planorbis and to find the environmental factors Which are responsible for interpopulation differences.

MATERIAL AND METHODS
In• order to study the infra-and interpopulation shell variability of P. planorbis living individuals of the species were collected from April to October in the following man-made water bodies in the Upper Siles~n Industrial Region: Magiera in ~wi~tochlowice, Janik in Bytom, Zag6rze in Sosnowiec-Zag6rze (sinkhoie ponds), ZOO in Chorz6w, Gliniak in Sosnowiec, Dziertno Male near Gliwice (sand pits), Paprocany in Tychy, power plant reservoir in Rybnik (storage reservoirs).Characteristics of the studied reservoirs are presented in Tables 1 and 2, their vegetation in Table J.
The breadth, height and thickness of the shell were measured in specimens of all probable age classes, with 0.02 mm accuracy.All the shells measured were al~o weighted (Tab.5).
In order to study the effect of enviromnental factors on the biometrical variability of the shell, about 200 shells of class IV from each reservoir were compared, Specimens at the age corresponding to that class were nu-• merous in all the studied populations and their infrapopulatlon variability index was comparatively low (iv = ~D • 100\), which means that they .wereleast varied in morphology (Tab.4).
Water chemistry (total hardness, ea••, Mg++, Fe++, NNH , NNO , sulphate and chlorid content) and pH were determined using standar8 meth~ds.The quantities given in the tables are the averages of three seasonal sampling resulls.

Estimation of individual age
Specimens of P. planorbis seem particularly convenient for biometrical analysis.Stripes (rest lines) appearing on their shells originate from periods of inhibited growth (Boag and Pearlstone 1979, Douglas 1981, Lutz and Phoads 1977,and Shoul and Goodwin 1982).Such stripes are commonly acknowledged to mark the termination of annual growth.However, this interpretation seems incorrect (Piechocki 1979) since it overestimates the lifetime length.Richardot (1976 and1979) obtained some results in solving the problem.She found the deposition of c~lcium carbonate in the shell to be inhibited not only during dormant periods.Also in the period of reproduction, when CaCO) contained in haemolymph i ~ used for egg capsules, thiC• ker stripes appear on the outer shell surface, which are very similar to those originating during a dormant period.
In the studied material there were specimens with 1-6 stripes on the shell.If the stripes were considered yearly growth lines the age of the individuals would be 6 years.However, in aquarium breeding individuals of the species live at most J years (Piechocki 1979).It seems therefore tnat the stripes should be interpreted as follows: stripe I -1st dormant period, stripe 11 -1st reproduction period, stripe Ill -2nd dormant period, etc.The above interpPetation seems to enable an age determination, observations of individual growth and biometrical comparisons of corresponding age classes.

RESULTS
The infrapopulation variability observed in the shell breadth was small there being statistically significant differences between specimens of corresponding age classes from different habitats.
The differences could be observed after the first dormant period.The individuals from the storage res~rvoirs were the smallest, and this difference was maintained in the next two age classes.All the differences were statistically significant (p= 0.005).
The individuals from the storage reservoirs were growing rapidly after the second dormant period, while in those from the other types of water bodies the growth rate was moderate.In classes V and VI the growth rate was similar in the storage reservoirs and sand pits (differences were insignificant, p = 0.5).The shells from the sinkhole ponds were markedly smaller and differed significantly from both the above types of water reservoirs (Fig. 1).
The specimens of class VI fro~ the sand pits; storage reservoirs, and sinkhole ponds attained on average 14.5 mm, 14.29 mm and 12.97 mm in diameter, respectively.The largest specimen 16.3 mm in diameter has been found in a storage reser~oir.

Shell height
The shell neight variab~lity among specimens from the studied water bodies was low.After the first dormant period individuals from the sand pits were the highest and this concerns all age classes, though in some cases the size differences were statistically insignificant (p 0.5).
Changes in shell height are most evident in class IV.A rapid increase in height was observed in the individuals from the storage reser-• voirs, the differences between them and the specimens from the sinkhole ponds being particularly sharp (F1g.2).The shell height of the specimens from the sand pits and storage reservoirs was similar.In classes IV -VI the differences were insignificant, except for the specimens from the sinkhole ponds.

Shell wall thickness
The shell wall thickness has rarely been regarded as a diagnostic character in Planorbidae.It seems, however, that it can provide information  about environ•ental conditions in various habitats.For this reason it has been used to study ecophenotype variation (Fig .3).In class !the shell wall thickness was not measured for technical reasons.
The meaSUI'eiiiBnts have revealed some 1'8!J.Ilar1ty: in each class the individuals fran the sinkhole ponds had themost thick -walled shells.The difference between them and the speciemns living in the other types of water bodies was highly significant (p = 0.001).The shells of classes II and III from storage reservoirs and sand pits differed insignificantly ( p = 0.5).In higher age classes, in the storage reservoirs significant differences appeared as a result of rapid increase in wall thickness.ln that period the shell wall thickness of the sand-pit individuals of class V had reached its maximum while that of the storage-reservoir ones of the same class was still increasing.It was •anifested in the latter having their shells much more thick-walled than those from the other reservoir types.

Shell weight
The shell weight increased parallelly to the shell diameter, though showing a greater infrapopulation variability.In classes I -III the lightest shells were found in the storage reservoirs.After the second dormant period the shells increased in weight and in class IV became the heaviest.In classes V and VI there was almost no difference in shell weight between This resulted in significant differences between them enGithe speci•ene from the remaining water bodies.The heaviest shells wer e found in sand pits (173 mg) and storage reservoirs (172 mg) while in sinkhole ponds the maximum observed was 110 mg (Fig. 4).
Effects of some environmental factors on shell biometrics (Tab.5) According to some authors the shells of the individuals living in hard water r i ch in calcium are larger and heavier than those of the ones occurring i n soft water.This has not been confirmed by the present study.The shell hei!tlt and breadth ~~~ere iOSiWJiU:antly correlated with total hardness and ea++ content.This is the shell thickness that much surprisingly has been found negatively correlated with the total hardness (r = -0.8~7)as well Ill as with the ea++ content (r • -0.717) • Some authors state that the shell size is influence~ by Mg++: ea•• ratio.This has been confirmed by the significant negative correlation calculated between the ratio and each of the measured shell parameters.The correlation coefficients are -0.76,-0.76, and -0.67 for the shell wall thickness, shell breadth, and ~hell height, respectively.
No significant correlation has been found between the total hardness, ea•• content, and shell weight.The latter para•eter, however, has been found to be negatively correlated with the Mg++ content (r = -0.68).The correl~tion found between the Mg++: ea++ ratio and shell weight is low (r = -0.56).The author supposes that the negative correlation suggests the negative influence of magnesium ions, being stronger than the positive effect of calciu• salts.
Relationships between the shell para•eters studied and the remaining physico-chemical factors measured are not clear considering the corresponding the correlations.9 The content of sulphates see•s to have influenced the shell breadth negatively (r = -0.583)..In the case of the Fe+++ content no apparent effect_ on the shell has~ found, there being a low, positive correlation between NNH content and shell size (r = 0.56).
It was more difficult to dete~•ine the effect of non-measurable factors, e.g.bottom type.The chi-square test having been used, only a small insignificant difference in shell breadth was found between the specimens from detritus and those inhabiting sand-clay bottom, while the individuals fro• muddy bottom differed significantly from the others (chi-square = 13.1 for mud and detritus, 30.4 for •ud and sand-clay).The specimens fro• muddy bottom were re•arkably smaller.
The individuals from the sinkhole ponds differed in a similar manner fro• those the other water bodies considered (ch1 2 = 14.3 for the sinkhole ponds and sand pits, 18.6 for the sinkhole ponds storage reservoirs).The difference between the individuals from the storage reservoirs and those from the sand pits was insignificant.

DISCUSSION
Most of the present results do not confirm the common opinion that some enviromnental factors have a strong effect on the shell of freshwater snails.Haley and Gibson (1971) ascribe an important role to the total hardness and calciu• content.They maintain that the amount of calcium stored by snails depends on its content in enviromnent.Our results suggest that the calcium content is not so iMportant factor.According to Zhadin (1952) snails inhabiting soft water have smaller shells.Thomas and Benjamin (197•) found that in water devoid of calcium snails can grow if their food is calciu•-rich.Van der Borght and van Pujmbroeck (1960) found in laboratory experiments that 80\ of calcium assimilated by ly•naea stagnalis come from water, only 20\ co•ing from food.Most authors agree with the latter view.Froemming (1936,1938,1953,1956) is the only one convinced that food is the only source of calcium.Russel-Hunter (1970) states that differences in calcium assimilation do not depend on calcium concentration in water but have a genetic basis.
My observations do not confirm the commonly accepted opinion that snails from calcium-rich water have heavier and more thick -walled shells (Boy-cott 1936, Franc 1968, McMahon 1975, Russel-Hunter 1978).Unfortunately the authors cited above do not give exact but only approximate weights of shells from various habitats.The shell weight in the studied water bodies was negatively correlated only with Mg++ content, no correlatiqn being found with ea++ concentration.
The shell wall thickness ln the man-made reservoirs studied was not correlated with the calcium content in the habitat.This might have resulted from the fact that in living organisms the magnesium act antagonistically to the calcium ions, in competing for the same binding sites in the cell membrane (Burton 1973).An excess of magnesium probably reduces the •ability to use calcium during the shell formation, though the former element has never been found a shell co~ponent (Strzelec unpublished data).Magnesium ions might inhibit the calcium transport by haemolymph, by occupying calcium binding sites.
Since calcium assimilated from enviromnent is transported by haemolymph but does not passively penetrate through the body integuMent as suggested by Rao and Goldberg (1959' Greenaway 1971, 1971a), it is obvious that the saturation of , body fluids with antagonistic ions would limit the amount of calcium transported and stored in the shell.There is also another possible explanation of the smaller size of the shells from magnesium-rich water.It was found that in vertebrates a surplus of magnesium inhibits the secretion of growth hormone (Base, Vale and Grant 1975).The same mechanism might occur in snails though no specific growth hormone has been detected in these animals so far.The present results are in agreement with those of Nduku and Harrison (1976) who found that a high Mg++ and ea++ ratio in environment is one of the •ost harmful factors for snails.Thomas, Goldworthy and Aram (1975) observed a positive correlation between the shell size of 8iomphalaria glabrata and the iron content.No such correlation has been found ln the present study.The suggestion that temperature has a positive effect on shell calcification (Stoor, Costa and Pravel 1982) has not been confirmed.The shells of the individuals fro• the storage reservoir of the power plant in Rybnik where the water temperature maintains over +5• C the year round (Krzytanek 1979) were neither heavier nor larger than those from the other studied water bodies of the same type.
Planorbis planorbis grows throughout life, as is shown by its size increasing with the number of rest lines.According to Pal11er (1982) growth is inhibited after the sexual maturity has been reached, a~d then variation in body weight is related only to sexual activity.That author meant probably the periodic decreases in body weight, which result from the use of part of the shell calciliR for the for~aation of egg envelopes (Richardot 1979).
Both the present results and literature data confirm the view of Odum (1763), that size variation in organisms living in various habitats reflects specific adaptations to the set of conditions, which characterizes each particular enviromnent.S t r e s z c z e n 1 e : Autorka badala wplyw n1ekt6rych czynn1k6w drodowiskowych na zmiennod6 biometryczn, muszli Planorbis planorbis w trzech typach sztucznych zbiornikdw wodnych w G6rno4l,sk1• Okr~gu Przemys~owym.