THE CONCHOLOGICAL VARIABILITY OF SPHAERIUM CORNEUM (LINNAEUS, 1758) IN THE POLISH WATERS

The conchological variability of Sphaerium corneum (Linnaeus, 1758) fr om Polish waters is manifested by the presence of two types of populations (Tab. 1). Ul the features mentioned in the diagnosis show great variabitity . For instance the shell outline oscillates from oval to triangular or trapeziform . · In some populations all the margins of the shells are much curved, in others the ventral margin is straight, joining the anterior and posterior ones at sharp angles. The variability of the shell colour is considerable, much li~e in other b~valve freshwater molluscs. The asymmetry of the shells, mentioned in the description of S. ~orneum "2nd type" is already distinctly marked in the appearance of the young. The regress! on lines w/1 ( 1) (width: length depe,nding on l!lngth) , w/h (1) (width,height depending on length), and w(l) (width depending on length) show that the first mentioned relation reflects more pronouncedly the differences between the manners of growth of the populations discussed (Fig. 1). The regressive slope b (y = a + bx) is the best index of these differences (Tab. 2). The values of that index change in the material studied from 1.5 (Ina river) to 7.28 (forest bogs Makruty). Tne changes of the non-metric features presented in the diagnosis (Tab. 1) are parallel to the changes in the shell proportions.

Sphaerium corneum (Linnaeus, 1758) shows the greatest concho1ogical variability among three Sphaerium species found in Poland.On account of the variability numerous forms, or even subspecies and species were desc~;ibed cZadin 1952, Ehrmann 1956, Ell is 1962, Herrington 1962, Zeiss1er 1971).The paper is an attempt at a biometrical analysis of some populations of 5. corneum from various types of water bodies.Some non-metric characters, for example: the shell outline, the position ~f the umbones, their shape, and the shell colour, have also been considered.

MATERIAL AND METHODS
The material was collected in 1975-1982; it was fixed in 4X formal or 75X ethanol.The shell dimensions: lengtb (1), height (h), and width (w) were measured with 0.1 mm accuracy,accordfng to the schems (fig.4A) by means of a scaled eyepice.

RESULTS
In the collected material the conchological variability is manifested in the presence of two types of populations.The diagnoses of the exemplary extreme ones are presented in Tab. 1, Fig.In some populations all the shell margins are much curved, in othe~s ~he ventral margin is straight and joins the anterior aRd posterior ones at sharp angles.The t\nterior and posterior shell margins are also ntarly straight.
The flatness of the ventral margin, which forms a keel, occurs with a various fl'S"qtrnncy in the material of each "2nd type" population.The variability of the shell colour is considerable, much like in other molluscs.In some.2ndtype" shells besides concentric bands colouring the gTowth lines, radial brown stripes occur with various intensity; they are o:rte11 absent.It is very difficult to describe the variability range of the hinge plate.However, concerning the previous data (e.g.Favre 1927) it is impossible to Tegard the hinge plate pattern as a crucial taxonomical feature in the systematics of Sphaeriidae, especially of the genus Sphaerium (Fig. 3).

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The shell asymmetry mentioned in the description of the S. corneum "2nd type" is already distinctly marked in the appearance of the young.It was observed, for instance, in the population from the River Nida, where three age classes of young (in respect of their dimensions at least) were found in one adult.The smallest ones showed a well marked asymmetry.

BIOMETRICAL ANALYSIS
In the populations of both types changes in width : length (w/1) and width : height (w/h), depending on length, were analysed to compare with each other the modes of the individual width growth.The length was accepted as most directly connected with the age of an indivi~ual.The regression lines w/1 (1) and w/h (1) in both cases are almost parallel to each other (Fig. 48).Hence, in the biometrical analysis of the otner populations height was omitted.It is confirmed by approximate values of the correlation coefficients of these parameters in the studied populations.For instace, they amount to 0. 706 (w/h) and 0. 745 (w/1) in the "2nd type" population, while in the "1st type" population they reach 0.9171 and 0.9467 respectively.In both cases the correlation coefficient of w/1 (1) reaches higher values.
The correlation of the shell width (w) and its length (1) is closer.It amounts to 0.9960 in the "1st type" shells and 0.9792 in the "2nd type" ones.The dependence w(l) is generally rather a quadratic function, however, within the range of the experimental values it approximates to a linear dependence and is• examined as such.Co~paTing the above with the w/1 (1) dependence one can see that the latter reflects more pronouncedly the differences between the studied populations in respect of their growth mode (Pig.5).The regressive slope {b) of the w(l) function is 1.5 times greater in the "1st type" populations than in the "2nd type" ones and several times greater respectively for the w/1 (1) function (Fig. 48).x S. c:1rst type Jeziorok Lake, VII 1976 ,, .The significance of the regression of the discussed dependence was verified using t • test (Parker 1978).The t values obtained here correspond, except three cases, to the probabilities 0.001; thus, the parameters discussed are highly significantly correlated.In the populations from the Wiepxz-Krzna Canal and the Ri ver Radunia the t values correspond to greater probabilities (0.02 p 0.05 for w/h (1) and 0.05 p 0.1 for w/1 (1) -Wieprz-Krzna Canal, 0.002 p 0.01 for w/1 (1) -the River Radunia) but they lay within the limits of error.
The values of the regressive slope (b; y =a+ bx) are listed in Table 2.They reflect the slope angle of the regression lines w/1 (1) and illustrate the width growth rate.In the studied material they vary from 1.5 (the River Ina) to 7.28 (forest bogs in Makruty).The poles of the variability are the populations from the ecologically antagonistic localities.In the closed, overgrown bog ~its in a pine forest the sphaeriids grow in width rather fast; their shells are very convex .On the contrary, on a well oxygenated locality (the bottom overgrown with sponges) the shells of the species studied are flat: the slope angles of the regression lines are far more _acute.The variab~li ty of the non-metric features listed in the diagnosis (TaQ. 1) is parallel to the variability of the shell proP.ortions.

DISCUSSION AND CONCLUSIONS
The interpre~tiqn of the above observ.ationsis very difficult for many years the re&ults of conchological and blamebeen applied in the mollusc systematics (Nilsson The main principles of the application as well as the criteria and methods used are differentiated and often criticized, considering at least the great conchologieal variability which in many cases makes the use of the shell features in the determination impossible (e.g.Lymnaeidae: Roszkowski 1915, Falniowski 1980;Zonitidae: Riedel 1957).However, examples of the usefulness of the morphometric and conchological characters in the 8ivalvia taxonomy are more numerous.Holopainen and Kuiper (1982) tried the application of morphometric indices in the identification of Sphaeriidae.They used the shell height: : shell length ratio and the shell width : double shell height ratio as indices.The indices have not been applied in the present paper.They are more weakly correlated with the specimen length than the w/1 ratio, moreover, relatively low values of the regression coefficient b indicate only a slight linear tendency and the regression illustrates poorly the variability of the mentioned parameters (Parker 1978).Similar values of these indices were found for S. corneum specimens from Poland.It was acknowledged that the regression w/1 (1) presents a more differentiated picture: it indicates better the differences between populations,additionally, the coefficients of this regression are pronouncedly different from zero; in the studied examples the regression is highly significant , The discussed S. corneum populations show a great conchological variability and differ between each other mainly in respect of their width growth rate.The differences are best iliustrated by the d~ dences w/1.The habitats of the studied populations are extremely differentiated in many respects.The water fecundity (i.e.food resources) does not seem the most important factor of the width growth.The size of gills which act as brood pouches determines the convexity of a speciMen.The close correlation between the length and width of the shells in the "1st type" populations (Fig. 5A) which live in eutrophic habitats under rather bad oxygen conditions, as well as parallelly the higher regressive slope (b) of the w/1 (1) dependence in this group phenomena remains open.It is commonly known that the studied variability was the basis of numerous descriptions of forms (f.firma Clessin, f. nucleus Studer, f. dianum (Normand,l844) of both the extreme lacustris Draparnaud).Perhaps evenS.corneum seal~ is one of the examples.The sympatric occurrence conchological types of S. corneum (Fig. •1) that might be identified with S. corneum corneum ("1st type") and S. corneum scaldianum ("2nd type") points out the necessity of a revision of their systema tic posi tio.n.However, such a revision would need thor.ough taxonomic studies based on modern cytological and biochemical methods.The diagnosis of S. scaldianum (see Zadin 1952) does not fully corre-spon~ with the description of the specimens from the "2nd type" populations.However, it seems that since the distinct character .of the two types of populations has oeen recognized, the diagnos~s of S. scaldiangm should c~nstitute a basis for the discussion on their .systematicZmienl)q~~ koncho1ogiczna Sohaerium corneum ( Unnaeus, 17 58) przej!lwia si~ obecno~Ci<!dw6ch . .typ6w populacji.Osobniki owalne, p~kate zamieszkuj(j male, si1nie zeutiofizowane zbiorniki wadne, osobniki tr6jkqtne i trapezoida1no-owalne, mniej p~kate iyjQ w akwenach o 1epszyeh warunkach tlenowych i ruchliwszej wodzie.Oobrym wska1nikiem tej zmienno~ci wydaje si~ by~ warto~t wsp6lczynnika regresji b(y= = a + bx).Zmienno~ci cech mierzalnych • towarzyszy • r6wnolegh :z•miennost cech niemierzalnych.