Population size and structure of three mussel species (Bivalvia ...

Hydrobiologia (2005) 537: 169–183 Ó Springer 2005
Population size and structure of three mussel species (Bivalvia: Unionidae) in
a northeastern German river with special regard to influences of
environmental factors
Eckhard Weber
Ernst-Moritz-Arndt-Universita¨t Greifswald, Zoologisches Institut und Museum, Johann-Sebastian-Bach-Straße 11/12,
17487 Greifswald, Germany (E-mail: webeck@mail.uni-greifswald.de)
Received 5 August 2002; in revised form 13 June 2004; accepted 27 August 2004
Key words: Anodonta, Unio, population structure, age structure, river, environmental factors
Abstract
Unionid mussels are of great ecological importance in running and standing waters, however their populations
declined continually during the 20th century. In order to collect more data on the situation of these
mussels in running waters, the populations of Anodonta anatina, Anodonta cygnea, and Unio tumidus
(Unionidae) were investigated in the northeastern German river Ryck from 1996 to 1998. At three sampling
stations along the river, the mussel stocks as well as sediment and water properties were analyzed. In this
river abundances of up to 138.7 indiv./m 2 were detected. The average population densities varied from 10.9
to 34.9 indiv./m 2 . More critically however, the age structures showed significant signs of irregular reproductive
success and overaging. Consequently, the mussel stocks are going to decline, and furthermore,
U. tumidus should be listed as an endangered species in the Ryck river. Negative influences on bivalves can
be expected from sporadically occurring low oxygen and high ion concentrations in the water. But after all,
the conditions within the sediments seem to be decisive for the survival of juvenile mussels and thus for the
age structure of the populations.
Introduction
Some of the most remarkable organisms of the
macrozoobenthos in fresh waters are the species of
the Unionidae. They can reach a considerable
body size and form dense populations. Due to
their filtering capacities, the unionids are of great
significance in the self-purification of waters
(Ostrovsky et al., 1993; Ogilvie & Mitchell, 1995;
Vanderploeg et al., 1995; Welker & Walz, 1998;
Soto & Mena, 1999).
Furthermore, another unique feature of the
unionid mussels emerges within their reproductive
biology. After the fertilized eggs have developed
into larvae (glochidia) in the gills of the females,
the larvae are shed and have to undergo a metamorphosis
as ectoparasites on host fish. Not until
then they are able to grow up as mussels in the
sediment (Heard, 1998; Wa¨ chtler et al., 2001).
In the 20th century, the Unionidae suffered
from enormous population declines, and the species
composition of mussel coenosises in lakes
and rivers shifted (Mentzen, 1926; Arter, 1989;
Lewandowski, 1991; Klemm et al. 1994/1995;
Krzyzanek, 1994; Zettler, 1996). Reasons for this
development can be an increasing eutrophication
as well as steps of river regulation and maintenance.
However, changes in water characteristics
do not affect mussels solely directly, but also
indirectly by influencing the density and the composition
of the ichthyofauna (Hochwald & Bauer,
1990; Scholz, 1991; Byrne, 1998).
Previous studies have documented the state of
European unionids especially in standing waters

170
(a)
M-V
Rienegraben
Greifswalder
Bodden
Horst
I
Groß Petershagen
Ryck
Jarmshagen
II
Wackerow
Wieck
Bachgraben
Boltenhagen
III
Heilgeisthof
Eldena
(b)
5 km
Greifswald
Figure 1. Maps showing the study area in northeastern Germany (a) and the sampling stations (b). M–V ¼ Federal State of
Mecklenburg-Vorpommern; I, II, III ¼ sampling stations.
(e.g. O¨ kland, 1963; Tudorancea, 1972; Lewandowski
& Stanczykowska, 1975; Arter, 1989;
Lewandowski, 1991; Krzyzanek, 1994; Mu¨ ller &
Patzner, 1996). Data on populations living in
running waters are infrequent (Negus, 1966;
Hickley, 1983; Lewandowski, 1990). Thus, the aim
of this work is to redress the balance and to provide
further information on unionids in running
waters. Moreover, the results should disclose
possible environmental factors, which may affect
the mussels. Data on the structure of populations
are required to take measures for their preservation
and to provide base-line data for further
investigations to reveal changes in the studied
river.
Study area
The river Ryck is a small coastal river located in
the northeast of Mecklenburg-Vorpommern
(northeastern Germany). Its catchment basin is of
an area of 230.7 km 2 (Amt fu¨ r Wasserwirtschaft,
1970) and is predominantly used agriculturally.
After a flowing distance of approximately 24.5 km
(VE Meliorationskombinat Rostock, 1968, 1970),
the river Ryck discharges into the Greifswalder
Bodden near Greifswald, i.e. into a bay of the
Baltic Sea. While the Ryck upriver of Greifswald is
of freshwater, the lower stretches carry brackish
water from the Bodden. In irregular intervals the
flowing direction of the river changes due to the
Table 1. Width and depth of the river Ryck at the sampling
stations I, II, and III
Station Width (m) Depth (m)
I 9.0 0.9
II 9.5 1.3
III 19.5 1.3
water level of the Greifswalder Bodden and the
wind conditions. Those changes are occasionally
connected with an influx of brackish water into the
stretch of the Ryck upriver of Greifswald. The
upper course of the river tables a slope of only
0.1& (VE Meliorationskombinat Rostock, 1968).
In the upper course, three sampling stations
were set up (Fig. 1). Depth and width of the Ryck
at these stations can be seen in Table 1. The shores
are fringed with reeds from Phalaris arundinacea
(station I) or Phragmites australis (stations II and
III). Among the hydrophytes in the river Nuphar
lutea, Potamogeton lucens, Potamogeton perfoliatus,
Ceratophyllum demersum, and Elodea canadensis
dominate.
Materials and methods
Environmental characteristics
In order to examine water properties and to
identify factors which could influence the mussels,

171
Table 2. Analyzed water properties and the used measuring
instruments and methods
Variable
NO 3 -N, NO 2 -N
NH 4 -N, PO 4 -P
O 2 , temperature
Conductivity
Current velocity
Seston
Instruments and methods
Ion chromatograph (Sykam Chromatographie
Vertriebs GmbH)
Photometer Nanocolor 200D (Macherey-
Nagel GmbH)
Oximeter Oxi 96 (WTW)
Conductometer LF 95 (WTW), temperature:
20 °C
Drift body method
Water samples (3 · 1 l) were filtered with
washed (Aqua dest.), dried and weighed GF/
C filters (Whatman), after that the filters
were dried (50 °C) and re-weighed
water samples were analyzed monthly between
April till October in the years 1996–1999. During
these months, the mussels reach their maximum
activity (e.g. growth, reproduction, metabolism,
translocation) and react therefore stronger to
environmental influences.
The concentration of oxygen, temperature, and
current velocity were measured at the sampling
stations. The other variables were analyzed in the
laboratory (Table 2).
To describe the sediment composition at each
station, 15 samples were taken across the width of
the river using a corer (inner diameter: 55 mm). In
this study, the upper 50 mm of the cores were
investigated. The organic content of the sediments
was determined by loss on ignition after heating
the samples for 2 h at 550 °C in a muffle furnace.
The percentages of sand, silt, and clay were measured
applying a laser particle sizer (Fritsch analysette
22).
Survey of the mussel populations
The samples were taken between 1996 and 1998. In
order to secure well-founded data on the abundance
of unionids, the entire sediment of defined
areas was sieved completely (Haukioja & Hakala,
1974; Hanson et al., 1988; Miller & Payne, 1988,
1993; Amyot & Downing, 1991; Richardson &
Yokley, 1996). Thus, a 0.25 m 2 plastic frame
(quadratic, height: 0.25 m) was pushed into the
substratum. The upper sediment layers within the
frame were removed with a net until more compact
layers, consisting of marl, sand, or peat, were
reached. The removed material was sieved (4 mm),
and the mussels were collected and placed in labeled
bags. With the intention to investigate different
ranges of the river bed, six of those samples
from the bank and further six from the middle of
the river were taken for every sample series. In the
middle of the river at station III, samples could not
be taken as described because of too soft sediments.
However, areas of 0.25 m 2 were investigated
using a dredge closable by a lid in a distance
of 5.5–6 m off the bank. The dredged material was
sieved, and the mussels were picked out at the
sampling station, too.
There were seven sample series taken at station
I, 3 at station II, and 4 at station III. Within one
sampling site, only different areas were investigated
by the several sampling series applying a preestablished
pattern.
In order to ascertain the total size of the mussel
populations at every station, four transects were
installed additionally across the river. In every
transect, one sample per meter was taken. The
sampling was carried out as described using frame
and net or the dredge, respectively.
The collected mussels were brought to the
laboratory for ageing and kept there in a large
vessel filled with river water. The age was identified
by means of dark annual rings observed on the
shells (e.g. Haskin, 1954; Crowley, 1957; Negus,
1966; Haukioja & Hakala, 1978; Neves & Moyer,
1988). Difficulties in counting these rings could
appear if mussels stopped their growing after disturbances
and formed pseudannuli. Those were
not easily to distinguish from true annual rings.
Furthermore, the age determination could become
difficult if several rings were very close. Such ring
concentrations occurred mainly in the fringe ranges
of shells of older mussels. Mistakes in the age
determination were avoided by means of repeated
controls and comparisons of shells. After the
investigations, the mussels were released at the
location of their origin.
The similarity of species compositions and age
structures ascertained at the sampling stations was
determined using three indices. First, for comparing
the presence or absence of age classes at the
stations, the set quotient was calculated (Mu¨ ller,
1978, 1987):

172
c
Sq ½%Š ¼100
a þ b þ c ;
where Sq is the set quotient; a, the number of age
classes only existing within stock A; b, the number
of age classes only existing within stock B; c the
number of age classes existing within both stocks.
Secondly, to calculate the similarity of the
dominance values of species or age classes, Renkonen‘s
coefficient was used (Mu¨ hlenberg, 1989):
Rc ½%Š ¼ Xs
i¼1
D min
i ;
where Rc is the Renkonen’s coefficient; D i min , the
smallest of the two dominance values of a species
or age class.
At last, set quotient and Renkonen’s coefficient
were combined to calculate Wainstein’s index,
which considers the presence of species or age
classes within different stocks and their dominance,
too (Mu¨ ller, 1987; Mu¨ hlenberg, 1989):
Sq Rc
Wi ½%Š ¼ ;
100
where Wi is the Wainstein’s index; Sq, the set
quotient; Rc, the Renkonen’s coefficient.
Results
Environmental factors
The results of water analyses are shown in Figure
2. Of all variables average values, highest and
lowest ones of the investigation period are presented.
The factors followed characteristic seasonal
trends during the years. Amongst nitrogen
ions the nitrate ions predominated (Fig. 2a–c).
The concentration of nitrate was 5–10-fold larger
than the ammonium one and represented the 25-
fold of the nitrite concentration. The concentration
minima were measured in summer. The
phosphate concentration was between 0.002 and
0.424 mg/l (Fig. 2d). Especially high concentrations
appeared at times of low oxygen saturation.
The concentrations of nitrogen and phosphate
ions slightly decreased downwards the river or were
lowest at station II. The gained values of the four
ions mirror a b-mesosaprobic to b-a-mesosaprobic
situation in the river (determination according to
LAWA, 1998). The concentration of oxygen declined
downwards the river (Fig. 2e). During the
year, concentration minima occurred in summer.
The lowest value of 1 mg/l (saturation: 11%) was
measured at station II. The temperature of the
water correlated with the air temperature and appeared
highest in summer. Down the river, the
mean water temperature rose (Fig. 2f). The conductivity
showed only little deviations (Fig. 2g).
The high value of 2.09 mS/cm at station III could
be explained by an influx of brackish water into
the upper river. The current was fastest at station
II (Fig. 2h). Its highest value was 0.4 m/s. The
concentration of seston was ascertained to quantify
the nutrient resources for mussels. The average
concentration at station III was nearly double-fold
as high as at the other two stations (Fig. 2i). No
significant differences between the sampling stations
were found regarding the water properties (ttest:
p > 0.05).
The sediment consisted of silty sand (stations I
and II) or silty loamy sand (station III). The
average content of organic matter varied between
3.6% (station II) and 17.8% (station III). The
sediment composition showed a close relationship
to the current velocity, as with an increase of current
velocity the percentage of sand in the sediment
increased, too. The content of organic material,
however, got reduced (Fig. 3). The percentages of
sand and organic matter in the sediment at station
III differed significantly from those found at the
stations I and II (t-test (sand): p I/II ¼ 0.375, p I/
III ¼ 0.049, p II/III ¼ 0.013; t-test (organic matter):
p I/II ¼ 0.308, p I/III < 0.001, p II/III < 0.001).
Species composition and population size
At all stations, mussels of the species Anodonta
anatina (L.), Anodonta cygnea (L.), and Unio tumidus
Philipsson were found. The abundance of
mussels, which got ascertained from particular
samples and sample series, showed considerable
deviations. In Figure 4 the mean values for the
sampling stations as well as the values of series
with highest and lowest abundances are given.
The highest population densities of the unionids
were observed in the bank regions of the
stations I (maximum: 138.7 indiv./m 2 ; April 1996)
and II (max.: 110.0 indiv./m 2 ; May 1997). In

173
(a)
NO 3 -N
(b)
NO 2 -N
(c)
NH 4 -N
5
0.27 0,27
0,8 0.8
Concentration [mg/l]
4
3
2
1
Concentration [mg/l]
0.18 0,18
0.09 0,09
Concentration [mg/l]
0,6 0.6
0,4 0.4
0,2 0.2
0
I II III
0.00 0,00
I II III
0.00
I II III
Station
Station
Station
(d)
0.5 0,5
PO 4 -P
(e)
18
O 2
(f)
25
Temperature
Concentration [mg/l]
0.4 0,4
0.3 0,3
0.2 0,2
0.1 0,1
Concentration [mg/l]
12
6
Temperature [°C]
20
15
10
5
0.00
I II III
0
I II III
0
I II III
Station
Station
Station
(g) Conductivity
(h) Current
(i)
2.5 2,5
0,4 0.4
25
Seston
Conductivity [mS/cm]
2.0 2,0
1.5 1,5
1.0 1,0
Velocity [m/s]
0,3 0.3
0,2 0.2
0,1 0.1
Concentration [mg/l]
20
15
10
5
0.5 0,5
I II III
Station
0.00
I II III
Station
0
I II III
Station
Figure 2. Average, highest and lowest values of the water properties which were analyzed in the river Ryck monthly from April to
October in the period of survey (1996–1999). I, II, III ¼ sampling stations.
comparison with these data, the highest abundance
at station III was very low (max.: 33.3 indiv./m
2 ; July 1997). The average population
density within the bank region at station III was
also distinctively lower than those at the stations I
and II (station I: 66.0 indiv./m 2 , station II:
61.1 indiv./m 2 , station III: 18.5 indiv./m 2 ).
The dominance values of the species are illustrated
in Figure 5. Anodonta anatina was the
predominant unionid at all stations (54.0–69.2%).
At the stations I and III, A. cygnea reached a higher
dominance (30.2%, 24.4%) than U. tumidus which
was only represented by relatively few individuals
(0.6%, 10.9%). The situation at sampling site II was
different, as the most balanced numerical ratio between
the three species existed at this station. Furthermore,
U. tumidus (28.2%) reached a higher
dominance than A. cygnea (17.8%).

174
Sand, org. matter [%]
90
60
30
0
Sediment
II I III
Station
0,15 0.15
0,10 0.10
0,05 0.05
0,00 0.00
Current velocity [m/s]
sand org.matter current velocity
Figure 3. Average percentages of sand and organic matter in
the sediments as well as the average current velocity measured
in the water column. I, II, III ¼ sampling stations.
A comparison of the dominance structures
observed at the three stations by means of
Renkonen’s coefficient underlined the special position
of station II (Table 3).
The mean abundances of the unionid mussels
are given in Table 4. However, the study showed
that the mussels were unequally distributed within
the river cross-section (Fig. 4). Thus, the determined
abundances only enabled to draw rough
conclusions about the total size of the unionid
populations. Therefore, the numbers of clams per
river meter (rm ¼ stretch of the river of 1 m
length) were calculated (Table 4). With 350.5 indiv./rm
the largest number of mussels per river
meter was found at station II. It even exceeded the
number of individuals at station III, though the
river Ryck was twofold wider there (comp.
Table 1).
Age structure
The age structures of A. anatina and A. cygnea
appeared very unbalanced within the period of
Station I (1996)
Station II (1997)
Abundance [N/m 2 ]
160
140
120
100
80
60
40
20
0
66.0
44.6
bank
20.7
0.7
middle
36.7 27.6
9.1
0.0
Un Aa Ac Ut Un Aa Ac Ut
Taxon
Abundance [N/m 2 ]
140
120
100
80
60
40
20
0
61.1
38.6
bank
13.8
8.7
42.2
middle
17.1 20.4
4.7
Un Aa Ac Ut Un Aa Ac Ut
Taxon
Abundance [N/m 2 ]
40
35
30
25
20
15
10
5
0
7.5
Station III (1997)
4.5
bank 18.5 5.5m
2.3 0.7
12.3
4.0 2.2
Un Aa Ac Ut Un Aa Ac Ut
Taxon
Figure 4. Abundance of the Unionidae at the sampling stations. Column and given number ¼ average value; vertical lines ¼ presentation
of maximal and minimal values; Un ¼ Unionidae; Aa ¼ A. anatina; Ac¼ A. cygnea; Ut¼ U. tumidus; 96, 97 ¼ years of
surveys (1996 and 1997); bank ¼ sample series taken within the bank region; middle ¼ sample series taken from the middle of the river;
5.5 m ¼ sample series taken in a distance of 5.5–6 m off the bank.

175
Dominance [%]
80
70
60
50
40
30
20
10
0
Station I
II
III
N 985 465 156
Figure 5. Species composition at the sampling stations I, II,
and III. Aa ¼ A. anatina; Ac¼ A. cygnea; Ut¼ U. tumidus;
n ¼ number of collected mussels.
Table 3. Similarity of the dominance structures according to
Renkonen’s coefficient (Rc)
Stations I/II I/III II/III
Rc [%] 72.44 89.71 82.73
I, II, III = sampling stations.
survey. At all stations, one or more age classes
were absent (Fig. 6). The youngest age classes
reached relatively low dominance values in the
stocks, and the older ones were very numerous and
partly even predominant. This tendency to overaged
stocks became clear especially regarding
A. anatina at station III.
At the three sampling stations, different age
classes of the Anodonta species were absent or
dominating. At the stations I and III, for example
the age classes of 1987, 1989, and 1995 were very
Aa
Ac
Ut
rich in individuals. At sampling site II, the age
classes 1988–1991 were represented by only few
individuals or they lacked completely. These differences
between the age structures suggest that
similar living conditions for mussels cannot be
presumed in the whole upper course of the river
Ryck.
In order to assess the similarity of age structures
more efficiently, set quotient, Renkonen‘s
coefficient, and Wainstein’s index were calculated.
The results showed that there was a higher similarity
between the age structures of both species
from the same station, than between the age
structures of the same species from different stations
(Fig. 7). This similarity between the age
structures of A. anatina and A. cygnea was greatest
at the sampling sites I and II. Amongst the stations,
the highest similarity was detected between
the sampling sites I and III (comp. Table 3).
For U. tumidus the partly very low abundances
and especially the age structures (Fig. 8) indicate
an instability of the population. At station I, a
very low population density of this species was
detected. But it has to be underlined that in this
certain river section some juvenile mussels were
also found. At station II, the population virtually
consisted solely of two age classes, one of them
was considerably young. The stock at station III
appeared overaged since no juveniles could be
collected there.
Discussion
The species Anodonta anatina, Anodonta cygnea,
and Unio tumidus which were found in the upper
course of the river Ryck inhabit running and
Table 4. Population size and mean abundance of the Unionidae in the river Ryck
Taxon I II III
Abundance (N/m 2 ) Pop. size (N/rm) Abundance (N/m 2 ) Pop. size (N/rm) Abundance (N/m 2 ) Pop. size (N/rm)
Unionidae 33.6 311.3 34.9 350.5 10.9 216.2
A. anatina 23.2 214.6 18.8 188.8 7.0 138.5
A. cygnea 10.2 94.7 6.2 62.4 2.8 55.0
U. tumidus 0.2 2.0 9.9 99.3 1.1 22.7
I, II, III = sampling stations; N = number of mussels; rm = river meter (stretch of the river of 1 m length).

176
40
Aa (stn.I 96)
N = 682
40
Ac (stn.I 96)
N = 297
30
30
N [%]
20
N [%]
20
10
10
0
0
0
2
4
6
8
10
12
14
0
2
4
6
8
10
12
14
Age [years]
Age [years]
40
Aa (stn.II 9 7)
N = 251
40
Ac (stn.II 9 7)
N = 83
30
30
N [%]
20
N [%]
20
10
10
0
0
0
2
4
6
8
10
12
14
0
2
4
6
8
10
12
14
Age [years]
Age [years]
40
Aa (stn.III 97)
N = 101
40
Ac (stn.III 97)
N = 38
30
30
N [%]
20
N [%]
20
10
10
0
0
2
4
6
8
10
12
14
Age [years]
Age [years]
Figure 6. Age structures of Anodonta anatina (Aa) and Anodonta cygnea (Ac). I, II, III ¼ sampling stations; 96, 97 ¼ years of surveys
(1996 and 1997); n ¼ number of collected mussels.
0
0
2
4
6
8
10
12
14
standing waters in large areas of Germany and
Europe (Falkner, 1990; Glo¨ er & Meier-Brook,
1994; Wolff, 1968; Zadin, 1938).
The abundance values of unionid mussels show
an enormous deviation range in running waters of
Central and Western Europe (Table 5).
The data in Table 5 give the impression that the
average and the maximum population densities of
unionid mussels in the river Ryck are very high.
The species composition corresponds with the type
of river, i.e. with a eutrophic slowly running water
having finer sediments. However, examining the
abundances and the age structures of the three
species at the several sampling stations carefully it
has to be said that the situation of unionids is not
as good as it appears at first sight.
First of all, the state of the U. tumidus population
has to be classified as critical. At station I,
this species reached only an abundance of ca.
0.2 indiv./m 2 . Regarding Unio crassus, Hochwald

177
Figure 7. Similarity of the age structures of both Anodonta species according to set quotient (Sq), Renkonen’s coefficient (Rc), and
Wainstain’s index (Wi). Aa ¼ A. anatina; Ac¼ A. cygnea; I, II, III ¼ sampling stations.
40
Ut (stn.I 96)
N =10
80
Ut (stn.II 97)
N =131
30
60
N [%]
20
N [%]
40
10
20
0
0
0
2
4
6
8
10
12
14
0
2
4
6
8
10
12
14
Age [years]
Age [years]
90
Ut (stn.III 97)
N =17
60
N [%]
30
0
0
2
4
6
Age [years]
Figure 8. Age structures of Unio tumidus (Ut). I, II, III ¼ sampling stations; 96, 97 ¼ years of surveys (1996 and 1997); n ¼ number of
collected mussels.
8
10
12
14
(1997) and Hochwald & Bauer (1990) verified a
reducing percentage of fertilized eggs in the clutch
during a period of a decreasing population density.
Downing et al. (1993) proved for Elliptio complanata
(Unionidae) that no successful fertilization is
possible at an abundance less than 10 mussels/m 2 .
Therefore, the stock at station I is possibly unable
to reproduce and depends on an import of glochidia
from other stretches of the river Ryck. In
the river section at station II, the stock was mainly

178
Table 5. Abundances of unionid mussels in European running waters
Author River Abundance (N/m 2 ) Sediment type
Franke (1993)
Main (banks) and its
2–4 Gravel, sand, mud
tributaries (Germany)
Hickley (1983) Eden (England) 73 Gravel and silt
Lewandowski (1990)
Szeszupa (lower stretches; 2–344 Sandy–muddy, muddy
Poland)
Libois & Hallet-Libois (1987) Meuse (Belgium) 18.1 Mud, sand, fine gravel
Negus (1966) Thames (England) 22.3 Different types (e.g. silt,
sand, muddy sand)
Zettler (1998)
Peene and its tributaries 7.6–75.5 Sandy–muddy
(Germany)
This study Ryck (Germany) Sandy–muddy
Station I 33.6 (max.: 138.7)
Station II 34.9 (max.: 110.0)
Station III 10.9 (max.: 33.3)
composed of representatives of two age classes.
Because of the low age of one of these age classes
and the relatively high population density (ca.
9.9 indiv./m 2 ), U. tumidus was less endangered at
this station and had the best chance of reproduction
there in the entire river. However, with respect
to the age structure, this part of the population has
also to be assessed as very vulnerable. At sampling
station III, the stock was considerably overaged
and will possibly disappear during the next years,
if no more juvenile mussels grow up there.
The Anodonta species are not so threatened,
though even they are also impaired. The age
structures prove the absence of some age classes
and an overaging of few stocks. The reproductive
success changes more or less annually and appears
unpredictable. However, because of the low percentage
of juveniles in the population, a reduction
of the number of individuals of all species is to
expect within the next years. This decline can have
certain effects on the future self-purification
capacity of the river.
Furthermore, the population structures found
at the three stations, their peculiarities and similarities,
are going to be discussed.
In the river Ryck, extensive works for river
maintenance are carried out annually. During
these works a lot of mussels is injured or killed by
machines or left on the bank, respectively. At
station II in 1997, no 7-year, 8-year, and 9-yearold
clams were found. The absence of some
successive age classes could lead to the assumption
that within this river section an annihilation of the
unionids occurred and later on, a resettlement.
Because of the fact that plenty individuals of older
age classes were collected, this possibility can be
excluded.
Furthermore, factors which influence the survival
of animals at different developmental stages
have to be considered. The glochidia of unionid
mussels are attached on host fish as ectoparasites
for a certain period of time. Species lists of the
ichthyofauna prove the existence of a lot of fish
species in the river Ryck from which several species
are potential suitable hosts for every mussel
species (see Appendix 1).
There are no hints for a complete collapse of
the fish population during the last years, which
could have been the reason for the absence of
different age classes. Kornmilch (pers. comm.)
observed the ichthyofauna in this river for several
years and found frequently shoals of juvenile fish.
If the state of the ichthyofauna was a reason of the
described situation of unionids, the same age
classes of mussels should be absent at all or
neighboring stations. But a comparison of the age
structures showed a higher similarity between the
stations I and III as well as a special position of the
amidst located sampling station . Therefore, a
certain influence of the fish population on unionid
mussels and their reproductive success could exist,
but it does not seem to be the most decisive one.

179
There can also be assumed connections between
the mussels and the water quality or the
sediment composition. Unio tumidus is considered
to be highly sensitive to phenomena of eutrophication
whilst the Anodonta species are less pretentious
(Agrell, 1949; Scholz, 1991, 1992; Jueg,
1993; Mouthon, 1996). Bahr (1994) showed a big
influence of oxygen concentration on U. tumidus.
Hereby, a concentration of 6.2 mg O 2 /l was given
as the limiting value below that the mussels were
impaired by oxygen deficiency. In the river Ryck,
oxygen concentrations lower than the abovementioned
limiting value got frequently measured
(Fig. 2e). Such shortage situations existed from
time to time for several weeks.
The Anodonta species suffer from low oxygen
concentrations less than U. tumidus. The different
tolerance of the species with regard to environmental
factors is closely connected with the type of
habitat the mussels live in. In this context e.g. the
stability of environmental conditions in waters is a
very important criterion. U. tumidus prefers larger
waters (e.g. rivers and lakes) with sandy sediments.
The Anodonta species inhabit additionally little
waters like ponds and peat turbaries with a frequently
muddy substratum. There they have to
endure extreme conditions which result from the
small water body and the accumulation of nutrients
from surrounding areas (e.g. extreme temperatures,
oxygen over- or undersaturation of the water).
Thus, at falling oxygen partial pressure A. cygnea
can keep the respiration rate constant or change to
an anaerobic metabolism (Holwerda & Veenhof,
1984; Massabuau et al., 1991). Therefore, this species
will react less sensitive to a shortage of oxygen.
Sheldon & Walker (1989) described similar
observations comparing two Australian naiad
species. Velesunio ambiguus can be found in billabongs
and creeks. When the oxygen concentration
is low, these mussels are able to stabilize their respiration
rate or they close the valves until the
oxygen content in the water rises again. In contrast,
the respiration rate of the species Alathyria jacksoni
which inhabits large rivers decreases with a
declining oxygen concentration. This species depends
on a relatively stable environment and is not
able to regulate the respiration rate under unfavorable
conditions (Sheldon & Walker, 1989).
Bahr (1994) declared furthermore that an
increase of nitrate, nitrite, and phosphate concentrations
can reduce the periods of activity of
U. tumidus to 82–38%. The ion concentrations
used in these experiments were repeatedly traced in
the river Ryck (see Fig. 2a, b, d):
NO ) 3 : 0.2 mmol/l (2.801 mg/l NO 3 -N, author’s
rem.);
NO ) 2 : 0.002 mmol/l, 0.01 mmol/l (0.028 mg/l
NO 2 -N, 0.140 mg/l NO 2 -N, author’s rem.);
PO/ 3) 4 : 0.005 mmol/l (0.155 mg/l PO 4 -P, author’s
rem.).
Perhaps U. tumidus in the Ryck is better
adapted to unfavorable conditions than the test
animals of Bahr (1994). But nevertheless, there
seems to be the possibility that the mussels of the
Ryck are being damaged or influenced at times by
low oxygen or higher ion concentrations. This can
enlarge the sensitivity of mussels towards further
stress-factors and in that way possibly cause a
higher mortality. Thus, a certain influence of the
properties of the water body on the stability of the
mussel populations can be expected. But as these
factors are relatively stabile or form gradients in
the course of the river they cannot explain the
similarity between the stations I and III as well the
peculiarities of station II.
Fleischauer-Ro¨ ssing (1990) and Engel (1990)
investigated influences on the survival of juveniles
of the Unio species. Because juveniles live within
the upper stratum of the sediment, sediment
composition and interstitial water were analyzed
by the mentioned authors. Both investigators explained
that ammonium and oxygen concentrations
and the content of organic and fine-grained
material determine the rate of mortality. Bauer
(1988) declared as well, considering the freshwater
pearl mussel that a high percentage of organic
material in the sediment influences the development
of juvenile mussels negatively.
The given factors are closely connected. During
the decomposition of organic matter in the sediment,
oxygen is consumed and poisonous ammonium
is accumulated. If fine-grained or organic
material clogs the pores of the sediment the exchange
of the interstitial water will become limited
and the conditions will aggravate. Furthermore,
since the water temperature influences the turnover
rate of organic matter and nutrients it can also
modify the situation in the sediment seasonally
(Fischer, 2000).

180
The above-described bearings seem to be
transferable to the river Ryck. At the sampling
sites I and II, where the content of organic material
in the sediments is lower, there were juvenile
U. tumidus found. Within the river stretch at
station II characterized by the lowest losses on
ignition of the sediments, U. tumidus reached its
highest abundance. At station III, where the
highest losses on ignition were measured juvenile
U. tumidus did not exist, and the stock was obviously
overaged.
In contrast to the results regarding U. tumidus,
juveniles of the Anodonta species were found at all
stations. The absence of some age classes can
probably be traced back to the same causes as for
U. tumidus, even if the clams of the genus Anodonta
are less sensitive to their environment.
Therefore, at station II the clearly higher current
velocity causes a diverging composition of the
sediment and creates living conditions for mussels
which are different from those at the other two
stations. The living conditions influence particularly
the survival of the juvenile unionids during
the first months after leaving the host fish which
is reflected in abundance, dominance, and age
structure of the species.
As mentioned above, brackish water flows
occasionally into the river stretch at station III. A
determinant influence of this water on the unionid
mussels there is not to be expected because of two
reasons. First, living A. anatina and A. cygnea were
found further down the river, too. Secondly, the
similarity of the mussel stock of station III to that
of station I which was not influenced by brackish
water rejects this thesis.
Summary and conclusions
The results of this survey showed that unionids
attained very high densities in the river Ryck.
Species composition and abundance corresponded
with this type of running waters, i.e. small and
slowly flowing rivers. But changes of stocks can be
predicted. The number of individuals is going to
decrease, and U. tumidus may disappear in several
river sections.
The data suggest that the dominance values of
the species as well as the peculiarities of their age
structures are influenced first of all by current
velocity and sediment composition, which change
seasonally and along the river. Especially the
juvenile unionids inhabiting the upper stratum of
sediment are dependent on the conditions in the
interstitium. A strong current ensures a better
ventilation of the sediment, prevents an enrichment
of poisonous substances, and reduces the
sedimentation of further fine material that clogs
the interstitium. In contrast, the concentrations of
ions and oxygen in the water body appear to have
only a low influence on the mussel populations.
Thus, the decline of unionid populations in the
Ryck seems to result from the eutrophication of
this river. The sediments that are largely enriched
with organic material will – for the longer term –
maintain the internal eutrophication of the river.
This situation will continue even if the influx of
matter into the river decreases. Therefore, there is
no rapid improvement in the living conditions of
the water organisms to be expected.
The presented results suggest to register not
only the presence of unionid species, but also the
age structure of their populations in monitoring
programs to characterize waters. Age structures
present important hints for the actual state of the
populations (e.g. present ability for reproduction),
their future stability and development (undisturbed,
impaired, or overaged populations).
Therefore, age structures appear as sensitive indicators
for the situation in the waters and as a
necessary supplement for a solely recording of the
species and their abundances.
Acknowledgements
The study was carried out at the Zoological Institute
and Museum of the Ernst Moritz Arndt University
of Greifswald and supported by grants of the
foundations ‘Studienstiftung des deutschen Volkes’
and ‘Fazit-Stiftung’. Thanks to Mrs M. Sandhop
for her encouragement and help in improving the
manuscript. Mr J.-C. Kornmilch kindly provided
data on the ichthyofauna of the river Ryck. I also
wish to thank Dr C. Weber for the translation of the
manuscript into English and Dr A. J. Anderson
(Daberkow) as well as S. Schubert (Greifswald) for
linguistic advice. I am grateful to an anonymous
reviewer for important comments on an earlier
version of this manuscript.

181
Appendix 1.
Fish species in the upper course of the river Ryck and their suitability for serving as host. Investigators of the ichthyofauna: a ¼ Hille
(1997); b = Kornmilch (pers. comm.); c ¼ Meyer (1962); d ¼ Wanke (1996).
Fish species Investigator Suitability of the fish species as host
Aa Ac Ut
Abramis brama (L.) Carp bream b, c +6 )5
Alburnus alburnus (L.) Bleak b, c
Anguilla anguilla (L.) Eel c, d )7 )7
Blicca bjo¨rkna (L.) White bream b, c, d +6 +6
Carassius carassius (L.) Crucian carp b, c, d )3, )7 )1, )7 )5
Cobitis taenia L. Spined loach a, b, c, d
Esox lucius L. Pike a, b, c, d 4, +7 +7, +6
Gasterosteus aculeatus L. Three-spined stickleback a, b, c, d +7 +7 +2
Gymnocephalus cernuus (L.) Ruffe c 4, )6 +1, )6 ±5
Leucaspius delineatus (HECKEL) Belica a, b, c +3
Leuciscus idus (L.) Ide d
Perca fluviatilis L. Perch a, b, c, d +3, 4, +6, +7 +1, +7 +2, +5
Pungitius pungitius (L.) Ninespine stickleback a, b, c, d
Rhodeus sericeus (BLOCH) Bitterling a, b, c )3, )6, )7 )1, )6, )7
Rutilus rutilus (L.) Roach a, b, c, d +3, 4 )2, +5
Scardinius erythrophthalmus (L.) Rudd b, c, d +7 +1, +6, +7 )2, +5
Tinca tinca (L.) Tench a, b, c, d +3 )1 +5
Vimba vimba (L.) Baltic vimba c
Suitability of these fish species for serving as host: 1 = Claes (1987); 2 = Fleischauer-Ro¨ ssing (1990); 3 = Franke (1993); 4 = Jokela
et al. (1991; finding of infected fish – no check, whether glochidia developed successfully); 5 = Maaß (1987); 6 = Nagel (1985);
7 = Niemeyer (1993). + = fish species that is suitable as host (metamorphosis of glochidia into juvenile mussels); – = unsuitable
species (glochidia were shed); Aa = A. anatina; Ac=A. cygnea; Ut=U. tumidus.
References
Agrell, I., 1949. The shell morphology of some Swedish unionides
as affected by ecological conditions. Arkiv fo¨ r zoologi
41A: 1–30.
Amt fu¨ r Wasserwirtschaft, 1970. Fla¨ chenverzeichnis der Flußgebieteder
DDR-Ku¨ ste. Verlag fu¨ r Bauwesen, Berlin,153 pp.
Amyot, J.-P. & J. A. Downing, 1991. Endo- and epibenthic
distribution of the unionid mollusc Elliptio complanata.
Journal of the North American Benthological Society 10:
280–285.
Arter, H. E., 1989. Effect of eutrophication on species composition
and growth of freshwater mussels (Mollusca,
Unionidae) in Lake Hallwil (Aargau, Switzerland). Aquatic
Sciences 51: 87–99.
Bahr, K., 1994. Untersuchungen zu Schalenbewegung und
Sauerstoffverbrauch einheimischer Großmuscheln unter
verschiedenen experimentellen Bedingungen. PhD thesis,
Tierärztliche Hochschule Hannover.
Bauer, G., 1988. Threats to the freshwater pearl mussel Margaritifera
margaritifera L. in Central Europe. Biological
Conservation 45: 239–253.
Byrne, M., 1998. Reproduction of river and lake populations of
Hyridella depressa (Unionacea: Hyriidae) in New South
Wales: implications for their conservation. Hydrobiologia
389: 29–43.
Claes, M., 1987. Untersuchungen zur Entwicklungsbiologie der
Teichmuschel Anodonta cygnea. Ms thesis, Tierärztliche
Hochschule Hannover.
Crowley, T. E., 1957. Age determination in Anodonta. Journal
of Conchology 24: 201–207.
Downing, J. A., Y. Rochon, M. Perusse & H. Harvey, 1993.
Spatial aggregation, body size, and reproductive success in
the freshwater mussel Elliptio complanata. Journal of the
North American Benthological Society 12: 148–156.
Engel, H., 1990. Untersuchungen zur Auto¨ kologie von Unio
crassus (PHILIPSSON) in Norddeutschland. PhD thesis,
Universität Hannover.
Falkner, G., 1990. Binnenmollusken. In Fechter, R. & G.
Falkner, Weichtiere-Europäische Meeres- und Binnenmollusken.
Mosaik Verlag, Mu¨ nchen: 112–273.
Fischer, H., 2000. The abundance and production of bacteria in
river sediments, and their relation to the biochemical composition
of organic matter. PhD thesis, Universita¨ t Freiburg.

182
Fleischauer-Ro¨ ssing, S., 1990. Untersuchungen zur Autökologie
von Unio tumidus PHILIPSSON
und Unio pictorum
LINNAEUS
(Bivalvia) unter besonderer Beru¨ cksichtigung der
fru¨ hen postparasitären Phase. PhD thesis, Universität Hannover.
Franke, G., 1993. Zur Populationsökologie und Geschlechtsbiologie
der Teichmuscheln A. anatina L. und A. cygnea L.
(Bivalvia: Unionidae). Ms thesis, Universität Bayreuth.
Glo¨ er, P. & C. Meier-Brook, 1994. Su¨ ßwassermollusken. DJN
(ed.), Hamburg, 136 pp.
Hanson, J. M., W. C. Mackay & E. E. Prepas, 1988. Population
size, growth, and production of a unionid clam, Anodonta
grandis simpsoniana, in a small, deep Boreal Forest lake in
central Alberta. Canadian Journal of Zoology 66: 247–253.
Haskin, H. H., 1954. Age determination in molluscs. Transactions
of the New York Academy of Sciences, Series II 16:
300–304.
Haukioja, E. & T. Hakala, 1974. Vertical distribution of
freshwater mussels (Pelecypoda, Unionidae) in southwestern
Finland. Annales Zoologici Fennici 11: 127–130.
Haukioja, E. & T. Hakala, 1978. Measuring growth from shell
rings in populations of Anodonta piscinalis (Pelecypoda,
Unionidae). Annales Zoologici Fennici 15: 60–65.
Heard, W. H., 1998. Brooding patterns in freshwater mussels.
Malacological Review, Supplement 7: 105–121.
Hickley, P., 1983. Ecological notes on three species of mussels
(Anodonta anatina, Unio pictorum, Unio tumidus) in the River
Eden, Kent. Transactions of the Kent Field Club 9: 71–81.
Hille, S., 1997. Untersuchungen zur Ichthyofauna des Rycks.
Author’s copy, Ernst-Moritz-Arndt-Universität Greifswald.
Hochwald, S., 1997. Populationsökologie der Bachmuschel
(Unio crassus). Bayreuther Forum Ökologie 50, Bayreuth,
181 pp.
Hochwald, S. & G. Bauer, 1990. Untersuchungen zur Populationso¨
kologie und Fortpflanzungsbiologie der Bachmuschel
Unio crassus (PHIL.) 1788. Schriftenreihe des Bayerischen
Landesamtes fu¨ r Umweltschutz 97: 31–49.
Holwerda, D. A. & P. R. Veenhof, 1984. Aspects of anaerobic
metabolism in Anodonta cygnea L.. Comparative Biochemistry
and Physiology 78B: 707–711.
Jokela, J., E. T. Valtonen & M. Lappalainen, 1991. Development
of glochidia of Anodonta piscinalis and their infection
of fish in a small lake in northern Finland. Archiv fu¨ r Hydrobiologie
120: 345–355.
Jueg, U., 1993. Die Situation der Großmuscheln in Mecklenburg-Vorpommern.
Naturschutzarbeit in Mecklenburg-
Vorpommern 36: 38–41.
Klemm, A., T. Ludwig, M. Opitz & M. Zschutzschke, 1994/
1995. Zur Bestandssituation charakteristischer Muschelarten
des Gu¨ lper Sees. Naturschutz und Landschaftspflege in
Brandenburg 4/1994–1/1995: 19–23.
Krzyzanek, E., 1994. Changes in the bivalve groups (Bivalvia-
Unionidae) in the Goczalkowice Reservoir (southern
Poland) in the period 1983–1992. Acta Hydrobiologica 36:
103–113.
LAWA, 1998. Beurteilung der Wasserbeschaffenheit von
Fließgewässern in der Bundesrepublik Deutschland-Chemische
Gewässergu¨ teklassifikation. Kulturbuchverlag, Berlin,
35 pp.
Lewandowski, K., 1990. Unionidae of Szeszupa River and of
the lakes along its course in Suwalski Landscape Park. Ekologia
Polska 38: 271–286.
Lewandowski, K., 1991. Long-term changes in the fauna of
family Unionidae bivalves in the Mikolajskie Lake. Ekologia
Polska 39: 265–272.
Lewandowski, K. & A. Stanczykowska, 1975. The occurrence
and role of bivalves of the family Unionidae in Mikolajskie
Lake. Ekologia Polska 23: 317–334.
Libois, R. M. & C. Hallet-Libois, 1987. The unionid mussels
(Mollusca, Bivalvia) of the Belgian upper River Meuse: an
assessment of the impact of hydraulic works on the river
water self-purification. Biological Conservation 42: 115–132.
Maaß, S., 1987. Untersuchungen zur Fortpflanzungsbiologie
einheimischer Süßwassermuscheln der Gattung Unio. PhD
thesis, Tierärztliche Hochschule Hannover.
Massabuau, J.-C., B. Burtin & M. Wheathly, 1991. How is O 2
consumption maintained independent of ambient oxygen
in mussel Anodonta cygnea? Respiration Physiology 83:
103–114.
Mentzen, R., 1926. Bemerkungen zur Biologie und Ökologie
der mitteleuropa¨ ischen Unioniden. Archiv fu¨ r Hydrobiologie
und Planktonkunde 17: 381–394.
Meyer, H.-J., 1962. Faunistisch-o¨ kologische Untersuchungen
des Ryck-Flusses bei Greifswald. Ms thesis, Ernst-Moritz-
Arndt-Universita¨ t Greifswald.
Miller, A. C. & B. S. Payne, 1988. The need for quantitative
sampling to characterize size demography and density of
freshwater communities. American Malacological Bulletin 6:
49–54.
Miller, A. C. & B. S. Payne, 1993. Qualitative versus quantitative
sampling to evaluate population and community
characteristics at a large-river mussel bed. The American
Midland Naturalist 130: 133–145.
Mouthon, J., 1996. Molluscs and biodegradable pollution in
rivers: proposal for a scale of sensitivity of species. Hydrobiologia
317: 221–229.
Mu¨ hlenberg, M., 1989. Freilando¨ kologie. Quelle & Meyer
Verlag, Heidelberg; Wiesbaden, 340 pp.
Mu¨ ller, D. & R. A. Patzner, 1996. Growth and age structure of
the swan mussel Anodonta cygnea (L.) at different depths
in Lake Mattsee (Salzburg, Austria). Hydrobiologia 341:
65–70.
Mu¨ ller, G., 1978. Parameter fu¨ r Carabiden-Sukzessionen auf
der Basis von Aktivitätsdichte-Werten. Pedobiologia 18:
442–447.
Mu¨ ller, G., 1987. Die Carabidenfauna der drei Nordbezirke der
DDR-eine o¨ kologische Analyse zum Problem der Faunenveränderungen.
PhD thesis II, Ernst-Moritz-Arndt-Universita¨
t Greifswald.
Nagel, K.-O., 1985. Glochidien und Fortpflanzungsbiologie
von Najaden des Rheins (Bivalvia-Unionidae-Anodontinae).
Mainzer Naturwissenschaftliches Archiv, Beiheft 5: 163–174.
Negus, C., 1966. A quantitative study of growth and production
of unionid mussels in the River Thames at Reading.
Journal of Animal Ecology 35: 513–532.
Neves, R. J. & S. N. Moyer, 1988. Evaluation of techniques for
age determination of freshwater mussels (Unionidae).
American Malacological Bulletin 6: 179–188.

183
Niemeyer, B., 1993. Vergleichende Untersuchungen zur bionomischen
Strategie der Teichmuschelarten Anodonta cygnea
L. und Anodonta anatina L.. PhD thesis, Universität
Hannover.
Ogilvie, S. C. & S. F. Mitchell, 1995. A model of mussel filtration
in a shallow New Zealand lake, with reference to
eutrophication control. Archiv fu¨ r Hydrobiologie 133: 471–
482.
Ökland, J., 1963. Notes on population density, age distribution,
growth, and habitat of Anodonta piscinalis NILSS. (Moll.,
Lamellibr.) in a eutrophic Norwegian lake. Nytt Magasin for
Zoologi 11: 19–43.
Ostrovsky, I., M. Gophen & I. Kalikhman, 1993. Distribution,
growth, production, and ecological significance of the clam
Unio terminalis in Lake Kinneret, Israel. Hydrobiologia 271:
49–63.
Richardson, T. D. & P. Yokley, 1996. A note on sampling
technique and evidence of recruitment in freshwater mussels
(Unionidae). Archiv fu¨ r Hydrobiologie 137: 135–140.
Scholz, A., 1991. Su¨ ßwassermuscheln der Familie Unionidae im
Regierungsbezirk Detmold-Gefa¨ hrdung und Schutz. Naturschutz-Landschaftspflege
im Regierungsbezirk Detmold
8: 8–14.
Scholz, A., 1992. Die Großmuscheln (Unionidae) im Regierungsbezirk
Detmold-Verbreitung, Biologie und Ökologie
der ostwestfälischen Najaden. Naturschutz-Landschaftspflege
im Regierungsbezirk Detmold, Sonderheft, Detmold,
73 pp.
Sheldon, F. & K. F. Walker, 1989. Effects of hypoxia on oxigen
consumption by two species of freshwater mussel (Unionacea:
Hyriidae) from the River Murray. Australian Journal of
Marine and Freshwater Research 40: 491–499.
Soto, D. & G. Mena, 1999. Filter feeding by the freshwater
mussel, Diplodon chilensis, as a biocontrol of salmon farming
eutrophication. Aquaculture 171: 65–81.
Tudorancea, C., 1972. Studies on Unionidae populations from
the Crapina Jijila complex of pools (Danube zone liable to
inundation). Hydrobiologia 39: 527–561.
Vanderploeg, H. A., J. R. Liebig & T. F. Nalepa, 1995. From
picoplankton to microplankton: temperature-driven filtration
by the unionid bivalve Lampsilis radiata siliquoidea in
Lake St. Clair. Canadian Journal of Fisheries and Aquatic
Sciences 52: 63–74.
VE Meliorationskombinat Rostock, 1968. Vorflutausbau Ryck,
Kreis Greifswald, Reg.-Nr. 13-1/05/LW/600/68.
VE Meliorationskombinat Rostock, 1970. Vorflutausbau Ryck,
Kreis Grimmen, Reg.-Nr. 13-1/07/LW/663/70.
Wächtler, K., M. C. Dreher-Mansur & T. Richter, 2001. Larval
types and early postlarval biology in naiads (Unionoida). In
Bauer, G. & K. Wächtler (eds), Ecology and Evolution of
the Freshwater Mussels Unionoida. Springer-Verlag, Berlin,
Heidelberg: 93–125.
Wanke, H., 1996. Fließgewässer im Landkreis Nordvorpommern-Informationen
der Kreisnaturschutzbehörde.
Grimmen, 19 pp.
Welker, M. & N. Walz, 1998. Can mussels control the plankton in
rivers? – a planktological approach applying a Lagrangian
sampling strategy.LimnologyandOceanography 43:753–762.
Wolff, W. J., 1968. The mollusca of the estuarine region of the
rivers Rhine, Meuse and Scheldt in relation to the hydrography
of the area. I. The Unionidae. Basteria 32: 13–47.
Zˇ adin, V. I., 1938. Molljuski semejstva Unionidae. Fauna SSSR
T. IV. Izdatel‘stvo akademii nauk SSSR, Moskva; Leningrad,
170 pp.
Zettler, M. L., 1996. The aquatic malacofauna (Gastropoda et
Bivalvia) in the catchment area of a north German lowland
river, the Warnow. Limnologica 26: 327–337.
Zettler, M. L., 1998. Die Wassermollusken im Einzugsgebiet
der Peene (Nordostdeutschland). Malakologische Abhandlungen
19: 127–138.