 Springer-Verlag 1997
Marine Biology (1997) 128: 257–266
U. Schulte-Oehlmann · J. Oehlmann · P. Fioroni · B. Bauer
Imposex and reproductive failure in Hydrobia ulvae
(Gastropoda: Prosobranchia)
Received: 28 September 1996 / Accepted: 7 November 1996
Abstract Hydrobia ulvae specimens collected from 16
stations along the German North Sea and Baltic coasts
exhibited imposex (occurrence of male parts in addition
to the female genital system). For the purposes of
comparison, a description of both the male and unaffected female genital systems is presented. Four different
imposex stages with two types of development were
identified and documented with scanning electron micrographs for the first time. The percentage of imposexaffected females (an average over all the localities sampled) was about 44.3%, and 12.9% were definitively
sterilized. The phenomenon of sex change was not observed. The vas deferens sequence index, imposex incidence, percentage of sterilized females and the average
female penis length are recommended as biological effect
monitoring parameters in response to tributyltin pollution.
Introduction
Imposex (Smith 1971), also known as pseudohermaphroditism (Jenner 1979), in prosobranch species is the
most sensitive biological effect in response to TBT
(tributyltin) pollution of the environment. Under the
influence of this biocide with androgenic activity, female
snails develop male sex characteristics, e.g. a vas deferens and/or a penis in addition to the female system.
Masculinisation effects can be described by using clas-
Communicated by O. Kinne, Oldendorf/Luhe
U. Schulte-Oehlmann (&) · P. Fioroni · B. Bauer
Institut für Spezielle Zoologie und Vergleichende Embryologie,
Universität Münster, Hüfferstr. 1,
D-48149 Münster, Germany
J. Oehlmann
Internationales Hochschulinstitut,
Lehrstuhl für Umweltverfahrenstechnik, Markt 23,
D-02763 Zittau, Germany
sification schemes (Gibbs et al. 1987; Oehlmann et al.
1991; Bauer et al. 1995; Schulte-Oehlmann et al. 1995)
based on different stages of virilisation culminating in
the functional sterilisation and ultimate death of females. Virilisation is induced by TBT, a substance used
in antifouling paints on ships, boats and off-shore installations, as a fungicide in agriculture and in the
preservation of wood, and is added to a variety of materials as a catalyst (e.g., polyurethane foams) and protectant against microbial decomposition (e.g., textiles,
dispersion paints, PVC and other plastics). During the
last decade, increasing environmental TBT concentrations have led to an increase of the number and intensity
of imposex occurrences in prosobranch populations. To
date, more than 120 prosobranch species are known to
be affected by these pathological alterations, which
demonstrates the widespread occurrence of this very
common problem. The degree of virilisation, because of
its dependence on the TBT contamination of the environment, may be fruitfully used for TBT effect monitoring purposes. Many investigations have already used
prosobranchs, especially the dogwhelk Nucella lapillus,
as sentinel organisms to assess the actual TBT contamination of the marine environment by means of pseudohermaphroditism intensities (Bryan et al. 1986; Gibbs
et al. 1987; Oehlmann et al. 1991, 1994; Minchin et al.
1995, 1996, 1997). Unfortunately, the distribution of
N. lapillus is restricted. Dogwhelks are absent in the
entire Baltic and, with few exceptions, also in the
Skagerrak, Kattegat and the southern part of the North
Sea. Therefore it is desirable to find a broader base of
indicator organisms which should be indigenous within
areas under investigation. The fact that OSPARCOM
(Oslo and Paris Commissions) included the intersex
phenomenon of Littorina littorea (Bauer et al. 1993) in
the JMP (Joint Monitoring Programme) underlines the
importance of such studies (Oslo and Paris Commissions
1996). As far as biomonitoring purposes within the
OSPARCOM convention area are concerned, the selection of the periwinkle and dogwhelk seems to be
sufficient. Problems would certainly arise if TBT effect
73.38
13.32
0.44
1.40
0.31
1.02
0.17
0.53
–
–
0.25
0.11
0.28
1.80
0.74
4.28
–
53.06
Females 946
72.30
82.27
77.57
66.67
100.00
43.75
50.00
50.00
0.00
0.00
75.87
17.56
11.20
8.33
14.29
30.77
0.00
47.06
33.33
43.75
62.50
100.00
0.41
0.40
0.47
0.30
–
0.37
0.38
0.62
0.28
–
–
–
1.36
1.45
1.60
1.54
1.50
1.22
1.05
1.24
0.94
0.60
0.25
0.44
0.26
0.23
–
0.28
0.19
0.25
0.00
–
–
–
0.98
1.03
1.13
1.09
0.90
1.00
0.84
1.08
0.75
ND
0.17
0.16
0.17
0.12
–
0.13
0.09
0.41
0.00
–
–
–
0.52
0.54
0.59
0.46
0.60
0.52
0.46
0.55
0.35
ND
–
–
–
–
–
–
–
–
–
–
0.30
0.99
–
–
–
–
–
–
–
–
–
–
0.00
0.06
0.33
0.58
–
0.00
0.08
0.56
0.41
–
0.48
1.79
0.00
0.11
0.43
0.65
2.00
0.00
0.07
0.41
0.53
0.60
0.28
0.26
0.29
0.26
–
0.20
0.30
0.19
0.26
–
0.26
1.77
1.77
1.81
1.89
1.94
1.70
1.80
1.81
1.71
1.84
2.50
0.73
0.66
0.79
0.91
–
0.47
0.77
0.55
0.53
–
4.17
4.27
4.61
4.61
4.70
4.31
4.56
4.08
4.61
6.30
55.71
21.56
15.54
1.37
0.11
1.80
1.27
1.69
0.84
0.11
–
–
–
–
–
–
–
–
–
–
46.94
4.20
–
527
204
147
13
1
17
12
16
8
1
0.71
%
sexually
mature
± SD Albumen ± SD Capsule ± SD Pallial ± SD %
gland
gland
oviduct
para(mm)
(mm)
(mm)
sited
± SD Prostate
length
(mm)
Aperture ± SD Penis
height
length
(mm)
(mm)
837
From proximal to distal the male genital system
(Fig. 1a) consists of testis, gonadial vas deferens, vesicula seminalis, renal vas deferens, prostate gland
(pallial vas deferens) and penis.
The yellow-coloured testis is clearly delimited from
the digestive gland and extends along the columellar side
of the visceral hump. The blind-ending seminiferous
tubules are separated from each other by delicate layers
of connective tissue. The coiled seminal vesicle runs
distally along the columellar side towards the prostate
0
1
2
3
4
5
6
7
8
9
Male genital system
Males
Results
± SD
In 1994 and 1995 Hydrobia ulvae samples from 16 different stations
along the German North Sea and Baltic coasts were collected. A
total number of 1783 mud snails was examined. The specimens
were analysed (sample size ≥ 30 individuals) and narcotised in 2 to
7% MgCl2 (according to salinity at sampling stations) dissolved in
distilled water. Before cracking the shell with a vice, shell and aperture height were measured, and after cracking, the external
properties were analysed with a stereo dissecting microscope and all
organs measured to an accuracy of 0.05 mm by eyepiece (Table 1).
For imposex measurements the snails were sexed, the penis length
measured and the imposex stage (according to Oehlmann et al.
1991 and Stroben et al. 1992) determined. As imposex indices the
VDS (Gibbs et al. 1987; Fioroni et al. 1991) (vas deferens sequence = average imposex stage of a population with values of 0 to
4 in Hydrobia ulvae) and the average female penis length (FPL) of a
sample were calculated and imposex incidences determined.
For histological studies, individuals were fixed with Bouin’s
fluid for 24 h and preserved afterwards in 70% ethanol. Serial
sections (5 lm), embedded in paraplast, were made and stained
with haemalun-chromotrop (Romeis 1989). Complete series of
histological sections exist for 15 males and 54 females. For scanning electron microscopy (SEM), individuals were also fixed in
Bouin’s fluid, dehydrated via graded ethanol series, critical point
dried, coated with gold and examined with a Hitachi scanning
electron microscope S-530.
% of
Shell
females height
(mm)
Materials and methods
Sex or
Number
% of
imposex of
total
stage
specimens
monitoring surveys are extended to the Baltic Sea or to
brackish estuaries. In this case the mud snail Hydrobia
ulvae would offer a very suitable alternative. The widespread distribution area of the mud snail includes the
North Sea, Baltic Sea and large parts of the Atlantic.
Furthermore, mud snails are known to be an important
part of the marine food web and possible effects of
xenobiotics on population size and/or reproductive
performance may well result in serious ecological problems. Generally this paper will give an idea of pathomorphoses which probably have a negative effect on
the fecundity of female mud snails. A preliminary description of the imposex development in Hydrobia ulvae
will be presented. The documentation of the variability
of imposex expression and the degree of TBT pollution
at different localities will be done in another publication
(Schulte-Oehlmann et al. in preparation).
Table 1 Hydrobia ulvae. Morphometrical data of males, females and different imposex stages. For definition of imposex stages see ‘‘Results – Imposex measurement and indices’’ (SD
standard deviation; ND not determined)
258
259
gland. In sexually mature individuals this organ is
characteristically white-coloured and sperm-filled from
spring until late summer. In the distal direction the vesicula merges into a short renal vas deferens section
which leads into the pallial glandular part of the prostate
gland. The latter is a kidney-shaped, closed structure,
with the exception of a small valve in the organ’s distal
region leading into the mantle cavity. This construction
represents a kind of safety precaution, guaranteeing a
decrease of overpressure resulting from interruption
during copulation. The gland’s slit-like lumen is lined
with a ciliated prismatic epithelium and surrounded by
voluminous subepithelial glandular tissue. The flask-like
cells are arranged in clusters; their ducts penetrate the
lumen’s epithelium to discharge their secretions. More
distally the prostate continues in the pallial vas deferens
section (Figs. 1a, 2a), which runs over the bottom of the
mantle cavity towards the massive copulatory organ.
The muscular penis (Figs. 1a, 2a) is situated in the neck
region. The coiled vas deferens, which serves as a penis
duct, extends throughout its whole length. Penis size
variations during the different phases of the reproductive
Fig. 1 Hydrobia ulvae. Reconstruction of the male (a) and
female (b) genital system without testis and ovary (ag albumen gland; bc bursa copulatrix;
c connective tissue; cg capsule
gland; god gonadial oviduct; gt
glandular tissue; oc ovipar
channel; ool orifice of the ovipar
channel; ovl orifice of the vaginal channel; p penis; pd penis
duct; pg prostate gland; pl lumen of the prostate gland; pv
prostate valve; rs receptaculum
seminis; vc vaginal channel; vd
vas deferens; vs vesicula seminalis)
cycle were not observed within the investigation period,
but the extension of the male copulatory organ can vary
considerably within the very same sample.
Female genital system
The proximal female genital tract (Fig. 1b) is, according
to its ontogenetic descent, composed of a gonadial and
renal section. Following the latter is a pallial region
which is divided into a proximal albumen and a distal
capsule gland by subdivided lumina. The tubular ovary
occupies a position similar to the testis in the male and is
clearly separated from the midgut gland. The branched,
blind-ending ovary diverticles are surrounded by a thin
layer of connective tissue and lined by a flat germinal
epithelium. During the spawning season the gonad has a
vivid white colour and is filled with oocytes. These are
visible through a stereo microscope after removal of the
shell, with no further dissection necessary. The tubules
fuse to form the gonadial oviduct which runs into the
proximal coiled renal portion. This section enlarges
260
Fig. 2 Hydrobia ulvae. a SEM
photograph of a male mud
snail. b Transverse section of
the male penis. c SEM photograph of a normal female (Stage
0). d Transverse section of the
capsule gland. e, f SEM photographs of sterilized females
without imposex; in f with split
capsule gland and abortive egg
capsules (ac abortive capsules;
cgo closed genital openings;
ct ctenidium; r rectum; t tentacle; other abbreviations as in
Fig. 1)
distally to form the loop-like receptaculum seminis. The
blind-ending translucent bursa copulatrix is embedded
in the proximal region of the albumen gland (Fig. 1b).
The duct of the organ is not visible through the glandular tissue, but in serial sections it becomes obvious
that it leads into the distal region of the coiled receptaculum. The black-pigmented renal part of the
oviduct is connected to the glandular tissue of the albumen gland and joins the most proximal section of the
capsule gland’s ventral channel (vaginal part of the organ’s lumen). The distal region of the renal oviduct is
lined by a cylindrical, ciliated epithelium. In this section
sperm are stored and orientated with their heads towards the epithelium and their tails towards the lumen.
The whole renal part is enclosed by a thin wrap of
muscular tissue. The pallial part of the female reproductive organ is represented by a glandular complex.
This can be divided into a proximal albumen gland
embedded in the tissue of the visceral hump and the
more distally situated capsule gland positioned in the
central region of the mantle roof (Fig. 1b). Both glands
fuse to form a mass with a common lumen. The division
of the lumen of the capsule, or nidamental gland, into a
dorsal ovipar and a ventral vaginal channel is a distinctive mark dividing the glands (Fig. 1b). The central
channel of the albumen gland does not resemble this
structure and has a nondivided lumen. Thus the ovipar
channel extends through the whole length of the gland
complex. Oocytes are discharged into the mantle cavity
from an inconspicuous opening hidden by the vaginal
261
lips (orifice of the vaginal part) (Figs. 1b, 2d). The
vaginal part runs from the proximal end, where it receives the receptaculum, in the distal direction as a
clearly separated ventral channel. The structure finally
ends in a second genital aperture which forms the clearly
visible lips of the vulva situated directly above the
opening of the ovipar channel (Figs. 1b, 2c, d). The
tissue of the albumen gland is composed of numerous
subepithelial gland cell clusters. Each of the cluster cells
is provided with its own duct which penetrates the epithelium to discharge its secretion into the central lumen
of the gland. The capsule gland exhibits a corresponding
histological composition. Different staining reactions of
the tissues of both organs demonstrate that qualitatively
different secretions are produced.
(average over all localities), i.e. almost half of all females
studied exposed clear signs of virilisation (Table 1) in
varying degrees. It should be mentioned that this value
includes considerable differences between single sampling stations (maximum values: 100%; minimum values:
6.3%). About 12.9% of all pseudohermaphrodites were
definitively sterilized (Table 1, Stages 5 to 9; compare the
following explanation). Table 1 also shows that the occurrence of Stages 1 and 2 in Hydrobia ulvae was significantly higher than of the more advanced stages (i.e.,
3 to 9) of imposex development.
In Hydrobia ulvae there were no signs of a sex change
of females as known for muricid species (Oehlmann et al.
1991).
Imposex measurement and indices
Imposex development
As imposex describes a morphological phenomenon
based on the masculinisation of dioecious prosobranch
species, females exhibit a superimposition of male sex
characteristics. In extreme cases this leads to sterilisation
and death of the affected specimens (Fig. 2e, f ). In
Hydrobia ulvae imposex development can be described
by a classification scheme (Stages 0 to 4 and in addition
Stage 5 which represents a sterile female without male
sex characteristics) (Fig. 3), including two different paths
of evolution and a gradual increase of masculinisation in
females. The a-types within this scheme are females
which first produce a penis and then gradually develop a
vas deferens by Stage 3a. In the b-path the occurrence of
male organs is vice versa; the formation of a vas deferens
portion initiates the process which is completed with the
appearance of a penis in Stage 3b.
Within the a-line, Stage 1 (Figs. 3, 4a) represents a
female with a penis primordium but without a penis duct
(Fig. 4b). In Stage 2a (Figs. 3, 4c,d) a penis duct is then
formed and the extension of the copulatory organ increases (Table 1). A distal vas deferens section appears
at the base of the penis in Stage 3a (Figs. 3, 4e) and
grows toward the genital apertures. Stage 4 sees the
completion of imposex development as a continuous vas
deferens connects the female orifices with the penis base
(Figs. 3, 4f ).
Directly in front of the vulva a gutter-like depression
can be found in Stage 1b (Figs. 3, 5a). This is modified
towards Stage 2b to become a completely closed vas
deferens running above the bottom of the mantle cavity,
ending at the corresponding male position at the mud
snail’s neck (Figs. 3, 5b). In contrast to the male vas
deferens, the corresponding female structure is not necessarily of an elevated (Fig. 5b) but can also be of a
lowered (Fig. 5c, d) appearance, making it more difficult
to identify the structure. In Stage 3b a penis primordium
without a penis duct, which increases in size and length
is developed; a penis duct then appears in Stage 4
(Figs. 3, 5d). In general the percentage of imposex-affected females in our investigation was about 44.3%
In Hydrobia ulvae the morphological appearance of the
female genital apertures is very variable. This means
that although most virilised mud snails exhibit open
orifices (Figs. 2d, 4a, e), some also possess closed orifices (Fig. 2e) irrespective of their imposex stage. The
VDS index represents a reliable parameter for imposex
intensities and its values exhibit a significant dependence on biocide concentrations (Gibbs et al. 1987;
Fioroni et al. 1991). This index represents the arithmetic mean value of all imposex stages in a sample. The
number of stages may differ according to species specificity. In H. ulvae four stages can be distinguished.
Although a Stage 5 (Figs. 2e, 3) is not explicitly included in the scheme, the value 5 should be added to
the imposex stage of all females which exhibit closed
genital apertures (Stages 6 to 9), similar to the original
schemes of Gibbs et al. (1987), Fioroni et al. (1991) and
Oehlmann et al. (1991), where sterile females have been
ranked as stages ≥5. According to the scheme here
suggested for H. ulvae a virilised female of Stage 2 with
closed female genital opening will be given the value
7. As already stated the VDS index should provide a
realistic assessment of the reproductive capability in the
populations. It should be mentioned at this point that
morphologically different pathological conditions can
lead to an occlusion of the female openings. Although
masculinisation will not automatically lead to sterility
in H. ulvae, proliferating vas deferens tissue can cause
vulva closure. In most cases this is detected in specimens which follow the b-path of imposex development.
In these cases sterility manifests the beginning of masculinisation. Figure 6 illustrates the correlation between
the percentage of sterile females and the degree of
imposex intensity measured as the VDS index. In
H. ulvae populations with VDS values <0.5, no sterile
females are usually found. In populations with a VDS
range between 0.5 and 2.0 the first infertile females may
occur. Sterile females are not found in every population
with VDS values in this range. In samples with VDS
values > 2.0, at least some females are sterilized by
vaginal blockade.
262
Fig. 3 Hydrobia ulvae. Imposex
development scheme with four
different stages. Stage 5 represents a sterilized female without
male characteristics (e eye; pp
penis primordium; other abbreviations as in Figs. 1 and 2)
Parasitism
The investigations of Krull (1935) and Rothschild (1938)
are important references for the ability of Hydrobia ulvae
to exhibit signs of imposex irrespective of TBT exposure.
These papers give evidence that the masculinisation of
H. ulvae females was already known well before environmental organotin pollution existed. Both authors
found that the occurrence of small nonfunctioning penes
in this species is closely related to the infestation with
parasites. Therefore in our investigation we considered
whether or not all cases of masculinisation in H. ulvae
are causally and exclusively correlated with parasitism.
Table 2 summarizes the incidence of parasites in males,
females and imposex-affected females. It is apparent that
there is only a slight increase of parasitism in imposex-
263
Fig. 4 Hydrobia ulvae. Different imposex stages of the apath. a SEM photograph of
Stage 1. b Transverse section of
the female penis primordium
of Stage 1. c SEM photograph
of Stage 2. d Transverse section
of the female penis of Stage 2.
e SEM photograph of Stage 3.
f SEM photograph of Stage 4
( pp penis primordium; r rectum; t tentacle; other abbreviations as in Fig. 1)
affected specimens in comparison with unaffected females (Chi-square test: v2 ˆ 8:24 , p < 0.01); in the
comparison between males and unaffected females
v2 ˆ 14:72 ( p < 1). The analysed infested H. ulvae
specimens harboured a great variety of larval trematodes; sporocysts, redia and cercaria were observable but
a determination of the different species was not performed.
These results do not negate the fact that virilisation
of female mud snails can also be caused by parasitism,
but on no account can this be the decisive and only
reason for imposex development. If this were so, 100%
of the imposex-affected females should have parasites.
As the percentage of parasitism in all analysed pseudohermaphrodites was only approximately 15.9% and
thus not much higher than in normal forms without
imposex, the reason for masculinisation of the other
84.1% cannot be explained by parasitism. On the other
hand, a high percentage of parasite infestation within an
analysed sample will lead to a slight increase of VDS
index values. Therefore for imposex analyses within
biomonitoring programmes parasitized specimens
should be excluded.
Discussion and conclusions
Detailed descriptions of the genital system of Hydrobia
ulvae have already been presented by Krull (1935),
Johansson (1948), Falniowski (1988) and Fretter and
264
Fig. 5 Hydrobia ulvae. SEM
photographs of the different
imposex stages of the b-path.
a Stage 1; b Stage 2; c Stage 3;
and d Stage 4 (cgo closed
genital openings; oga open
genital apertures; pp penis
primordium; other abbreviations as in Fig. 1)
Graham (1994). Although the present results are basically in agreement with those of the authors above, there
are minor differences in respect to the female reproductive tract. While Krull (1935), Falniowski (1988) and
Fretter and Graham (1994) described the bursa copulatrix and the receptaculum seminis as two pouch-like
bulges of the proximal oviduct section, we were unable
to discover a receptaculum in this form. Sperm are
stored in the distal region of the coiled part of the renal
oviduct. In transverse serial sections sperm could also be
found in the vaginal channel of the nidamental gland
which continues in the coiled sperm-storing receptaculum. Hershler and Davis (1980) reported that in
Hydrobia truncata sperm are also stored within these
coils although a separated receptaculum was identified.
They furthermore pointed out that in various species of
the European hydrobiid genus Pyrgula, the seminal receptacle is reduced to a simple bulging of the coiled
oviduct. Johansson (1948) and Fretter and Graham
(1994) concluded that the closure of the pallial duct is
delayed in juvenile H. ulvae, and a ciliated groove runs
posteriorly to form the vaginal channel. In contrast to
this no individual analysed in the present study displayed a complete closure of the pallial tract; we found
two orifices in juveniles and adults alike (Figs. 2d, 4e).
As this phenomenon was detectable in all analysed
samples it does not seem to be a pathological condition
comparable with intersex development in Littorina littorea (Bauer et al. 1995), where masculinisation of females is introduced by an inhibition of the ontogenetic
Table 2 Hydrobia ulvae. Occurrence of parasites in males, females
with and without imposex characteristics
Fig. 6 Hydrobia ulvae. Relationship between the percentage of sterile
females and the VDS index. The results of population analysis (data,
points) and the calculated exponential regression (line) are documented: y ˆ 2:79 1 e 0 984 x ÿ 2:78; n ˆ 64; r2 ˆ 0:444; p < 0:0005
… :
1
†
Number of specimens
Number parasitized
Relative amount (%)
Males
Females without Females with
imposex
imposex
837
147
17.6
527
59
11.2
419
66
15.9
265
closure of the pallial oviduct. It is also worth noting that
Krull (1935) and Rothschild (1938) described parasitized
but typical male mud snails with a reduced copulatory
organ or missing vas deferens. If a correct gonadial sex
determination can be assumed, these results emphasize
that males also face the possibility of feminisation when
they are affected by parasites. Therefore it might be
possible that under the influence of TBT as a xenobiotic
with androgenic activity, the phenomenon of feminisation of males caused by gonadial castration can no
longer be observed. Within our investigation such individuals could not clearly be identified. However, an
analysis of preserved museum species sampled before
1960 when organotin compounds in antifouling paints
became available, would elucidate this problem.
For Hydrobia ulvae the imposex classification scheme
is in accordance to the findings of Oehlmann et al. (1991).
The stages of the a-path of H. ulvae are comparable with
those of muricids and buccinids, while the b-path in the
mud snail represents a mixture of the b- and c-lines reported for muricids and is less complicated. Oehlmann
(1994) reported phenotypical imposex variabilities for
the dogwhelk Nucella lapillus according to a Robertsonian polymorphism previously described by Staiger
(1957). Chromosome dimorphism of dogwhelks from
different geographical areas in Brittany is correlated with
the determination of imposex development paths a and
b. Corresponding to the results for dogwhelks, all
H. ulvae exhibiting the b-path of masculinisation came
from sampling stations on the Kiel Bight. For biomonitoring studies the VDS index (Gibbs et al. 1987; Fioroni
et al. 1991) is best suited for giving a realistic measure of
the virilisation and reproductive capability of females as
well as for the corresponding TBT concentrations. For
H. ulvae imposex incidences, percentage of sterilized females and average female penis length may be added to
the list of biological effect monitoring parameters. A
combination of all parameters will guarantee the most
precise results. As male penis size in H. ulvae does not
vary during phases of the reproductive cycle, the RPS
index [(average female penis length3 ÷ average male penis length3) · 100] introduced by Gibbs et al. (1987), as
well as the RPL index [(average female penis
length) ÷ (average male penis length) · 100] (Stroben et al.
1992) are further biological pollution parameters.
However, both parameters are of restricted practicability in Hydrobia ulvae especially in only slightly polluted areas. At these stations the average female penis
length values are very small compared to the extensions
of the copulatory organ of males. As a result, RPS and
RPL indices are near 0%, whereas it is possible to differentiate VDS values at lower pollution levels as well. It
is important to notice that the majority of the established imposex classifications refer to neogastropods
(Gibbs et al. 1987; Fioroni et al. 1991; Oehlmann et al.
1991; Stroben et al. 1992). These descriptions indicate
that sterility in female prosobranchs is likely to be seen
as closely related to the masculinisation phenomenon.
Sterility in these species is always the end result of
imposex development. In H. ulvae, on the other hand,
infertility does not necessarily correlate with pseudohermaphroditism; the mathematical parameters for assessing imposex intensities within mud snail populations
should therefore be adjusted. Former studies do not deal
with sterile females without male characteristics. The
VDS index was developed to measure imposex intensities in prosobranch populations and to give an estimation of the reproductive capability of females (Gibbs
et al. 1987); through this data TBT levels in the environment may be determined. For the analysis of mud
snail data, however, this system is unsuitable without
adaption in order to account for sterile females without
any male characteristics (Stage 5 at the beginning of the
imposex development).
Hydrobia ulvae’s life span of approximately 2 years
(Fish and Fish 1974; Lassen and Clark 1979) assures
that the biological and chemical data thus gathered
represent the current situation if the species is used in a
TBT effect monitoring programme. In this context the
problem of floating mud snails (Armonies and Hartke
1995) has to be considered. H. ulvae juveniles, for the
most part, exhibit high mobility (Armonies personal
communication); adult mud snails are more or less stationary, so that this species meets the requirements for a
bioindicator.
High abundance of mud snails with population densities up to 70 000–90 000 individuals m)2 (Walters and
Wharfe 1980) may lead to the opinion that imposex and
female sterility will have no serious effect on population
levels. As already mentioned, an average over all the
localities sampled showed that >44% of mud snails exhibit signs of masculinisation. Although imposex incidences at different sampling stations varied
considerably, the biological indices at single stations
remained quite constant (Schulte-Oehlmann et al. in
preparation). In various laboratory experiments with
marine and limnic prosobranch species it has been
proven that above all the occurrence of female sterility is
a question of the degree of TBT pollution (SchulteOehlmann et al. 1996). Considering the fact that Hydrobia ulvae is a typical soft-sediment inhabitant and the
TBT half-life of aerobic and anaerobic harbour sediments is up to 15 years (de Mora et al. 1989), the ecological consequences should not be underestimated.
Reise (1985) has already reported the important role of
hydrobiid species for secondary production, and the
occurrence of mud snails in the digestive tract of sea
birds and fish has been documented (Clay 1960). Furthermore, Drake and Arias (1995) described a significant
positive correlation between population abundance of
Hydrobia species and the biomass of benthic macroalgae. Although Wilhelmsen and Reise (1994) have
pointed out that it still remains uncertain whether
Hydrobia species feed on macroalgae themselves or
whether they consume microflora covering its surface,
the possible effects on coastal ecosystems if population
densities of the mud snail H. ulvae decrease should be
considered.
266
Acknowledgement This study was supported by the Umweltbundesamt, Berlin (R & D Project 102 40 303).
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