International Journal of
Salt Lake Research 4: 251-263, 1996. © 1996 Kluwer
Academic Publishers. Printed in the
Netherlands.

The salinity tolerance
of benthic invertebrates of the Aral Sea

A.A. FILIPPOV1
and A.YU. KOMENDANTOV2

1Department of
Aral Sea Biology, Zoological Institute of the Russian Academy of Sciences, Universitetskaya naber. 1, St Petersburg, 199034,
Russia, 2Department of Marine Biology, Zoological
Institute of the Russian Academy of Sciences, Universitetskaya
naber. 1, St Petersburg, 199034, Russia

Abstract. Experiments to determine
the upper limits of potential salinity tolerance of four species of Aral Sea
benthic macroinvertebrates were carried out.
Invertebrates of marine origin tolerate salinity increases up to 70-90 g L-1,
and the gastropod Caspiohydrobia, up to
100-110 g L-1. It was concluded that the 'express-method' based on
the estimation of salinity resistance of isolated tissues cannot be used to
estimate the limits of tolerance. The prediction is made that bivalve molluscs and polychaetes will
remain in the Aral Sea until the salinity increases to 60-70 g L-1, but
perhaps gastropods will tolerate salinity increases to 100-110 g L-1.

Key words: Aral Sea, salinity
tolerance, zoobenthos

Introduction

Salinity was always an
important factor determining species composition in the Aral Sea. The comparatively low salinity, which characterised
this large lake until 1960 (about 10 g L-1), supported the existence
of both fresh-and brackish-water organisms. From 1960 to 1990, the
salinity increased from 10 to 20—40 g L-1 (to different
degrees in different areas), and as a result the species composition of the
Aral Sea was changed completely. The fresh- and brackish-water organisms
inhabiting the Aral Sea until the 1960s disappeared entirely while
invertebrates originating from hyperhaline
water-bodies and original and acclimatized animals of marine origin have
survived and spread (Aladin and Kotov,
1989). By 1993, the salinity of the Large Sea had reached 41 g L-1
and was continuing to increase, thus providing a threat for the present
inhabitants of the Aral Sea. The biological prognosis required the
determination of the maximal salinity in which the present inhabitants of the
Aral Sea could live.

Explorations
of salinity tolerance of Aral Sea benthic invertebrates (including those
species presently dwelling there) began at the end of 1940s. But most
experiments undertaken used extremely poor methods. In the simplest cases,
animals collected from the Aral Sea were merely put into aquaria with Aral Sea
water of defined salinity and their state observed. From the results of these
observations, conclusions concerning the influence of various salinities were
drawn. Investigations of this type were performed by Karpevich
(1947, 1953a,b), Khusainova (1958), Bekmurzaev (1969,
1970), and Andreev and Andreeva
(1990). Understandably, results of this kind could not provide any confident
estimates of the salinity tolerance of animals investigated.

At the end of the 1940s, Karpevich employed a
new method, namely 'fiziologicheskaya adaptaciya (physiological adaptation)'. This was based on
gradual salinity increase (Karpevich, 1947). Most
subsequent experiments on Aral Sea invertebrates used this method (Karpevich, 1947, 1953b; Khusainova,
1958; Bekmurzaev, 1970, 1971; and others). On the
basis of these experiments, it was concluded that the benthic animals now
inhabiting the Aral Sea are not able to survive salinities > 35-42 g L-1.
Note that field investigations (Filippov, 1995) showed
that animals inhabiting the Aral Sea in 1993 occurred at a salinity of 41 g L-1
and there were then no signs of depression in their populations. Moreover,
published data on the distribution of these animals in water-bodies with high
salinity (Behning, 1937; Vorobjev,
1940; Pora, 1946 (cited after Dajoz,
1972)) indicated that they could tolerate much higher salinities. It is clear
therefore that determination of salinity tolerance by using 'physiological
adaptation' has its own disadvantages; it too does not provide precise
estimates.

In the 1970s, based on the 'physiological adaptation' method, a more
advanced method was elaborated. Known as the 'stepwise acclimation' method, it
was advanced by Khlebovich and Kondratenkov
(1971, 1973). The recent investigations of Andreev
and Andreeva (1983,1990) on
bivalves of the Aral Sea used this method. The experiments indicated the high
salinity tolerance of those animals investigated but again did not determine
the upper! limits of their potential tolerance.

Also
in the 1970s, another method for the determination of salinity tolerance was
proposed. This was the 'express method' and was based on estimating salinity
tolerance of excised tissues (ciliated epithelium, muscular tissue, etc.)
without previous acclimation of the animals (Yaroslavtseva,
1976; Yaroslavtseva et al., 1981). It was
shown that on certain occasions the minimal salinity which did not damage
tissues did not differ from the lower limit of potential salinity tolerance for
whole organisms determined by stepwise acclimation. This method was also used
to estimate the potential euryhalinity of benthic
invertebrates of the Aral Sea (Andreev and Andreeva, 1983,1990). We have
serious doubts concerning the biological basis and appropriateness of this
method. This is particularly the case because in the 1960s Theede
and Lassig (1967) showed that tissue tolerance is
closely related to salinity of acclimation and thus experiments on isolated
tissues have exactly the same peculiarities and deficiencies as experiments on
whole organisms.

In general and in spite of a great number of experimental studies
undertaken since the 1940s, we still do not have any reliable data on the
salinity tolerance of benthic animals of the Aral Sea. This makes it very
difficult to predict the evolution of the Aral Sea ecosystem as it becomes more
saline. It is for this reason that further experiments on the salinity
tolerance of Aral Sea invertebrates using modern methods of investigation are
required.

The aim of the present work was to evaluate experimentally the salinity
tolerance of four important taxa of the Aral Sea macrozoobenthos (Syndosmya segmentum Recluz, Cerastoderma isthmicum Issel, Caspiohydrobia spp., and Nereis diversicolor O.F. Miiller), and, based on the results of this evaluation, to
predict possible changes in the Aral Sea macrozoobenthos
with increasing salinity.

Material and methods

Experimental animals
were collected from various parts of the inshore zone of the Aral Sea at depths
of 0.5-1 m. They were brought to the laboratory in St Petersburg. For the
experiments, Aral Sea water collected from where the animals lived was used.
Water of higher salinity was prepared by evaporating original samples to a
given salinity; water of lower salinity was prepared by diluting with distilled
water. Chlorinity was determined using the argento-metric method. Salinity was calculated using the
equation:

5 = 1.3959+ 2.4411C1 +0.0251C12

where Cl
is chlorinity in g L-1 and S is salinity
in g L-1 (Bortnik and Chistyaeva,
1990).

Temperature during
experiments varied from 18-25 °C.

Salinity
tolerance was investigated using the method of stepwise acclimation (Khlebovich and Kondratenkov, 1971)
and the 'express method' using the salinity tolerance of ciliated epithelium (Yaroslavtseva et al., 1981) (for bivalve molluscs only).

Bivalve
molluscs (S. segmentum, known
earlier as Abra ovata
(Phil.)) were collected in the Bay of Butakov at
a salinity of 35 g L-1. In the stepwise acclimation experiments,
animals were kept in five aquaria (3 experimental, 2 control) each containing 25
individuals. Each aquarium had a thin layer (2 cm) of sandy silt and contained 1.5
L of water. The molluscs used in the experiments were
8-11 mm long. Algae on the substrate served as food.

During
acclimation, animals were transferred step by step to water of higher salinity.
The time at each salinity was two weeks. The extent of
changes in salinity between steps was determined in preliminary experiments. In
the case of S. segmentum, it was 5 and 10 g L-1.
After acclimation at each step, the number of live molluscs
was determined.

For the determination of the salinity tolerance of the ciliated
epithelium of S. segmentum, the mouth blades
were excised and put into separate containers of 5 mL
volume containing water of the experimental salinity. The beating of cilia was
observed under the microscope. Water was changed every 12 hours. The survival
times of the ciliated epithelia were determined. The salinity at which ciliary movement ceased first and was then restored was
also recorded (this phenomenon is known as the 'double function stop' (Zirmunsky, 1962) or 'salinity shock' (Orlova,
1990)).

Bivalve molluscs (C. isthmicum)
were collected in the Bay of Tshe-Bas at a
salinity of 41 g L-1. In stepwise acclimation experiments, the molluscs were kept in two 10 L aquaria each with 30 specimens.
The individuals used were 15-22 mm long. Experimental aquaria had sandy
substrata and were aerated. It was not possible to maintain controls because of
a shortage of material. Salinity steps were 10 g L-1. Molluscs were fed using a concentrated culture of a green
alga.

To
determine the salinity tolerance of ciliated epithelium from C. isthmicum, molluscs were
collected in the Bay of Butakov at a salinity of 35 g
L-1. Excised gills were used. The experimental technique was the same
as for S. segmentum.

Gastropods (Caspiohydrobia) were
obtained in the Bay of Tshe-Bas at a salinity of 41 g
L-1. Because of taxonomic difficulties with regard to Caspiohydrobia, experiments involved a
species mixture. During the experiments on stepwise acclimation, individuals
were kept in ten 1-L aquaria (5 experimental, 5 control) each containing 50 individuals.
Filamentous green algae, and periphyton
on the wall of the aquaria served as food. The salinity steps were 10 g L-1.

Polychaetes (N. diversicolor)
were collected in Tshe-Bas Bay at a salinity of 41
g L-1. During experiments, animals were kept in five 10 L-aquaria (3
experimental, 2 control) each containing 30 animals. No artifical
substrata were provided, but glass tubes on the bottom served for shelter. The
animals were fed with chironomid larvae. Salinity
steps were 10 g L-1.

Results

S. segmentum

In the stepwise acclimation to high
salinity, complete mortality was observed at a salinity
90 g L-1 (Fig. 1). The highest salinity at which no signs of
depression were noted was 75 g L-1. At a salinity of 80 and 85 g L-1,
depression was expressed as a significant decrease in feeding and movement. The
large mortality of control animals (cumulatively, about 35 per cent near the
end of experiments) indicated unfavorable conditions in experimental aquaria.
For this reason, there is no certainty that a salinity of 85 g L-1
is in fact the upper salinity limit to which the molluscs
can adapt. Note that the percentage survival values given in Figs. 1, 3, 5 and 6
are not values for all animals but percentages of animals alive at the
end of each step (i.e. the number of animals alive at the end of each step x 100
/ the number of animals alive at the beginning of each step).

Fig. 1. Survival
of 5.
segmentum in experiments
on stepwise acclimation. 1, experimental animals; 2, control
animals; 3, salinity in experimental containers.

In
the experiments to determine the salinity tolerance of ciliated epithelium of S.
segmentum, three groups of animals were used. The
first consisted of animals acclimated for one month to a salinity of 20 g L-1,
the second, of animals acclimated to 35 g L-1, and the third, of
animals acclimated to 60 g L-1.

In
the first group, ciliary activity was noted from 20 (the
minimal experimental salinity) to 60 g L-1 (Fig. 2). Salinity shock
was observed at a salinity 50 and 60 g L-1 and continued for about 0.5
and 1 hour, respectively. In the salinity range where movement of cilia was
observed, the duration of their beating reduced gradually from low to high
salinity. The movement of tissue pieces due to ciliary
activity was remarkable up to a salinity of 50 g L-1.

Fig. 2. Duration of functioning of isolated epithelium of S.
segmentum. 1, epithelium of molluscs
acclimated to the salinity 20 g L-1; 2, acclimated to 35 g L-1;
3, acclimated to 60 g L-1. Vertical lines, 95 per
cent confidence intervals.

In the second group, ciliary beating was
observed up to the highest experimental salinity, viz. 90 g L-1 (see
Fig. 2). Salinity shock was observed at a salinity of 80 and 90 g L-1,
and its duration was about 1 and 2 hours, respectively. The survival time of
the epithelium was greatest at salinities between 20 and 50 g L-1,
appeared to decrease slightly at 40 g L-1, but at other salinities
diminished smoothly from low to high values. The movement of the mouth blades
in experimental containers was noted up to a salinity of 70 gL-1.

In the third group, ciliary activity was also
noted up to the highest experimental salinity (see Fig. 2), but salinity shock
was not registered. As in the previous groups, the longest time during which
cilia functioned was at low salinities and the shortest at high salinities.
However, the differences between the duration of ciliary
beating at various salinities were small and statistically insignificant.
Movement of tissue pieces was observed at all experimental salinities

Fig,
3. Survival of C. isthmicum in
experiments on stepwise acclimation. 1, experimental
animals, 3, salinity in experimental containers.

In general, the results of the experiments with isolated epithelium were
in accord with acclimation histories. Thus, note the extremely high salinity
tolerance of mollusc epithelium from individuals
acclimated to a salinity of 60 g L-1,
a tolerance apparently exceeding that of whole animals.

C. isthmicum

In the
experiments on stepwise acclimation, individuals remained alive up to 90 g L-1.
In one case, the molluscs lived more than a month in
water with a salinity of 90 g L-1 during which time there was no
obvious change in behaviour: individuals appeared
healthy and actively filtered algae from the water column. Mortality did not
exceed that of the previous acclimation step (Fig. 3). However, after
transference to the next salinity (100 g L-1), all animals were dead
within two weeks. Thus, it appears that the tolerance range for adult molluscs is up to 90 g L-1.

Fig. 4. Duration of functioning of isolated epithelium of C.
isthmicum. 1, epithelium of molluscs from salinity 35 g L-1; 2, epithelium
of molluscs from 20 g L-1. (After Andreeva and Andreev,
1990). Vertical lines, 95 per cent confidence
intervals.

The
salinity tolerance of isolated epithelium of C. isthmicum
was explored in the salinity range 1 to 90 g L-1. Ciliary movement was noted from 6 g L-1 to the
highest salinity investigated (Fig. 4). Salinity shock was recorded at a
salinity of 80 and 90 g L-1. In the salinity range 10-70 g L-1,
the epithelium survival period changed insignificantly with maximal
continuance recorded at 50 g L-1. At the extreme points of the range
(at a salinity 10 and 70 g L-1), the duration of ciliary
beating was slightly less but did not differ much from that at the salinity of 35
g L-1 in which the molluscs were kept
before the experiments.

Notably, the upper range of salinity which did not damage ciliated
epithelium was higher than the range obtained by Andreev
and Andreeva (1990) for animals which lived at a
salinity of 20 g L-1 (Fig. 4). Thus, in general for C. isthmicum (as well as S. segmentum),
considerable distinctions in the salinity tolerance of epithelia were noted
in animals acclimated to different salinities.

Fig.
5. Survival of Caspiohydrobia
spp. in experments on
stepwise acclimation. 1, experimental animals; 2, control
animals; 3, salinity in experimental containers.

Caspiohydrobia spp.

The experiments on
stepwise acclimation showed that these animals can adapt to extremely high
salinity: they survived more than two weeks at a salinity of 110 g L-1 (Fig.
5). However, some changes in behaviour began after
the salinity exceeded 70 g L-1, but acute depression occurred only
in the maximal salinity range of 100-120 g L-1. The results of these
experiments suggest that these gastropods can tolerate salinity increases up to
100-110 g L-1.

Nereis diversicolor

In stepwise acclimation,
negative effects on behaviour were not recorded until
a salinity of 70 g L-1. At 40 and 50 g L-1, mortality was
even less than in the control (Fig. 6). As dead individuals in aquaria were not
recorded, this probably reflects greater degrees of cannibalism in the control.
At 80 g L-1, individuals lived more than a month, but their state was obviously depressed and by the end of that period all
were dead. In general, experimental results indicate that the upper
limit of potential salinity tolerance for adult N. diversicolor
lies in the range 70-80 g L-1.

Fig.
6. Survival of N. diversicolor
in experiments on stepwise acclimation. 1, experimental
animals; 2, control animals; 3, salinity in experimental containers.

Discussion

The experimental results
show that the upper limits of potential salinity tolerance for the Aral Sea
fauna of marine origin, namely the clams and polychaetes,
is in the range 70-90 g L-1. The gastropods on the other
hand, which are derived from inland saline water-bodies, can tolerate higher
salinities, up to 100 g L-1. The salinity limits for animals of
marine origin are similar to those for marine animals in hyperhaline
water-bodies found by others (e.g. Hedgpeth, 1959,
1967; Khlebovich, 1962; Tseeb,
1961, 1982), viz. 70-80 g L-1. This corroborates our values.

Our upper limits for the
potential salinity tolerances of Aral Sea benthic invertebrates differ from
limits obtained by other researchers by 30-50 g L-1. The main reason
for this is probably because differing methods were used. But it is also
possible that during the past 30 years, when the Aral Sea has become more
saline, there has been a change in the genetic composition of invertebrate
populations and consequently an upward shift of potential salinity tolerances
as a result of selection.

Note that
estimates of the salinity tolerance of isolated tissues cannot apparently be
used with confidence to evaluate the salinity tolerance of whole organisms.
This is because such estimates are influenced by the salinity conditions at which
the whole aimals were kept (i.e., preliminary
acclimation). In this respect, the results of our experiments correspond well
with others (i.e., Zirmunsky and Vinogradova,
1965; Theede and Lassig, 1967;
Shlakhter, 1968; Berger, 1986). Moreover, our research
shows that the range of salinity over which the ciliated epithelium is not
damaged is likely to be wider than the salinity range to which it is possible
for intact organisms to adapt. All these data reveal that the use of tissue
tolerance to estimate the tolerance of whole organisms is not valid and may be
used only in comparative studies.

To
make predictions about changes of the fauna following increases in the salinity
of the Aral Sea it is necessary to take into account the fact that the survival
of an organism in nature, unlike the survival of an organism in an experiment,
demands additional energetic expenditure on searching for food, competition,
growth, reproduction, and so on. Moreover, different life-cycle stages often
have different salinity tolerances. Obviously, populations of the species
investigated by us will have disappeared from the Aral Sea before its salinity
reaches the lethal limit for adult organisms. On the basis of our experiments,
and taking into consideration published data on the distribution of marine
species in hypersaline water-bodies, it is possible
to suggest that organisms of marine origin will not lose their dominant
position in the Aral Sea macrozoobenthos until
salinity reaches 60-70 g L-1.

Investigations
of the fauna of hypersaline water-bodies (e.g., Tseeb, 1961; Hammer, 1986; Timms et
al., 1986) have shown that few organisms occur in the macrozoobenthos
at salinities more than 70 g L-1. And as a rule, they are organisms
of freshwater origin and among which the larvae of the Chironomidae
and Ephydridae are dominant. Apparently, therefore,
as the salinity of the Aral Sea further increases, the benthic animals of
marine origin will be gradually replaced by halophilic
organisms of inland origin. When the salinity of the Sea reaches 80-90 g L-1,
dipteran larvae will dominate bottom communities.
Nevertheless, it is possible that, until the salinity exceeds 100 g L-1,
gastropods (Caspiohydrobia) will remain in the
Aral Sea fauna.

Further
changes in the macrozoobenthos will be determined by
the extent of salinity adaptation of these halophilic
organisms. As noted by various authors (e.g., Kinne, 1971;
Hammer, 1986), even halophilic benthic invertebrates
cannot as a rule tolerate a salinity of more than 200 g L-1, and
occurrences at a salinity of 200-300 g L-1 are seldom. There are no
reasons to consider that the situation in the Aral Sea will be different. It is
likely that when its salinity exceeds 200 g L-1 all of its benthic macroinvertebrates will have disappeared.

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