Invasions of New Morphotypes of Corbicula fluminalis and Corbicula fluminea (Bivalvia: Corbiculidae) to the Basin of the Dagestan Region of the Caspian Sea

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ISSN 2075-1117, Russian Journal of Biological Invasions, 2023, Vol. 14, No. 2, pp. 251–261. © Pleiades Publishing, Ltd., 2023.
Russian Text © The Author(s), 2023, published in Rossiiskii Zhurnal Biologicheskikh Invazii, 2023, No. 1, pp. 166–179.
Invasions of New Morphotypes of Corbicula fluminalis and Corbicula
fluminea (Bivalvia: Corbiculidae) to the Basin
of the Dagestan Region of the Caspian Sea
M. V. Khlopkovaa, *, R. M. Barkhalova, b, **, K. M. Guseynova, ***,
A. Sh. Gasanovaa, c, ****, and U. D. Zurkhaevaa, *****
a Precaspian Institute of Biological Resources, Dagestan Federal Research Center,
Russian Academy of Sciences, Makhachkala, 367025 Russia
b Dagestan State Nature Reserve, Makhachkala, 367010 Russia
c Makhachkala Branch, Moscow Automobile and Road Construction
State Technical University, Makhachkala, 367000 Russia
*e-mail: hlopkovam@mail.ru
**e-mail: barkhalov.ruslan@yandex.ru
***e-mail: kais61@mail.ru
****e-mail: aicha67@yandex.ru
*****e-mail: zurkhaeva81@mail.ru
Received February 21, 2022; revised February 18, 2023; accepted February 23, 2023
Abstract—The article reports the discovery of populations of Corbicula mollusks in the south of the European
part of Russia (43° N, 47° E). Representatives of the genus Corbicula are included in the 100 most actively
spreading alien species. The aim of the study was to identify species and morphotypes in the Caspian Sea
basin. For the first time on the territory of the Russian coast of the Caspian Sea at the mouth of the Sulak
River in November 2019, two live specimens of the new morphotype B/Rlc of Corbicula fluminea (eastern
Corbicula, Asian clam) (O.F. Müller, 1774) were found. In November 2021, 26 live individuals of the A/R
C. fluminea morphotype were found in the Yuzbash-Sulak Reservoir; 13 individuals of the A/R morphotype
and 90 specimens of the C/S morphotype of Corbicula fluminalis (O.F. Müller, 1774) in the Prisulak Canal
were found. A study of the morphometric parameters of the shells showed that the Asian clams found
belonged to different morphotypes, with well-distinguishable conchiological signs. The data obtained expand
the understanding of the morphological variability of Asian clams. A study of the species and size–age com-
position of populations was carried out. It was revealed that C. fluminea and C. fluminalis discovered on the
Dagestan coast were new species and morphtypes of Asian clams for the Russian sector of the Caspian Sea.
The studied samples were dominated by mollusks of size and age groups corresponding to the age from 1.5 to
3 years, which indicated a recent settlement into a new water body for them. The populations of C. fluminalis
studied from 2017 to 2021 were dominated by mollusks aged 5–6 years, which indicated an earlier invasion of
this species. High physiological tolerance combined with different breeding strategies of the representatives
of the genus Corbicula gives grounds to assume a high probability of their successful naturalization in estua-
rine and freshwater ecosystems of the Dagestan region of the Caspian Sea.
Keywords: Corbicula (Asian clam), invasions, Caspian Sea basin
DOI: 10 .11 3 4/ S 2 0 75 11172 3 0 20 0 5 4
INTRODUCTION
In the 21st century, in the context of globalization,
climate change, and an increase in anthropogenic
pressure, hidden biological threats from the spread of
alien species are becoming more dangerous.
Representatives of the family Corbiculidae live in
fresh and brackish waters of the subtropics and tropics,
being a clear indicator of warm climatic conditions
(Son, 2007; Tiemann et al., 2017). Corbicula fluminea
(O.F. Müller, 1774) (Veneroida: Corbiculidae), also
known as the eastern Corbicula (Asian clam), with a
typical habitat in China and a native range in freshwa-
ter bodies of Africa and Southeast Asia (Zhadin,
1952), is one of the actively spreading alien organisms.
C. fluminea and C. fluminalis are capable of a vari-
ety of reproductive strategies, which explains their
successful colonization of new territories in North and
South America and Europe (Araujo et al., 1993; Raja-
gopal et al., 2000; Skuza et al., 2009; Pigneur et al.,
2011, 2014; Crespo et al., 2015). In 1938, the f irst

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KHLOPKOVA et al.
expansion in North America was noted; further,
C. fluminea penetrated into South America (McMa-
hon, 1982), and beginning in the 1980s, it settled in
Europe (Morton, 1987; Elliott and Ermgassen 2008;
Khabenov et al., 2013).
C. fluminea were first found on the European part
of the Atlantic coast, including in the rivers of France
and Portugal. In 1988, the mollusks were found in the
North Sea basin, then in the rivers of other European
countries, the Black Sea (in Romania, Bulgaria,
Ukraine), and the Mediterranean Sea (in Northern
Italy) (Rajagopal et al., 2000; Son, 2007; Elliott and
Ermgassen, 2008). In recent years, C. fluminea
actively settled in freshwater estuarine ecosystems
throughout Eurasia. Currently C. fluminea is included
in the group of the 100 most dangerous invasive spe-
cies in Europe (Son, 2007; Skuza et al., 2009; Crespo
et al., 2015).
On the territory of the Russian Federation, only
one species of Corbicula lived in the Far East—Corbic-
ula japonica. From 2015 to 2017, the invader C. flu-
minea was noted in the European part of Russia. This
alien species was also found in the White Sea basin, in
the Northern Dvina River (Bespalaya et al., 2018,
2021).
In 2015, a live juvenile specimen was found for the
first time at the site of the Gorky Reservoir. C. flu-
minea; in 2017, there was a large settlement. According
to morphological and genetic features, the mollusk
was identif ied as C. fluminea (O.F. Müller, 1774)
(Voroshilova et al., 2020). In 2017, C. fluminea was
recorded in the lower reaches of the Don, which
reveals a new vector of its further expansion—the
development of river systems connected through the
Volga-Don Canal (Zhivoglyadova and Revkov, 2018).
Along this migration corridor, C. fluminea expands its
range and quickly develops new territories for habita-
tion.
Aquatic invaders spread with maritime navigation
in ballast waters (at the stage of pelagic larvae), in foul-
ing, or in the thickness of the ship’s hull (McMahon,
1982). Invasion of alien species into marine ecosys-
tems usually occurs in estuaries and main channels of
large river basins. Accidental entry of invasive species
into the Caspian Sea occurred during the Holocene
and continues at the present time.
After the opening of the Volga-Don Canal, the
Caspian Sea loses its uniqueness as an isolated brack-
ish water body owing to the constant penetration of
alien organisms along the northern invasion corridor.
Suffice it to recall the invasion of the ctenophore Mne-
miopsis leidyi into the Caspian Sea since the end of the
last century, which led to the destruction of the struc-
ture of benthic ecosystems. When integrating alien
species, stenobiont native species are the first to be
affected. Invaders are usually euryhaline, colonizing
the same biotopes and displacing the autochthonous
fauna in many complexes (Khlopkova and Gasanova,
2017).
Marine invaders, when they appear in the Caspian
Sea, find themselves in favorable biotopic conditions,
and the absence of serious competition from the native
fauna allows them to occupy a dominant position
(Karpinsky, 2009).
The Caspian Sea was used as a testing ground for
the introduction of new fish species and valuable food
objects in order to increase the fish productivity of the
water body. Seven species of planktonic invertebrates,
six species of fouling, nine species of benthic organ-
isms, and two species of fish have been introduced into
the sea, have successfully naturalized, and are found.
About 18 introduced species of fish and invertebrates
have not taken root in the Caspian Sea. The most suc-
cessful example of deliberate introduction was the
introduction into the Caspian Sea of Abra ovata and
Nereis diversicolor in the 1940s–1950s to increase the
food base of fish (Karpinsky, 2009; Zarbaliyeva et al.,
2016). Currently, alien mollusks Abra ovata, Mytilaster
lineatus, and Cerastoderma glaucum dominate in the
Caspian biocenoses, being the main food components
of commercial fish (Karpinsky, 2009; Khlopkova
et al., 2018).
Settlement of Mytilaster lineatus (1918) led to the
displacement of two endemic species—Dreissena elata
and D. caspia, which by now have completely disap-
peared in the Caspian Sea. Under the influence of
Mytilaster, the abundance and ranges of 13 species of
Caspian cardiids decreased (Karpinsky, 2009;
Zarbaliyeva et al., 2016; Khlopkova and Gasanova,
2017).
As a result of the introduction of Acartia tonsa,
Calanoida and Cladocera have disappeared in the
coastal ecosystems of the Dagestan region. But despite
the tangible damage to the biodiversity of the hydro-
fauna of the Caspian Sea, the crustacean A. tonsa is the
main food item for fish (Osmanov et al., 2017).
Unintentional introduction Balanus improvisus led
to a significant decrease in crustaceans in the sea; this
is a characteristic type of fouling community of
hydraulic structures (it grows in tons to the bottoms of
sea vessels, leading to costs for their cleaning and mil-
lions of losses). But in addition to the negative impact,
along with crabs and bivalves, this species has a pelagic
larva, which makes up the bulk of the zooplankton,
thus being an important food item for fish.
For many years, there have been attempts to intro-
duce various species of fish into the Caspian Sea,
including flounder Platichthys flesus (1930s–1940s)
and salmon Oncorhynchus keta and O. gorbusche
(1960s–1970s). Significant costs for growing juveniles
were not justified; about 1 million fry were released,
but they did not take root (Shikhshabekov and
Gadzhimuradov, 2009).
Also, unintentionally introduced species are unde-
sirable components of ecosystems that harm the local

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INVASIONS OF NEW MORPHOTYPES 253
fauna by feeding on eggs and larvae of commercial fish
species. There are the stone moroko (Pseudorasbora
parva), which came from China to the basins of the
Black Sea and Sea of Azov, single specimens of which
were also found in the rivers of the Caspian Sea basin;
needlefish (Syngnathus abaster), which settled in res-
ervoirs and rivers; and three-spined stickleback Gas-
terosteus aculeatus (Zarbaliyeva et al., 2016).
Only in the last 3 years in the Caspian Sea basin,
along with representatives of Corbiculidae, infiltrated
Japanese shrimp Macrobrachium nipponense, cteno-
phore Beroe cf. ovata, and polychaete Marenzelleria
arctia (Afanasiev et al., 2020; Vostokov et al., 2020;
Mikhailova et al., 2021).
In 2015–2017, empty shells of Corbicula fluminalis
were found on the coast of the Caspian Sea
(Nabozhenko, M.V. and Nabozhenko, S.V., 2016;
Khlopkova et al., 2018). In 2018, in the basin of the
Dagestan sector of the Caspian Sea, we first discov-
ered living individuals of C. fluminalis (O.F. Müller,
1774) in the northern part of the Agrakhan Bay
(Khlopkova et al., 2019). Detection of living different-
sized individuals of a closely related species C. flu-
minea in 2021 indicates the expansion of the range of a
new, more aggressive invader on the Dagestan coast of
the Caspian Sea.
In connection with the foregoing, the purpose of
our work was to describe and analyze the morpholog-
ical features and size–age structure of species and
morphotypes of C. fluminea and C. fluminalis that
penetrated into the Middle Caspian basin.
MATERIALS AND METHODS
The work is based on the results of hydrobiological
surveys carried out from April to November in 2017–
2021 in the coastal zone of the Dagestan coast of the
Caspian Sea—in the Kara-Murza region of the north-
ern part of the Agrakhan Bay, in the Kubyakinsky
Bank (migration path), in Lake Kuznechonok, in the
mouth of the Sulak River, in the Yuzbash-Sulak Col-
lector, and in the Prisulak Canal, which supplies Lake
Mekhteb with water. Mollusks were collected using a
rectangular dredge at a depth of up to 2 m and a hyd-
robiological scraper. The collected samples were fixed
in 96% alcohol.
The morphometric and allometric parameters of
the shells were studied on the basis of the analysis of
about 250 specimens. Laboratory processing was car-
ried out according to generally accepted methods
(Skarlato, 1981). The shell length of mature individu-
als Lad, the shell length of young mollusks Ljuv, the
height H, and the convexity of two shells dv2 were
measured with a vernier caliper, with an accuracy of
0.01 mm. The coefficients of elongation, L/H, and
convexity, dv2/L, characterizing the shape of the shell,
and the number of ribs per 10 mm of the shell length
were calculated. A study of the size–age composition
of mollusks was carried out. In order to identify the
discovered morphotypes, various shell features were
used, such as sculpture, the number of ribs on the shell
surface, general habitus, and the color of the outer and
inner surface of the shells.
We have used the term “morphotype” as an intra-
specific category denoting a variety of one species that
differs in its morphology, the isolation of which is not
genetically confirmed. The description of Corbicula
morphotypes was carried out using the European and
American classifications (Britton and Morton, 1986;
Korniushin, 2004; Marescaux et al., 2010; Tiemann
et al., 2017). We designated the morphotypes of C. flu-
minalis as C/S, C/Rlc, and C/S1 and the morphotypes
of C. fluminea as A/R and B/Rlc.
RESULTS
In benthic samples collected during the November
2021 survey in the area under investigation, mollusks
of the genus Corbicula were detected. Their shells are
triangular-rounded, with concentric ribs, almost
equilateral. Each valve contains three cardinal teeth
and long, transverse, lamellar anterior and posterior
lateral teeth; mantle line entire or with very short
sinus.
Locations of finds of new species and morphotypes
and coordinates are marked on the map (Fig. 1). In the
Yuzbash-Sulak Collector (Fig. 1, station 4), 26 living
specimens of a new species, previously not found in
the biocenoses of the Middle Caspian basin, were
found in bottom sediments. All mollusks analyzed by
us are assigned to the A/R morphotype of C. f luminea,
with massive shells, with convex ribs, 10–13 per
10 mm of valve surface. The shell length of the largest
specimen found, weighing 7.23 g, was 26.2 mm. The
age of the mollusks ranged from 1 to 4 years. At the
same time, two one-year-old individuals were
encountered, Ljuv = 12–13 mm and H = 10–12 mm.
The average age of the population was 2 years.
Among 103 living mollusks from the samples of the
Prisulak Canal (Fig. 1, station 5), we identified 13
individuals as the A/R morphotype of C. fluminea.
The shells had coarse ribs, 10–14 per 10 cm of shell
length. The maximum length of an adult individual
weighing 5.3 g is 23.1 mm. Shells of C. fluminea are tri-
angular-rounded, L/H = 1.03–1.08 (n = 13).
The remaining mollusks from the Prisulak Canal
are identified as C/S morphotype of C. f luminalis. The
shells had thin, weakly expressed, crowded ribs, with
small gaps, in the amount of 17–23 per 10 mm of shell
length (the number of individuals studied, n = 90).
Maximum Lad = 29.1 mm, H = 26.5 mm, dv2 =
18.3 mm; maximum age of mollusks is 5 years. It
should be noted that in June 2017, May 2018, and Sep-
tember 2021, more than 100 empty shells of the C/S
morphotype of C. fluminalis were also found on the
coast of the sea, 25 km north of Makhachkala.

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KHLOPKOVA et al.
In benthic samples in June and September 2021 in
Kubyakinsky Bank (Fig. 1, station 1), at a depth of up
to 2 m, 6 living specimens and 24 empty shells of C/S1
morphotype of C. fluminalis were found. Maximum
Lad = 27.4 mm, H = 27.3 mm, dv2 = 17.1 mm. In the
area of Kara-Murza and Lake Kuznechonok (Fig. 1,
stations 2, 3) in November 2021, specimens of C/S1
morphotype of C. fluminalis were found.
In November 2019, in the mouth of the Sulak
River, two living specimens of Corbicula of a new
B/Rlc morphotype of C. fluminea were found.
Corbicula mollusks live at depths from the water’s
edge to 10 m, both in fast currents and in low-flowing
water bodies. The decisive role is played by tempera-
ture, oxygenation, rheophilicity, the nature of the soil,
and the salinity of the water. Sharp fluctuations in
salinity are a natural barrier to the wide distribution of
Corbicula (Yavnov and Rakov, 2002; Paunović et al.,
2007; Skuza et al., 2009). The brackish bivalve mol-
lusk C. fluminalis can live in water with a salinity of up
to 5‰ and can withstand up to 14‰ for a short time;
it tolerates temperatures in the range of 2 to 34°С (the
optimum temperature for the reproduction of mol-
lusks Corbicula is from 6 to 15°C) (Yavnov and Rakov,
2002; Skuza et al., 2009); therefore, its introduction to
the northern part of the Dagestan coast of the Caspian
Sea is quite probable in the near future.
BRIEF DESCRIPTION
OF THE IDENTIFIED MORPHOTYPES
C/S morphotype of Corbicula fluminalis
(O.F. Müller, 1774) was discovered by us for the first
time on the coast of the Caspian Sea, 25 km north of
Makhachkala (June 2017; 43.08568° N, 47. 28512° E)
(Fig. 2a). Periostracum dark brown. Mollusk valves
have all features of estuarine eastern Corbicula—high,
triangular, less often triangular-rounded shells, with
wide cardinal plates, symmetrical, massive, with lilac
hypostracum (Fig. 3a). Lateral teeth thickened, ser-
Fig. 1. Map showing occurrence of mollusks Corbicula in Dages tan, Sou th of t he Europ ean pa rt of Rus sia; a utumn survey o f 2021
(space image). Stations: (1) Kubyakinsky Bank (43°51′29.2″–43°51′31.9″ N, 47°30′38.4″–47°31′30.1″ E); (2) Kara-Murza region
in the northern part of the Agrakhan Bay (43°49′42.7″–43°49′57.4″ N, 47°32′22.5″–47°32′40.3″ E); (3) Lake Kuznechonok
(43°45′45.2″–43°46′06.5″ N, 47°29′57.4″–47°30′07.1″ E); (4) Yuzbash-Sulak Collector (43°24′31.3 ″–43°25′09.5″ N,
47°21′29.3″–47°22′29.0″ E); (5) Prisulak Canal (43°19′36.5″–43°19′41.4″ N, 47°20′30.5″–47°21′09.6″ E).
Astrakhan
Makhachkala
MakhachkalaMakhachkala CASPIAN SEA
CASPIAN SEACASPIAN SEA
CASPIAN SEACASPIAN SEACASPIAN SEA
Novaya Kosa
Novaya KosaNovaya Kosa
Novyi Bryuzyak
Novyi BryuzyakNovyi Bryuzyak
Starotherechnoe
StarotherechnoeStarotherechnoe
Terek River
Terek RiverTerek River
1
1
1
2
2
2
3
3
3
4
4
4
5
5
5
Sulak River
Sulak RiverSulak River

RUSSIAN JOURNAL OF BIOLOGICAL INVASIONS Vol. 14 No. 2 2023
INVASIONS OF NEW MORPHOTYPES 255
rated. The crown is swollen, central. L/H = 1.05–
1.08, convex shell vp2/L = 0.57–0.80. Concentric ribs
are thin, slightly protruding, closely spaced; their
number per 10 mm is 17–22 (n = 109). Lad = 13–
23 mm (on average 18.6 mm). Ljuv = 9–12 mm. H =
15–20 mm (on average 17.1 mm; n = 109). The ante-
rior muscular imprints are longitudinally oval; the
posterior ones are rounded. In November 2021, an
expansion of the range of this species was noted; living
individuals were found (n = 90) in the Prisulak Canal.
The maximum age of individuals in the sample was
5 years, with Lad 29.1 mm.
C/S1 morphotype of C. fluminalis (Fig. 2b). The
place of the first discovery is the northern part of the
Agrakhan Bay, Lake Kuznechonok (April–June 2018;
43.45452° N, 47.29574° E). Periostracum black lus-
trous, rarely with a dark brown area, closer to the
umbo; the inner surface of the shells is grayish blue.
The shells are large, triangular-rounded, f lattened,
not high, less massive compared to the first morpho-
type; the umbo is central; the maximum dimensions of
the shells are L = 27.2 mm, H = 25.3 mm. Convexity
factor (vp2/L) is 0.74–0.78; elongation factor L/H =
1.0–1.13. The number of concentric ribs per 10 mm is
18–20 (n = 25). The maximum age of mollusks is
6 years. In November 2021, an expansion of the range
was observed in the northern part of the Agrakhan
Bay, found in the Kubyakinsky Bank.
C/Rlc morphotype of C. fluminalis. The place of the
first discovery is the northern part of the Agrakhan
Bay, Kubyakinsky Bank (June 2018; 43.51292° N,
47.30384° E); the number of ribs per 10 mm is 14–17
(n = 3). The shell is oval-triangular, with a pointed
prosogid central umbo and thin cardinal teeth, flat-
tened vp2/L = 0.72–0.74 (Fig. 2c). Periostracum light
brown, lustrous; inner surface pale lilac; valves light
brown in lumen. The shell is elongated, L/H = 1.19–
1.25; the limiting age of individuals in the sample was
6 years. In November 2021, the C/Rlc morphotype of
Fig. 2. Corbicula fluminalis. (a) Morphotype C/S, in 2017 was first discovered 25 km from the city of Makhachkala. Expansion of
the range—Prisulak Canal, 2021; (b) morphotype C/S1, Lake Kuznechonok, 2018. Expansion of the range—Kubyakinsky Bank,
2021; (c) morphotype C/Rlc, Kubyakinsky Bank, 2018. Expansion of the range—Kara-Murza, 2021.
(a) (b)
(c)
Fig. 3. Corbicula fluminea. (a) Morphotype V/Rlc, the
region of the mouth of the Sulak River, 2019; (b) morpho-
type A/R, Yuzbash-Sulak Collector, 2021.
(a)
(b)

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KHLOPKOVA et al.
C. fluminalis was found in the northern part of the
Agrakhan Bay—in the Kara-Murza area (n = 5).
B/Rlc morphotype of C. fluminea (O.F. Müller,
1774) (Fig. 3a). The place of the first discovery is in the
mouth section of the Sulak River (November 2019;
43.25936° N, 47.54503° E). The shells are thin-walled,
symmetrical; the periostracum is olive yellow; the
hypostoracum is violet-purple, with purple rings
before and after the mantle line. The shape of the shell
is oval-triangular. The crown is small, central, gray
marsh color. Dimensions of the largest copy: L =
15.3 m, H = 13.0 mm, vp2 = 10.0 mm. The number of
ribs is 14 per 10 mm, L/H = 1.15, vp2/L = 0.65; the
maximum age is 3 years.
A/R morphotype of C. fluminea (Fig. 3b). The place
of the first find was noted in the Yuzbash-Sulak Col-
lector (November 2021; 43.24313° N, 47.21293° E).
Lad = 26.2 mm, H = 25.3 mm, vp2 = 20.1, and maxi-
mum age is 4 years. The shells are thick-walled,
round-triangular, high; the outer layer is yellow-olive
or gray-olive, shiny, with distinct protruding ribs, in
the amount of 10–13 per 10 mm of the shell surface;
hypothoracum bright purple, L/H = 1.0–1.2, vp2/L =
0.67–0.79 (n = 26). A/R morphotype of C. fluminea
discovered by us in the Prisulak Canal (November
2021; 43.19365° N, 47.20305° E), dark olive shells
Lad = 23.1 mm, H = 22.5 mm, vp2/L = 16.0 mm; the
maximum age of the mollusks is 3 years.
DISCUSSION
The study of the species composition and distribu-
tion of freshwater bivalve mollusks in the water bodies
of Dagestan, as well as the analysis of published data
from 1960 to 2015, showed that here 15 species of
bivalve mollusks live (Israpov et al., 2008; Abdulme-
jidov et al., 2017) but on representatives of the genus
Corbicula before 2016 (Nabozhenko, M.V. and
Nabozhenko, S.V., 2016), there was no information.
Taxonomic status. In this work, we studied the
morphotypes of Corbicula fluminea and C. fluminalis
identified by us in the water bodies of the Dagestan
coast of the Caspian Sea from 2017 to 2021. It is known
that the genus Corbicula includes about 14 species.
The taxonomy of the genus is complicated by the fact
that clear species characteristics have not been estab-
lished, and the presence of hermaphroditic invasive
lines makes it difficult to distinguish between individ-
ual morphotypes.
Along the borders of native ranges, the western
Corbicula C. fluminalis and eastern Corbicula C. flu-
minea were distinguished (Zhadin, 1952; Son, 2007),
noting that the first species belongs to typical estuarine
species, and the second one belongs to freshwater spe-
cies, but despite this, their ranges often overlap, as
studies in Europe and America show (McMahon,
1982; Renard et al., 2000). In the Dagestan region of
the Caspian Sea basin, in the Prisulak Canal, we also
observe that these euryhaline species live together.
Therefore, it is currently impractical to distinguish
species on the basis of the degree of halophilicity of
these inhabitants of the littoral zone.
The taxonomic status of Corbicula is complex and
often changes; researchers in their discussions did not
come to a consensus on the number of species or on
the origin of certain invasive lines recorded in America
and Europe. Thus, some authors suggest that species
such as C. leana, C. manilensis, C. consobrina, and
C. africana belong to the subspecies C. fluminea, while
C. japonica belongs to the group C. fluminalis (Kam-
burska et al., 2013 Komaru et al., 2013). It should also
be mentioned that, when comparing the morphology
and ecology of Corbicula species, some researchers
suggest that C. fluminalis, C. japonica, and C. sandai
are the same species or three related species, as they
are often difficult to distinguish (Morton, 1987; Kor-
niushin, 2004; Skuza et al., 2009).
It is known that sympatric populations of C. flumi-
nalis and C. fluminea exist in Europe; they may have
the same habitat requirements (Karatayev et al.,
2007). So, in Poland in the lower part of the Oder
River and in the Italian Lake Garda, C. fluminalis
meets with C. fluminea (Paunović et al., 2007; Skuza
et al., 2009). Three taxa of the genus Corbicula were
found in French rivers; however, the data of morpho-
metric and molecular biological methods did not
match (Renard et al., 2000). Often, the same haplo-
type is found in mollusks, but they differ in shell color
and other morphological features, which indicates a
different origin of the populations. Cases with a pre-
dominance of the alien haplotype are known in many
invasive Corbicula populations (Pigneur et al., 2014).
A similar situation was observed with the morphotypes
of Corbicula in the Northern Dvina River, in the White
Sea basin. “The first line belonged to the Rlc morpho-
type in terms of shell morphology, but has a haplotype
that is usually registered in the R morphotype, and the
second line shows the shell morphology of the R mor-
photype, but the haplotype that is usually found in
individuals of the S morphotype” (Bespalaya et al.,
2018, 2021). Undoubtedly, in-depth genetic studies
will shed light on the existence of a variety of Corbicula
morphotypes.
Invasive populations seem to consist exclusively of
asexual clonal lines, which makes it difficult to give
them a taxonomic status (Renard et al., 2000; Mares-
caux et al., 2010; Komaru et al., 2013; Tiemann et al.,
2017; Bespalaya et al., 2018, 2021; Voroshilova et al.,
2020). Thus, the issue of the diversity of Corbicula
morphotypes is problematic in scientific terms, which
requires careful study. Nevertheless, the use of mor-
phological and allometric parameters also makes it
possible to determine the morphotypes of Corbicula
with a high degree of probability.
Our study of the morphometric parameters of the
shells showed that the found Corbicula belong to dif-

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INVASIONS OF NEW MORPHOTYPES 257
ferent morphotypes, with well-distinguishable con-
chological features. As can be seen from Figs. 2 and 3
and the description of morphotypes, they clearly differ
in the outer and inner color of the shells, the number
of ribs on the surface, and the coefficients of elonga-
tion and convexity.
Corbicula morphotypes C and S are known in
North and South America and in Europe. “Shells of
C. fluminalis have thinner ridges and a purple inner
surface, while C. fluminea has coarser ridges with a
pale inner surface” (Kamburska et al., 2013). The
main species differences of shells of C. fluminalis and
C. fluminea are the nature and number of concentric
ribs of the shell surface: in the first species from 13 to
28, in the second from 7 to 14 ribs per 10 mm. Thus,
mollusks from the Volga and Don river basins had pro-
truding ribs with wide intervals, no more than 10–
12 per 1 cm of the shell surface (Zhivoglyadova and
Revkov, 2018; Voroshilova et al., 2020).
In our research, C. fluminea from two samples has
from 10 to 14 convex large ribs on the surface of the
shells. Shells from the Yuzbash-Sulak Reservoir (n =
26) and the Prisulak Canal (n = 13) correspond to the
earlier described C. fluminea found in the Don and
Volga rivers. The absence of underyearlings and large
individuals in the samples may indicate a recent intro-
duction of mollusks.
The mollusks studied by us corresponded to the
A/R morphotype, which prevails in most samples of
C. flaminea from Europe and North and South Amer-
ica (Morton, 1987; Renard et al., 2000).
In samples of the A/R morphotype of C. fluminea
from the Yuzbash-Sulak Reservoir, size groups of 11–
15 and 16–20 mm predominated, which corresponds
to an age of 1.5–2 years. (Fig. 4a). The A/R morpho-
type sample from the Prisulak Canal is dominated by
individuals with a shell length of 16–20 mm, which
corresponds to an age of 2 years (Fig. 4b). The small
number of Corbicula with L > 26 mm may be
explained by the small population size in the initial
period of introduction.
In the sample of the C/S1 morphotype of C. flumi-
nalis from the Prisulak Canal, size groups with a shell
length of 16–20 mm and 21–25 mm also dominate,
which corresponds to an age of 2–3 years (Fig. 4c).
Thus, taking into account the discovery of mol-
lusks of different ages, we assume that Corbicula pop-
ulations begin to form in the Yuzbash-Sulak Collector
and the Prisulak Canal.
Studies by a number of authors show that genetic
analysis does not always accurately reflect differences
in morphotypes of Corbicula that are quite different in
appearance (Renard et al., 2000; Pigneur et al., 2011;
Tiemann et al., 2017; Bespalaya et al., 2018; Voroshi-
lova et al., 2020). Therefore, an integrated approach is
needed that takes into account the phenotypic, geno-
typic, and biochemical characteristics of the studied
mollusks.
Effect of abiotic conditions on population density
and shell size. Corbicula fluminalis is encountered both
in rivers, lakes, and canals and in saline water bodies
(Zhadin, 1952). We found morphotypes of Corbicula
fluminalis in the saline Lake Kuznechonok and
C. flaminea in freshwater canals and collectors.
According to our results, the morphotype of C. flu-
minea quickly adapted to local environmental condi-
tions, while the morphotypes of C. fluminalis are
found in small numbers despite the fact that their
introduction occurred earlier.
Fig. 4. Dimensional structures of samples, November
2021. (a) Morphotype A/R of Corbicula fluminea from the
Yuzbash-Sulak Collector; (b) morphotype A/R of C. flu-
minea from the Prisulak Canal; (c) morphotype C/S of
C. fluminalis from the Prisulak Canal.
60
40
20
0
11–15 16–20
21–25 >25
N, %
L, mm
60
40
20
0
11–15 16–20
21–25 >25
N, %
L, mm
40
20
10
30
0
11–15
6–10
16–20 21–25 >25
N, %
L, mm
2
25 34
23
46
31
28
11
42.3 38.5
15.4
3.8
(a)
(b)
(c)

258
RUSSIAN JOURNAL OF BIOLOGICAL INVASIONS Vol. 14 No. 2 2023
KHLOPKOVA et al.
An important factor is the type of substrate. Any
type of soil is suitable for the habitation of Corbicula—
from silty mud to small pebbles and rubble (Yavnov
and Rakov, 2002; Skuza et al., 2009). It should be
noted that, in our study, the population density of
mollusks was higher up to 25 ind./m2 for rheophilic
A/R morphotype of C. fluminea and C/S morphotype
of C. fluminalis, living in the Prisulak Canal sympatri-
cally, at a depth of 30–80 cm, on the ground with fine
gravel. We assume that the active circulation of water
in the channel saturates the sediments with oxygen,
which contributes to the rapid growth of the abun-
dance of mollusks.
In the low-flowing Yuzbash-Sulak Reservoir on
dense silty to fine sandy soil, at a depth of 40–90 cm,
the population density of C. fluminea was about
12 in d./m 2. For comparison, the density of Corbicula
in Lake Maggiore is 87–1249 ind./m2 (Kamburska
et al., 2013). A number of authors write that the den-
sity of C. fluminea usually exceeds 1000 ind./m2
(McMahon, 1982; Dulenina and Dulenin, 2009;
Scuza et al., 2009). The nature of the soil as an indica-
tor of the habitat affects the density and structure of
the population. So, in Kubyakinsky Bank in the north-
ern part of the Agrakhan Bay and in dense silty ground
and on soft silty to fine sandy deposits in
Lake Kuznechonok, the density of the C. fluminalis
morphotype С/S1 was 2–3 ind./m2, and in the Prisu-
laksky canal on dense sandy soil, the density of
C. fluminalis morphotype С/S was 6–8 ind./m2. In
Lake Kuznechonok, we found mollusks buried in the
ground at a depth of 12–15 cm. It is known that they
can burrow from 8 to 45 cm (Yavnov and Rakov,
2002). Mollusks are tolerant to hypoxia; they can tol-
erate the drying up of a water body for a short time.
С/S1 and C/Rlc lived in the saline Lake
Kuznechonok and in Kubyakinsky Bank sympatri-
cally. Morphotype C. fluminalis С/S1 is similar to
C. japonica, which is the most famous of the five spe-
cies of the genus living in the Far East.
The maximum dimensions shells of Corbicula
found by us in the samples in the Yuzbash-Sulak Res-
ervoir L = 26; in the Agrakhan Bay, L = 27.2 mm.
Large Corbicula mollusks have been found in the Bul-
garian Danube, L = 40.79 mm. Large specimens of
C. fluminea have been recorded in the United States,
L = 50–65 mm (McMahon 1982; Hubenov et al.,
2013). C. japonica reaches in the south of Primorye
L = 62 mm, while in the estuary of the Amur River,
individuals with L > 30 cm are rarely encountered
(Dulenina and Dulenin, 2009). Corbicula mollusks
from different habitats differ in growth characteristics,
which follows from a comparison of our and published
data. Mollusks grow at different rates according to the
variability of environmental factors in a given habitat.
The maximum sizes are achieved under conditions
close to optimal in temperature, as a result of high
growth rates. The water temperature in the channel
and collector in November 2021 generally met the
environmental requirements of Corbicula.
The question of the ways of penetration of the stud-
ied morphotypes of C. fluminalis and C. fluminea to
reservoirs and canals of the Dagestan coast of the Cas-
pian Sea is open. River navigation is considered to be
the main route for the spread of invaders (primarily by
changing the ballast water), as well as along with the
introduction of Asian fish species. The distribution of
Corbicula can be natural (transport with the current),
carried by birds, and be associated with recreational
activities: transporting sand and gravel and stocking
fish in reservoirs. The passive upstream movement as
a possible mechanism of propagation of C. fluminea
was also reported in Elba River in the Czech Republic,
in the Rhine River in Switzerland, and in streams in
the United States (McMahon, 1982; Hubenov et al.,
2013; Kamburska et al., 2013; Crespo et al., 2015).
However, the Prisulak Canal and the Yuzbash-Sulak
Collector are not navigable, so we can assume that the
most probable propagation vectors of C. fluminalis
and C. fluminea are an upstream movement, and the
transport of sand and gravel is not excluded. And in
the drying Lake Kuznechonok, transport by migratory
birds is possible.
Considering reports of finds of C. f luminea by dif-
ferent author s from 2015 to 2019 in the river systems of
the Don and Volga and that, as a rule, C. f luminalis
penetrates with this species, we assume that the pene-
tration of the A/R and B/Rlc morphotypes of C. flu-
minea and the C/S, C/S1, and R/C morphotypes of
C. fluminalis occurs namely along the northern inva-
sion corridor through the Volga-Don Canal.
The impact of new invaders on the ecosystem is
ambiguous. Corbicula, like all bivalve mollusks, have a
positive effect on the ecosystem of the water body, as
they are biofilters, they are used as bioindicators of
pollutants, and larvae and juveniles are food objects
for fish. Failure to respond quickly to infestations of
C. fluminea and C. fluminalis may lead to the displace-
ment of a number of native mollusks and changes in
the structure of communities. Taking into account the
high physiological tolerance of Corbicula, the diversity
of breeding strategies, and the presence of planktonic
larvae, a rapid growth in populations of new invaders
and their penetration into other water bodies of the
Dagestan coast is possible in the near future. The eco-
logical role of morphotypes of C. fluminea and C. flu-
minalis in the functioning of coastal food chains
remains to be seen. Therefore constant monitoring of
the basin of rivers and canals of the Dagestan coast of
the Caspian Sea is necessary for the timely detection
of invasive species.
CONCLUSIONS
(1) In the Dagestan region of the Middle Caspian,
self-settled C. fluminalis and C. f luminea (O.F. Müller,

RUSSIAN JOURNAL OF BIOLOGICAL INVASIONS Vol. 14 No. 2 2023
INVASIONS OF NEW MORPHOTYPES 259
1774) appeared—they are new species of bivalves for
the Russian coast of the Caspian Sea, as well as for the
entire European part of Russia. For the period from
2017 to 2021, in the process of monitoring the Caspian
Sea basin, in particular, in the northern part of the
Agrakhan Bay, the Prisulak Canal, and the Yuzbash-
Sulak Collector, and the mouth of the Sulak River, the
authors identified five morphotypes belonging to two
species of representatives of Corbiculidae.
(2) On the basis of the results of our studies, it is
assumed that the C/S, C/S1, and C/Rlc morphotypes
of C. fluminalis settled about 10 years ago, during
which time a population was formed in the northern
part of the Agrakhan Bay. Mollusks of the A/R mor-
photype of C. fluminea apparently penetrated rela-
tively recently into the Yuzbash-Sulak Collector and
the Prisulak Canal and formed new populations there
in 2–3 years. Mollusks of the C/S and A/R morpho-
types were found in the Prisulak Canal. Single individ-
uals of the B/Rls morphotype were found at the
mouth of the Sulak River. At this point in time, the
latent stage has been passed—the identified morpho-
types of species C. fluminea and C. fluminalis partially
naturalized in the water bodies of Dagestan studied by
us (sometimes their ranges overlap) and can become a
source of further invasion in their new areas.
(3) What will the introduction of new invaders lead
to? At this stage, it is difficult to predict. But consider-
ing previous invasions, it is safe to say that this will not
be reflected in the native species in the best way. The
freshwater clam C. fluminea multiplies and grows very
quickly, increasing its biomass, and fills collectors,
fish-breeding channels, and cooling channels of the
state district power station, completely clogging all
water bodies, getting into new habitat conditions for it,
without meeting competitors.
(4) Of the positive aspects of invasion, it should be
noted that bivalves are filter feeders and accumulate
harmful substances from bottom sediments. It is also
known that the main food object of the Amur sturgeon
Acipenser schrenskii (Brandt, 1869) is C. japonica, a
species related to C. fluminalis; therefore, the appear-
ance of a new food object for sturgeons living in the
Caspian Sea is not excluded.
(5) In our opinion, the most likely source of popu-
lations of morphotypes of C. fluminea and C. f lumina-
lis discovered by the authors in the coastal zone of the
Dagestan coast of the Caspian Sea are mollusks that
have penetrated from the water bodies of the Volga
River basin. The high invasive potential of representa-
tives of Corbiculidae determines the need for further
monitoring of the waters of the Russian sector of the
Caspian Sea and investigation of the influence of these
invaders on autochthonous Caspian species, and the
diversity of morphotypes leads to the need for their
genetic analysis.
ACKNOWLEDGMENTS
We express our gratitude to the inspectors of the Agra-
khansky subordinate reserve of the Dagestan Nature
Reserve, who provided full technical support at the work
sites.
FUNDING
This study was carried out within the framework of topic
no. AAA-A19-119101590064-1: “Resource Potential and
Structure of Commercial Fish Populations, Current Trends
in the Dynamics of Ecosystems in the Dagestan Region of
the Caspian Sea, and Patterns of Their Formation” accord-
ing to the State Assignment of the RF Ministry of Science
and Higher Education to the Caspian Institute of Biological
Resources of the Dagestan Federal Research Center of the
Russian Academy of Sciences, in the laboratory of marine
biology.
COMPLIANCE WITH ETHICAL STANDARDS
Conf lict of interest. The authors declare that they have no
conf licts of interest.
Statement of the welfare of animals. The article does not
contain any studies involving animals in experiments per-
formed by any of the authors.
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