Abra ovata, (Phill.)

The shell is thin, fragile, translucent, equally valved, rounded-triangular (the height is equal to 0.62-0.82 of the length), flattened (the convexity is equal to 0.38-0.5 of the length and 0.5-0.7 of the height). The anterior margin is rounded, the posterior one is taper. The apexes are very narrow, less pronounced. The surface is covered with thin lines of accretion. The cardo of the right valve consists of two small cardinal and two lateral teeth. In the left valve there is a small cardinal tooth. The length is not more than 25 mm.
Intraspecific forms. In the Sea of Azov A. ovata forms variations distinguished from each other by the thickness of the shell and size. Individuals inhabiting shell rock and silt �shell rock develop a more massive and thick shell and reach 25 mm in length. Individuals living in silt rarely reach 20 mm and have a very thin translucent shell. Individuals that occur in the shell rock are supposed to be ecological varieties and identified by N.N. Chugunov (1926) as A. ovata f.crassa (Vorobyev, 1949).
Related forms. Information is not available.

Distribution of species within the Caspian Sea

After 1955 the mollusk disseminated throughout the Northern Caspian and since 1958 the range limits of A. ovata have hardly changed. The range covers the entire zone of the steep slope of Bolshaya Zhemchuzhnaya Bank. South-west of Kulaly Island and on the border of the Middle Caspian the biomass ranges between 100 and 400 g/m². The eastern part of the Northern Caspian is inhabited by A. ovata only in certain years. In 1976 and 1999 it occupied rather a vast area of Uralsky Deep Trench where the biomass exceeded 100 g/m² (Osadchikh, 1985, unpubl.).
By the end of 1959 the mollusk became established at the western coast of the Middle and Southern Caspian and in 1962 A. ovata range covered the entire inshore area of the central and southern parts of the sea. In the mid-1960s A. ovata dominated the benthic fauna of Dagestan inshore area and formed the basis of the benthic fauna of the eastern inshore area of the Southern Caspian in the Gorgan Bay reaching 600 g/m² with the density of 2500-3000 ind./m² (Aligadjiyev, 1963, 1964; Epshtein, 1964; Vladimirskaya, 1973; Romanova, 1977).
In the 1990s A. ovata reached a maximum weight of 70-90 g/m² at a depth of 20-50 m east of the Agrakhan Peninsular and north-east of Derbent City. The maximum biomass in the Southern Caspian amounted to 13 g/m² south of Ogurchinsky Island and east of the bank Gryazny Vulkan (Kochneva, unpubl.).
Status as per International Red Data Book. N/A
Status as per National Red Data Books. N/A
First record for the Caspian Sea. 1955 at Kulaly Island (Sayenkova, 1956)
Redescription of species. V.M. Zhadin, 1952 � Freshwater Mollusks of the USSR. Atlas of Invertebrates of the Caspian Sea, 1968, edited by Ya.A. Birstein.

General characteristics of species

Ecological-taxonomic group. Zoobenthos
Origin. Atlantic-Mediterranean
Abra ovata was brought into the Caspian Sea in 1940 in order to improve the food supply of benthic feeding fish. The first batch of 18 000 individuals was stocked in the area of Peshnoy Island and on Guryevskaya Deep Trench. Despite annual surveys, A. ovata was not detected. Therefore, the mollusk was planted again: 44 000 individuals were stocked in 1947 and 43 000 in 1948. They were released in the Tyub-Karagansky Bay and at the southern end of Kulaly Island (Karpevich, 1956; Sayenkova, 1956).
World distribution. Abra ovata inhabits the Atlantic Ocean at the coasts of England, the North, Mediterranean, Marmara, Adriatic, Black and Azov Seas (Vorobyev, 1949).
Biotope. Abra ovata is a representative of the infauna. They bury themselves in the ground to 5 cm.

The mollusk leads a mobile life, can bury itself completely in the ground leaving its very gaunt siphons exposed. A. ovata occurs in almost all bottoms, but prefers soft grounds covered with silt or sand and silt where a fraction of small organisms, less than 1 mm in length, accounts for 50% (Vorobyev, 1949; Stark, 1951; Aligadjiyev, 1963; Romanova, 1963; Osadchikh, 1965).
Migrations. Information is not available.

Relation to abiotic environmental factors

Relation to salinity. Marine euryhaline species
Abra ovata tolerates salinity variations from 3 to 30^o/oo, but optimum salinity is 9-11^o/_oo (Vorobyev, 1949). It occurs in the Northern Caspian at salinity between 2-3 and 13^o/oo (Osadchikh, 1965). According to L.A. Zenkevich (1947), its saline optimum lies within 9-12^o/_oo. Experimental findings showed that the most favorable conditions for A. ovata in the Sea of Azov are found within the range of 10-25^o/oo (Karpevich, 1955). From our data, the highest density is recorded at a salinity of 8-10^o/_oo.
Relation to temperature. Eurythermic mollusk tolerates low temperatures in winter and a rise in temperature to 28-30⁰C in summer.
Vertical distribution. Stenobathic-coastal organism occurs in all seas at a depth not more than 30-40 m. It colonizes bottom areas from 1 m to 13 m in the Sea of Azov. The largest densities were recorded at a depth of 8-9 m. The maximum frequency of occurrence is recorded at depths 10-11 m.
Abra ovata colonizes bottom areas at a depth from 3 to 25-30 m, occurs more often above 6-7 m. The highest densities are noted in areas as deep as 8-10 m.
Relation to oxygen conditions. Euryoxybiont organism tolerant of hypoxia can survive 5-8 days in anaerobic conditions. A.ovata is rather tolerant of hydrogen sulfide and ammonia contained in water, but die during long suffocation periods in summer (Vorobyev, 1949). It is noted on a massive scale in the Northern Caspian in the areas with oxygen content ranging at the bottom from less than 1 to 5 ml/l. According to L.I. Yakubovich and E.I. Malm (1930), A. ovata can survive without oxygen 96 and even 168 hours at a temperature 18⁰C and only 10 hours at 25-26⁰C (Karpevich, 1962).
Relation to fluctuations of the sea level. In the 1980s-1990s A. ovata biomass varied from 1.5 g/m² in 1994 to 31.8 g/m² in 1987. During the period of sea level rise between 1986 and 1996, the average long-term biomass amounted to 15.6 g/m² as compared to 7.6 g/m² recorded during the period of sea lowering between 1997 and 1999. Large volumes of spring flood favored an increase in the biomass of A. ovata in the early 1990-s (Smirnova, unpubl.).

Annual changes in Abra ovata biomass in the north-western area of the Caspian Sea, g/m²
(Smirnova, unpubl.)

Feeding

Feeding types. Heterotrophic
Feeding behavior. . Grasps food particles from the surface of the bottom (Romanova, 1963; Yablonskaya, 1955).
Food spectrum. Stenophagous
Supply of food. Abra ovata refers to detrivorous organisms. It uses detritus deposited in the ground and unicellular algae (Vorobyev, 1949; Karpevich, 1962; Yablonskaya, 1976).
Quantitative characteristics of feeding. Information is not available.

Reproduction

Reproduction type. Gamogenesis. A. ovata is dioecious.
Reproduction areas. There are no specific areas of reproduction, they are probably determined upon optimum salinity value (9^o/_oo). V.F. Osadchikh (1965) and G.E. Galperina (1972) suggested that reproduction depended on the temperature of water at the bottom in A. ovata habitats.
Terms of reproduction. Abra ovata reproduces in the Black Sea in May and September (Zernov, 1913), in the Sea of Azov from May to September, most actively in June, July and September (Vorobyev, 1949). Recently settled young A. ovata are recorded in the northern part of the Caspian Sea from April to October while their largest numbers (less than 3 mm in length, occur in April and July (Osadchikh, 1965).

Abra ovata abundance in different size groups in 1958
(according to V.F. Osadchikh, 1965)

Mass reproduction of A. ovata in the Middle Caspian in the area of Khudat-Kilyazi, according to N.N. Romanova (1977), is likely to occur in early spring (March - early April) and in late autumn (November). The time of settling in the central part of the Caspian Sea is strongly influenced by the rise in water temperature at the bottom to 8-11⁰C in spring and its decrease to the same level in autumn. The temperature above 20⁰C delays the settling of A. ovata larvae (Romanova, 1977).
Fecundity. A female can produce several hundred thousand eggs (Vorobyev, 1949).
Limiting factors. Obviously, water salinity is the main factor affecting reproduction. The largest number of A. ovata of younger age groups from 1 to 9 mm in length occur in water salinity 8-10^o/oo (unpubl.).

Life history and development

Life history stages. Development of a pelagic veliger larva. The development of A. ovata includes transformation stage. Freely swimming veliger larvae leave the membranes 56 hours after fertilization. 10 days later, swimming larvae begin to develop an apex of the shell and then settle onto the bottom (Karpevich, 1962).
Relation to environmental factors. The settling of larvae to the bottom occurs immediately after they finish their development in mid-water, regardless of the bottom they are above at the moment. They colonize all grounds more or less evenly. But the density is higher in certain grounds and at a salinity of 8-10^o/oo. The biocenosis of Abra ovata develops in the grounds where small organisms account for more than 50%. The only competitor for habitat is Cerastoderma lamarcki that supercedes A.ovata from the area with good aeration of the bottom.
Age of maturity. A. ovata becomes mature in the Sea of Azov in the third summer of its life cycle and only some individuals mature in the second summer. By that time the mollusk reaches 9 mm in length. There are two mass settlings of larvae recorded in A. ovata. In November one-summer olds vary from 1 to 4 mm in length (autumn settling) and from 4-5 to 8 mm (summer settling).
Thermal conditions of development. Young mollusks less than 3 mm in length appear in the Northern Caspian between April and October at a temperature range from 12 to 25⁰C (Galperina 1972).
Quantitative characteristics of growth. In May after they overwinter and become one-year olds, they vary in length from 2-5 to 5-9 mm. In autumn one-year olds reach 8-9 and 9-13 mm. Two-year olds are 9-13 and 13-15 mm in spring and 13-15 and 15 �17 mm in autumn. Three-year olds are 15-17 and 17-20 mm in spring and more than 17-20 mm in autumn.
The maximum length of A. ovata (20-21 mm) in the Northern Caspian is less than that in the Sea of Azov (24-25 mm).

Structural and functional population characteristics

Sex ratio. Information is not available.
Age-size structure. The population of Abra ovata is based on sexually mature two-three-year olds 7-9 and 9-11 mm in length and young mollusks not more than 3 mm which vary in number from 1 100 to 26 400 individuals.

Abra ovata abundance in different size groups in the Northern Caspian in July
(L.V. Smirnova, unpubl.)

Note: *data on August
Quantitative characteristics. The biomass and abundance of Abra ovata increased steadily between 1955 and 1958 and was noted to stabilize during 1959-1961. The proportion of A. ovata varied from 15.5 to 35.5% of the Northern Caspian benthic fauna depending on year. In 1987 there was recorded the maximum biomass of Abra ovata in the north-western part of the Caspian Sea � 31.8 g/m² or 235 ind./m².
In 1999 because of large biomass in the eastern part of the Northern Caspian, benthophages fed mostly on Abra ovata as a result its biomass decreased by a factor of 5 in 2000 (Smirnova, unpubl.)

Average number and biomass of Abra ovata depending on year and season
(Osadchikh, 1965)

Due to a large biomass in the eastern part of the Northern Caspian in 1999, benthic feeders used mainly A. ovata. As a result, its biomass was reduced to 1/5 (L.V. Smirnova, unpubl.).

Population trends. In preceding years a trend of reduction in population number was outlined.

Interspecific relations

Quantitative changes in Abra ovata development occur because of the trophic impact of benthophagous fish. The thin shell and a large amount of flesh in relation to the total weight of the mollusk considerably increases the food value of A. ovata as compared to other mollusks. It is one of the favorite food organisms of all benthic-feeding fish. The greatest intake of A. ovata in the Caspian and Azov Seas is recorded in Russian sturgeon and bream. A. ovata amounted to 12.6%- 74.0% of the Russian sturgeon diet in 1992 and 1999, respectively.

Importance of species to bioresources production of the Caspian Sea

Economic significance of species. The successful acclimatization of A. ovata contributed to the food supply of benthophagous fish in the Caspian Sea (Romanov, Osadchikh, 1965). Due to its food value, high abundance and large biomass, A. ovata became the main food item in sturgeon diet (Karzinkin, Makhmudov, 1968; Tarverdiyeva, 1965; Polyaninova, Molodtsova, et al., 2000). As for protein and fat content, A. ovata takes third place after Adacna and plankton. This mollusk contains 13% of the protein, 1.14% of the fat, 71.54% of the ash (Bokova, 1946).
The intake of A. ovata by fish in the Caspian Sea is very high. In the late 1990s when the importance of Caspian endemic species Hypanis angusticostata (Borcea), Hypanis vitrea (Eichwald, 1838), Dreissena polymorpha andrusovi (Andrusov, 1897) diminished, Abra ovata became very significant for sturgeons and all benthic feeders (Polyaninova et al., 2000).
Commercial characteristics of species, catches. Information is not available.
Fishing gears and fishing zones. Information is not available.

Impact of fisheries on the population status

Information is not available.
Human impact/Threats. A sharp decrease in Abra ovata abundance may result from oil pollution.
Conservation measures. None.

References

Aligadzhiyev. G.A. 1963. Invasion of Dagestan waters of the Caspian Sea by Syndesmya ovata. DAS of USSR. V.149, 3 (in Russian).
Aligadzhiyev, G.A. 1964. On acclimatization of the Azov-Black Sea fauna in the Caspian Sea. Zool. J., 43, 3 (in Russian).
Atlas of invertebrates of the Caspian Sea. 1968. Moscow. Food Industry (in Russian).
Bokova, E.N. 1946. Food value of benthos in the Northern Caspian. Zool.J. XXV, 6: 523-528 (in Russia).
Chugunov,N.L. 1926. Preliminary results of studies on the production capacity of the Sea of Azov. Proceedings of the Azov-Black Sea research commercial expedition, 1 (in Russian).
Epshtein, B.M. 1964. Benthic fauna and fish feeding in the Kura-Caspian area of the sea. P.p. 99-166. In: Proceedings of AzerbNIRKH, 4. Moscow. Food Industry(in Russian).
Galperina, G.E. 1972. On the problem of reproduction of fish food resources in the Northern Caspian (mollusks). P.p. 37-38. In: Book of Abstracts of TSNIORKH. Astrakhan. Volga Press (in Russian).
Karzinkin, G.S. and A.M. Makhmudov. 1968. Evaluation of the Caspian zoobenthos as fish food resources in terms of chemical parameters. Voprosy Ikht., 8, 2 (49): 325-331 (in Russian).
Karpevich, A.F. and B.G. Polyakova. 1956. Acclimatization of Syndesmya ovata in the Caspian Sea. Ryb. Khoz., 8: 66 (in Russian).
Karpevich, A.F. 1962. Effect of water with an increased content of calcium on the Azov Sea mollusks. VGBO Proceedings, XII (in Russian).
Malm, E.N. and L.I.Yakubova. 1930. Phenomenon of temporary anaerobiosis in some representatives of the benthos in the Black Sea. In: USSR AS Reports. Leningrad (in Russian).
Osadchikh, V.F. 1965. Mollusk Syndesmya ovata (Philippi) in the Northern Caspian. P.p. 35-46. In: KaspNIRO Proceedings, XX (in Russian).
Polyaninova, A.A., A.I. Molodtsova et al. 2000. Feeding conditions for sturgeons in the Northern Caspian during the present period. P.p. 164-175. In: Biodiversity of aquatic ecosystems in the south-eastern part of European Russia. Part 2. Volgograd (in Russian).
Romanova, N.N. 1963. Feeding behavior and benthic invertebrates in the Northern Caspian. VGBO Proceedings (in Russian).
Romanova, N.N. 1977. Seasonal changes in quantitative distribution and some characteristics of A. ovata (Molluska, Bira evia) ecology in the western zone of the middle part of the Caspian Sea. Zool.J. LVI (8): 1150-1160 (in Russian).
Sayenkova, A.K. 1956. New in the Caspian fauna Zool.J. 5 (in Russian).
Stark, I.N. 1951. The status of the food supply of benthic feeding fish in the north-western part of the Sea of Azov. In: AzerNIRO Proceedings, 15. Krymizdat (in Russian).
Tarverdiyeva, M.I. 1977. Importance of acclimatized organisms for Russian sturgeon and stellate sturgeon feeding in the Caspian Sea. In: Changes in biological complexes in the Caspian Sea. Moscow. Nauka (in Russian).
Vorobyev, V.P. 1949. Benthos of the Sea of Azov. In: Proceedings of the Azov-Black Sea Institute of Marine Fisheries and Oceanography, 13 (in Russian).
Vladimirskaya, E.V. Plankton and benthos at the southern coast of the Caspian Sea. In: Proceedings of the Azov-Black Sea Institute of Marine Fisheries and Oceanography, 59 (3):40-47. Moscow. Food Industry (in Russian).
Yablobskaya, E.A. 1976. Studies of trophic relations between benthic communities of southern seas. P.p. 117-144. In: Biosphere Resources, 2. Leningrad. Nauka (in Russian).
Zenkevich, L.A. 1947. Fauna and biological productivity of the sea. II. Sov. Nauka (in Russian).
Zhadin, V.I. 1952. Freshwater mollusk of the USSR. Moscow. USSR AS Press (in Russian).

Compiled by:

L.V. Smirnova (Caspian Fisheries Research Institute, Astrakhan, Russia)

Acknowledgements:

The author is grateful to Dr. Valentina N. Belyaeva (CaspNIRKH) and Dr. Alevtina A. Polyaninova (CaspNIRKH) for valuable comments.