Dreissena rostriformis, LogvStar

The subspecies of Dreissena rostriformis, lateral and ventral view:1 - D.r. grimmi Andr. 1890; 2 - D. r. distincta (Andr., 1897); 3 - D.r. compressa Logv. Et Star. 1966; 4 - D.r. pontocaspica (Andr., 1897).

Taxonomic description of species

The shell has no pearl layer, it is sharp in front with an epical crown moved strongly forward, bill-shaped, oval or triangulate with a smooth keel often hardly visible, located in the middle of a valve and always closer to the dorsal rather than ventral margin of the apical part. Cardo has no teeth, ligament is exterior. The pedal part has bissus. (Logvinenko, Starobogatov, 1968).
Intraspecific forms. There are 4 subspecies inhabiting the Caspian Sea. All of them have intermediate forms. 
D. r. distincta (Andr., 1897) (synonyms: Dreissena rostriformis (Desh.) Eich-wald, 1885; Dreissena caspia Issel, 1866; Dreissena rostriformis (Desh.) Grimm, 1876; Dreissena rostriformis var. distincta Andrusov, 1897; Dreissena distincta (Andr.) Kolesnikov, 1950). The shell is oval or bill-shaped (height is 0,42-0,55 of its length), hardly convex (convexity is 0,3-0,4 of its length and 0,68-0,82 of its height). Valves are unequal. The exterior surface of the shell is brown-red, sometimes even white with thin lines of growth. Dorsal and posterior margins form a convex arch. The ventral side is evenly convex. The keel is strongly smoothed, almost invisible. Bissus lacuna is not pronounced. The length is up to 23 mm (Logvinenko, Starobogatov, 1968).
D.r. grimmi Andr. 1890 (synonyms: Dreissena brardi Eichwald, 1885; Dreissena brardi var. caspia Grimm, 1877; Dreissena grimmi Andrusov, 1890, 1897). The shell is comparatively small, strongly convex (convexity is 0,4-0,6 of its length and 0,75-0,96 of its height), sharp in front, widened in the posterior part (the height is 0,53-0,67 of its length), with unequal valves. The exterior surface of the valves is white with sharp and frequent lines of growth. The ventral side is slightly S-shaped. Its keel is more pronounced than that in the previous subspecies. The length is up to 16 mm.
D.r. compressa Logv. Et Star. 1966. The shell is oblong (height is 0,48-0,57 of its length), flat (convexity is 0,28-0,38 of its length and 0,58-0,70 of its height) with slightly unequal valves. The exterior surface is grayish-white covered with thin lines of growth. The dorsal and posterior margins form a smooth arch. The ventral side is evenly and slightly convex, almost straight near the apical crown so that concavity typical of other subspecies is almost invisible. Keel is strongly smoothed. Length is up to 18 mm. 
D.r. pontocaspica (Andr., 1897) (synonyms: Dreissena tschaudae var. ponto-caspica Andrusov, 1897; Dreissena pontocaspica (Andr.) Nalivkin, 1915). The shell has unequal valves, it is very convex (convexity is 0,55-0,70 of its length and 0,88-1,12 of its height), thick-walled. Exterior surface is white, sometimes with a brown pattern, smooth with thin and hardly visible lines of growth. The dorsal and posterior margins form a smooth arch or a rounded obtuse angle. The posterior margin turns smoothly into the lower one. The ventral side is strongly curved. Keel is well-developed at apical crowns, smoothed at the back. It differs from previous subspecies by a strongly curved ventral side and wing-shaped widening of the posterior part of the shell. Length is up to 18 mm. 
The fifth subspecies D.r. bugensis (Andr.) inhabits the Dnieper-Bug estuary, from which it has recently started its expansion into fresh waters. 
Thirteen subspecies (D.r. exigua, D.r. bipartita, D.r. simplex, D.r. rostriformis, D.r. inaequivalvuis, D.r. akmanaica, D.r. colchica, D.r. tortuosa, D.r. utvensis, D.r. tschaudae, D.r. decipiens, D.r. huoti, D.r. latiuscula) are known only from tertiary sediments (Starobogatov, 1994).
Related forms. Two species of the similar subgenus Dreissena: D. elata (Andr.) and D. caspia Eichv. inhabited the Middle and Southern Caspian. They were forced out by Mytilas-ter linneatus and are not encountered now (Logvinenko, 1965). The members of the same subgenus, the subspecies Dreissena polymorpha andrusovi (Andr.), replace Dreissena rostriformis in the Northern Caspian whereas in rivers of the Aral-Ponto-Caspian basin as well as in most European rivers another subspecies Dreissena polymorpha polymorpha (Pall.) is distributed widely (Starobogatov, Andreeva, 1994).

Distribution of species within the Caspian Sea

The major parts of the species range are the Middle and Southern Caspian where the species inhabits depths more than 10 m (near the eastern coast) and 15-20 m (at the western coast). The maximal depth where it was found is about 300 m though it occurs probably at greater depths. Shallow water areas (from 10-25 to 45-50 m) are inhabited by the subspecies D. r. distincta that forms large densities along both coasts of the Middle Caspian and along the steep slope of the Northern Caspian. The subspecies D.g.grimmi is widespread in deeper areas of the Middle Caspian (at depths from 40-50 m to 70-80 m). These two subspecies are replaced at the same depths (from 25-35 m to 70-80 m) in the Southern Caspian by the subspecies D.r. pontocaspica which is not abundant. And finally, depths from 70-80 m to the lowest limit of distribution are inhabited by another rare subspecies, D.r. compressa.
 
Status as per International Red Data Book. Not included.
Status as per National Red Data Books. Not included.
First record for the Caspian. Dreissena rostriformis (Desh.)Eichwald, 1855.
Redescription of species. Andrusov, 1897.

General characteristics of species

Ecological-taxonomic group. Macrozoobenthos.
Origin. Caspian autochthons.
World distribution. Ponto-Caspian endemic species.
Habitat. Not too hard grounds, mostly shell sediments, shell sediments containing soft mud or shell sediments containing silt; water velocity at the bottom is rather high.
Migrations. Organisms are fixed to substrate, almost immobile, spread by pelagic larvae.

Relation to abiotic environmental factors

Fixed species.
Dreissena rostriformis inhabits relatively large depths with rather stable condi-tions, so its distribution does not change considerably. The species forms large densi-ties on shell sediments with soft mud in zones with intensive water movement.
Relation to salinity. Brackishwater stenohaline species occurs at a salinity more than 7-10�. 
Relation to temperature. The species occurs at temperatures from 6.5� to 26�C in August and from 2.3� to 10,6� in October. 
Vertical distribution. Dreissena rostriformis inhabits depths of 10-300 m forming large densities at depths 25-50-60 m, sometimes at 100 m. 
Relation to oxygen conditions. The allied species, Dreissena polymorpha, can survive about 5 days at a tem-perature of 18�C without oxygen though its resistance decreases as temperature rises (Karpevitsh, 1953). It forms the largest densities at an oxygen content in water not less than 5 cm³/L (Ossadchikh, 1988).
Relation to fluctuations of the sea level. Indifferent as it lives at depths more that 10 m.

Feeding

Feeding type. Heterotrophic
Feeding behavior. Fixed sestonophage, filtration of bottom water by organisms, fixed over the sediment surface. 
Food spectrum. Stenophagous species 
Supply of food. Phytoplankton, phytoplankton detritus (Yablonskaya, 1971,1975).

Reproduction

Investigations were not performed. The description is given on the basis of the allied species Dreissena polymorpha (Pall.) (Galperina, Lvova-Kachanova, 1972; Lvova, Makarova, 1994).
Reproduction type. Gamogenesis. 
Reproduction areas. The same as the species range
Terms of reproduction. The development of gonads and spawning take place during the warm season from the end of May to August-September, in winter the growth of sexual cells stops. Spawning is intermittent. As eggs are laid, they grow and mature rapidly. 
Fecundity. It depends on the size of animals, in general may be described as high.
Limiting factors. Available places with favorable conditions for settlement (oxygen, salinity, sediments, currents).

Life history and development

Life-history stages. Fertilized eg hatches into a pelagic larvae, trochophore, that turns into a veliger (rotiger, the size is 50-110 �m). A pediveliger larva may crawl using a pedal and re-tains the ability to float. The final larval stage is veliconch that has the size up to 225 �m. The larval development period depends on water temperature (Lvova et al., 1994 a).
Relation to environmental factors. Larval pelagic stages are the most vulnerable. These larvae are eaten by pil-chard and young herring (Ignatova, Khodkina, 1972), but the impact of various envi-ronmental factors has not been estimated.
Age of maturity. The sexual maturity of the allied species Dreissena polymorpha (Pall.) occurs at a size 5-9 mm reached by the mollusk after 5-6 months of existence as a bottom organism.
Thermal conditions of development. Studies were not conducted
Quantitative characteristics of growth.

Curves of lineal growth of Dreissena polymorpha (Pall.) in different basins. 1 - Tsimlianskoye reservoir; 2 - Uchinskoye reservoir; 3 - Northern Caspian; 4 - Lukomskoye lake (heating); 5 - Lukomskoye lake (control) (Lvova et al., 1994 b).

Structural and functional population characteristics

Sex ratio. Information is not available.
Age- size structure. Average life-span is 3-5 years.
Quantitative characteristics. The highest biomass and quantity are reached by Dreissena rostriformis in the north-eastern part of the Middle Caspian, in the area of the most intensive upwelling. In August 1986 the biomass reached there 640 g/m². It was also rather high along the eastern coast and in the zone of steep slope in the Northern Caspian (50-200 g/m²) while at the western coast of the Middle Caspian and in the Southern Caspian it rarely reached 10 g/m². In August 1998 the biomass in the northern part of the Middle Caspian reached 1600 g/m² (Kochneva, unpubl.).
Population trends. Considerable changes were not recorded, though in 1971 the overall decline in benthic biomass occurred mainly because of Dreissena biomass decrease (Yablonskaya, 1975).

Interspecific relations

The mollusk is used as feed by sturgeons (about 2-3% of the diet, Tarverdieva, 1965, 1982) and gobies. Dreissena larvae are used by planktivorous fish.

Importance of species to bioresources production of the Caspian Sea

Economic significance of species. The species has no economic significance.
Commercial characteristics of species, catches. It is not used commercially.
Fishing gears and fishing zones. None.

Impact of fisheries on the population status

None.
Human impact/Threats. Not defined.
Conservation measures. Not required.

References

Andrusov N.I. 1897. Fossil and living Dressensiidae of Eurasia. In: Proc. of SPb. society of naturalists. Dept. of geology and mineralogy. V. 25 688 p. (in Russian)
Eichwald E. 1855. Zur Naturgeschichte des Kaspischen Meeres. - Nouv. Mem. Soc. Natur. de Moscou. - T. 10 (16). - Pp. 288-823.
Galperina G.E., Lvova-Kachanova A.A. 1972. Some peculiarities of reproduction of Dreissena polymorpha polymorpha (Pall.) and Dreissena polymorpha andrusovi (Andr.). In: Complex investigations of the Caspian Sea. MSU. Vol. 3: 61-73. Moscow (in Russian)
Ignatova V.V., Khodkina I.B. 1972. Feeding of pilchard (Clupeonella delicatula) at the western coast of the Middle Caspian. In: Complex investigations of the Caspian Sea. MSU. Vol. 3: 99-101. Moscow (in Russian).
Karpevitsh A.F. 1953. The relation of bivalve mollusks of the Northern Caspian and Aral to salinity changes. P.p.21-305. In: Diss. of Doctor of Biol. Sc. M.: VNIRO, 1998 (in Russian). 
Logvinenko B.M. 1965. On changes in the fauna of Caspian mollusks of the genus Dreissena after invasion of Mytilaster linneatus (Gmel). Sc. rep. of higher school. Biol. sciences, N 4: 14-19. 
Logvinenko B.M., Starobogatov Ya.I. 1968. Phylum mollusks Mollusca. P.p. 308-385. In: The Atlas of invertebrates of the Caspian Sea. Pischevaya promyschlennost. Moscow (in Russian). 
Lvova A.A., Makarova G.E. 1994. Gametogenesis, reproductive cycle. P.p. 138-148. In: Dreissena Dreissena polymorpha (Pall.) (Bivalvia, Dreissenidae). (Species of Russian fauna and adjoining countries). Nauka. Moscow (in Russian).
Lvova A.A., Makarova G.E., Karataev A.Yu., Kirpitshtnko M.Ya. 1994 �. Plankton larvae. P.p. 149-155. In: Dreissena Dreissena polymorpha (Pall.) (Bivalvia, Dreissenidae). (The fauna species of Russia and adjoining countries). Nauka. Moscow (in Russian). 
Lvova A.A., Makarova G.E., Alimov A.F., Karataev A.Yu. 1994 �. Growth and production. P.p. 156-179. In: Dreissena Dreissena polymorpha (Pall.) (Bivalvia, Dreissenidae). (The fauna species of Russia and adjoining countries). Nauka. Moscow (in Russian).
Ossadchikh, V.F. 1967. Seasonal dynamics of the northern Caspian bivalved mollusks. CaspNIRKh Proceedings Vol. 23: 80-90 (in Russian).
Ossadchikh, V.F. 1988. Long-term dynamics of the quantitative development of Dreissena polymorpha (Pall.) in the Northern Caspian. P.p.22-42. In: Quantitative and qualitative distribution of benthos. Food supply of benthos-feeding fish. VNIRO. Moscow (in Russian).
Romanova N.N. 1960. Benthos distribution in the Middle and Southern Caspian. Zoological journal. V. 39, 6: 811-825.
Romanova N.N. 1985. Zoobenthos.P.p. 120-167. In: Caspian Sea. Fauna and biological productivity. Nauka. Moscow (in Russian). 
Starobogatov Ya.I. 1994. Systematics and paleontology. P.p. 18-46. In: Dreissena Dreissena polymorpha (Pall.) (Bivalvia, Dreissenidae). (The fauna species of Russia and adjoining countries). Nauka. Moscow (in Russian). 
Starobogatov Ya.I., Andreeva S.I. 1994. Range and its history. P.p. 47-55. In: Dreissena Dreissena polymorpha (Pall.) (Bivalvia, Dreissenidae). (The fauna species of Russia and adjoining countries). Nauka. Moscow (in Russian). 
Tarverdieva M.I. 1965 The role of acclimatized organisms in the feeding of Russian and stellate sturgeon in the Caspian Sea in 1962. P.p. 234-256. In: Changes in biological complexes of the Caspian Sea during the past few decades. Nauka. Moscow (in Russian). 
Tarverdieva M.I. 1982. The feeding of Russian and stellate sturgeon in the Caspian Sea. In: Feeding and use of food supply by sturgeons of the Caspian Sea. VINITI. 375 (82): 6-164. Moscow (in Russian). 
Verneuil E., Deshayes G.P. 1838. Memoire geologique sur la Crimee; observations sur les fossiles de cette peninsule. // Mem. Soc. geol. France. T. 3, pt 1. P. 1-69.
Yablonskaya �.�. 1971. Feeding of benyjic invertebrates and the trophic structure of benthos in the Caspian, Azov and Aral Seas. Moscow. 146p. (in Russian). 
Yablonskaya �.�. 1976. Investigation of trophic relations between bottom communities in southern seas. Resources of biosphere: (The results of soviet investigations on the International biological program). Vol. 2: 117-144. Nauka. Leningrad (in Russian). 
Yablonskaya �.�. 1975. Annual changes in biomass of different trophical groups of benthos in the Northern Caspian. VNIRO Proceedings. V. 108: 50-64 (in Russian).

Compiled by:

M.G. Karpinsky, Caspian Fisheries Research Institute, Astrakhan, Russia