Exuviaella cordata

Taxonomic description of species

Cells on the shell side are heart-shaped or rounded, nearly spherical, from elongated-elliptical to elliptical or rounded-quadrangular in longitudinal section, 6.5-20 � long, 5-16 � wide, 3-10 � thick. The anterior end of the cell is rounded or nearly flat with a shallow cavity in the middle. The shell is thick, structural, finely porous or warty. Very small porous canals are arranged in rows and are quite noticeable on the margin of the valve (13-19 pores 10 � each). Warts are arranged in noticeable transversal rows or are randomly scattered along the margin of the shell or all over its surface. The chromatophore is isolated or consists of several golden-brownish lamellae with a pyrenoid. The nucleus is in the center of the cell hardly noticeable.
Intraspecific forms. Exuviaella cordata var. aralensis Kisselew differs from the species in length (it is shorter) and in length-width ratio which is almost equal to 1. The length is 17-20 �, width is 17-19 �. The Aral Sea.
Related forms. E. asymmetrica Wislouch (1924) occurs not far from Sax Lake in the Crimea. E. baltica Lohmann (1908) is a typical specimen of the surface plankton, usually occurs in the Barents, Black, Baltic and North Seas, in Skagerrak and Kattegat, Kiel Bay. E. caspica Kisselew (1940) occurs in the bays of the Caspian Sea: Komsomolets (Dead Shallow Bay) and Kaidak. E. cassubica Woloszynska lives in the littoral plankton of the Baltic Sea along with Peridinium balticum and purple bacteria. E. compressa Ostenfeld occurs in the Atlantic Ocean, Black, Japan and Mediterranean Seas. E. marina var. marina Cienk occurs in the Southern Caspian, in the Krasnovodsk Bay at Ogurchinsky Island, at the eastern coast of the Middle Caspian and in Kara-Bogaz Gol, according to I.A. Kisselev, in the north-eastern part of the Northern Caspian. Ex. marina var. lima Schiller is widely distributed in the littoral plankton of seas, can live in sand in the tidal zone as well as in saline continental water bodies (the Caspian, Aral, Japan Seas). E. ostenfeldvi Schiller occurs in the Aral Sea, E. perforata in the Black, North, Baltic (Skagerrak) and Japan Seas, E. pusilla Schiller in the Black and Adriatic Seas, E. vaginila (Stein) Schütt in the Black and Mediterranean Seas, Atlantic Ocean.

Distribution of species within the Caspian Sea

This is a most abundant species in the Caspian plankton. It inhabits the entire sea from the surface layer to the depth of 75-100 m at a salinity 1.4-13 PSU and temperature 6-26⁰C.
Status as per International Red Data Book. Not defined
Status as per National Red Data Books. Not defined
First Record for the Caspian Sea. C. Ostenfeld, 1901.
Redescription of species. Genkel, 1909; Makarova, 1961

General characteristics of species

Ecological-taxonomic group. Phytoplankton
Origin. Caspian autochthon
World distribution. Neritic, brackish water, euryhaline and eurythermic species
It is probably an endemic to southern seas (the Caspian, Azov and Black Seas).
Habitat. E. cordata inhabits the water column from the surface layer to the depth of 75-100 m.
Migrations. Daily vertical migrations of the species are known. Proliferation of cells occurs mainly during the daytime, while a decline in cell abundance was recorded during the nighttime (Morozova-Vodyanitskaya, 1954).

Relation to abiotic environmental factors

Relation to salinity. Brackishwater euryhaline species
It inhabits the Caspian waters at a salinity 1.4-13 PSU.
Relation to temperature. Eurythermic species
It vegetates on a mass scale in the Middle and Southern Caspian throughout a year and in the zone of increased salinity of the Northern Caspian during the period of the maximum warming up of water (Kuzmicheva, Levshakova, 1996).
Vertical distribution. Eurybathic species
It occurs in good vegetative state throughout the water column from surface to the depth of 100 m. Live cells of E. cordata were sampled at a depth of 500 m (unpubl.)
Relation to oxygen conditions. E. cordata prefers waters with a large content of dissolved oxygen (Kisselev, 1950)
Relation to fluctuations of the sea level. When the sea level dropped and salinization of its northern part occurred, the size and biomass of E. cordata population were estimated to increase. In the Middle and Southern Caspian with a more stable hydrological regime, the effect of sea level fluctuations on E. cordata development was different.

Quantitative characteristics of Exuviaella cordata development in the Caspian Sea at different
(unpublished data.)

Feeding

Feeding type. Holophytic (autotrophic)
Feeding behavior. E. cordata utilizes solar energy and mineral substances in the process of photosynthesis.
Food spectrum. Biogenic elements/ nutrients
Supply of food. Information is not available
Quantitative characteristics of feeding. Information is not available

Reproduction

Reproduction type. Longitudinal division of cells in in active or resting state with subsequent regeneration of a missing half of the shell.
Reproduction areas. There are no specific areas of reproduction in the Caspian Sea.
Terms of Reproduction. It vegetates on a mass scale in the Middle and Southern Caspian throughout a year and in the zone of increased salinity of the Northern Caspian during the period of the maximum warming up of water. Cell division usually occurs at night, but most stages of division are recorded in the early morning while the regeneration of the shell comes to an end by noon during warm summer months (Kisselev, 1950). The day light is used by vegetative cells for photosynthesis whereas the nighttime is used for reproduction. Regenerating halves of a cell require a lot of light and usually stay in top layers of water in contrast to other individuals that are not involved in division.
Fecundity. Information is not available
Limiting factors. Temperature, salinity, availability of biogenic elements in water.

Life history and development

Life history stages. Information is not available
Relation to environmental factors. The species is variable both in the form of cells and structure of the shell and is affected by water salinity. Cells are larger in size (16-20 μ long and 13.2-16.5 μ wide) in areas with more saline water (12-13.3 PSU). If the salinity is lower (1.2-7 PSU), cells are smaller: 6.5-16.5 μ in length, 5-13 μ in width.
Age of maturity. Information is not available
Thermal conditions of development. Warm water alga
Quantitative characteristics of growth. Information is not available

Structural and functional population characteristics

Sex ratio. Information is not available.
Age- size structure. Information is not available
Quantitative characteristics. In the 1980s-1990s, the most favourable conditions for the development of the alga were recorded in the Northern Caspian in the zone of mixing Northern and Middle Caspian waters. Exuviaella cordata was most abundant in the area of Chechen and Tyuleny Islands, not far from Makhachkala and Bautino Cities. The biomass during the years under consideration varied from 100 to 1, 000 mg/m³. E. cordata was found in small quantities everywhere in the Middle and Southern Caspian. Its biomass was less than 50 mg/m³.

Population trends. Exuviaella cordata dominated the phytoplankton (33-58% of the total biomass) before 1935. Its biomass reached 6 g/m³ at a density of 100-130 million cells per m³. In the 1970s the species occurred in vast amounts reaching 90-96% of the total quantity of phytoplankton. During the 1980s-1990s the proportion of the alga in phytoplankton was estimated to decrease. It did not exceed 40% while the biomass was only 2% of that total.

Interspecific relations

Exuviaella cordata is of great importance for feeding plankton and benthic invertebrates as well as for fish. That alga serves as food for bivalved mollusks Hypanis angusticostata, Didacna trigonoides, Cardium edule (Babayev, 1965) and lower crustaceans (Petipa, 1968). Invertebrates predation on E. cordata is responsible for fluctuation in its quantities (Morozova-Vodyanitskaya, 1954).
The most abundant species of the Caspian phytoplankton are the pyrrophyte E. cordata and diatom Rhizosolenia calcar-avis. Exuviaella cordata is an autochthon of the Caspian Sea while Rhizosolenia calcar-avis was introduced into the sea not long ago.
Exuviaella cordata exceeds Rhizosolenia calcar-avis in the number of cells, but its biomass is considerably lower as its cells are very small (15-18 μ as compared to 600-1000 μ).
Locations and timing of maximum densities of Exuviaella cordata and Rhizosolenia calcar-avis do not coincide. The time of maximum density of Exuviaella cordata is recorded in summer and autumn while that of Rhizosolenia calcar-avis is observed in spring and summer.
Maximum densities of E. cordata (100-130 million cells per m³) occurred at a salinity 9-12 PSU while those of Rhizosolenia calcar-avis (about 60 million cells per m³) at a salinity 6-7 PSU. Therefore, Rhizosolenia calcar-avis can not limit the development of E. cordata or displace it as it was suggested (Levshakova, 1972).

Impact on the ecosystem

E. cordata is an autochthon of the Caspian Sea. Along with other peridinium algae and chemosynthetic bacteria, it supplies the animal population of the water body with necessary organic matter. E. cordata is of great importance for the turnover of substances in the water body: O₂, CO₂, P, N. It was proved that they are of great significance for the feeding of larvae, fish fry, plankton and benthic invertebrates that serve, in their turn, as food for fish and other commercially exploited animals (Kisselev, 1950).

Importance of species to bioresources production of the Caspian Sea

Economic significance of species. None
Commercial characteristics of species, catches. It is not a commercially important species
Fishing gears and fishing zones. None

Impact of fisheries on the population status

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

References

Babayev, G.B. 1965. Importance of phytoplankton for feeding some benthic invertebrates of the Southern Caspian. P.p. 54-63. In: Hydrobiological and ichthyological studies of the Southern Caspian and internal waters of Azerbaijan. Azer.SSR AS Zool. Institute. Baku.
Genkel, A.G. 1909. Materials on the phytoplankton of the Caspian Sea. Bot. Notes, 27. S.-Petersburg (in Russian).
Kisselev, I.A. 1950. Testaceous Flagellata. USSR AS Press. Moscow-Leningrad. 280 p. (in Russian).
Kuzmicheva, V.I. and V.D. Levshakova, 1996. Impact of ocean conditions on the development of phytoplankton. In: Caspian Sea. Vol. 6. 2: 249-254. Gidrometeoizdat. S.-Petersburg (in Russian).
Levshakova, V.D. 1972. On the most important species of Caspian phytoplankton, their quantitative development and relationships. P.p. 100-101. In: Biological resources of the Caspian Sea. Astrakhan (in Russian).
Makarova, I.V. 1961. On the phytoplankton of the Northern Caspian. Bot. J., 46, 11: 12-25 (in Russian).
Morozova-Vodyanitskaya, N.V. 1954. Phytoplankton of the Black Sea. P. II. In: Proceedings of the Sevastopol Biological Station, 8: 11-100. Moscow-Leningrad. USSR AS Press (in Russian).
Ostenfeld, C.H. 1998. Phytoplankton from the sea around the Faeroes. Bot of the Faeroes, 2.
Petipa, G.S. 1968. Feeding of plankton crustaceans in the Black Sea.P.p. 67-71. In: Biological studies of the Black Sea and its commercial resources. Nauka. Moscow (in Russian).
Proshkina-Lavrenko, A.I. and I.V. Makarova, 1968. Plankton algae of the Caspian Sea. Leningrad. Nauka. 291 p.(in Russian).

Compiled by:

O.V. Terletskaya (Caspian Fisheries Research Institute, Astrakhan, Russia)

Acknowledgements:

The author is grateful to Dr. A.G. Ardabiyeva, and senior research worker T.A. Tatarintseva for providing data on the Northern and Middle Caspian.