Acartia tonsa, Dana, 1849

The body is elongated, slender.
Female. The abdomen of the female is very short, somewhat shorter than ¼ of cephalothorax that has rounded posterior margins. The abdomen is armed with thin bundles of hairs on the sides of the anal segment. The first antennae reach the middle of the genital segment.The distal segment of the fifth pair of female�s podites has a rounded projection on the inner margin. The apical seta is coarsely serrated at the distal end, it is almost equal in length to the lateral seta. 
Male. The lateral part of the genital segment has bundles of short setae. The caudal branches are asymmetrical, with bundles of setae on the inner margin. The fifth pair of the male�s podites is markedly asymmetrical, uniramous. The third segment of the right podite has a large inner projection. The forth segment is elongated, curved, armed with thin long setae. Basipodite (left) has a large projection.

Intraspecific forms. Unknown
Related forms. Unknown

Distribution of species within the Caspian Sea

The species occurs all over the Caspian Sea.
Status as per International Red Data Book. Not defined
Status as per National Red Data Books. Not defined
First record for the Caspian Sea. Acartia tonsa Dana was first found in the Middle Caspian - in 1982, and in the Northern Caspian - in 1983. It is suggested that this species was earlier erroneously identified for the Caspian Sea as Acartia clausi Giesbrecht.
Redescription of species. Dana, 1848; Giesbrecht, 1892; Esterly, 1924

General characteristics of species

Ecological-taxonomic group. Zooplankton
Origin: Mediterranean invader
World distribution. Acartia tonsa is a widely spread species: Indian Ocean, Malay Archipelago, Cayenne, Atlantic and Pacific coasts of the North and South Americas , Baltic Sea - up to the Gulf of Finland, Black, Azov and Mediterranean Seas. At present this species is present in the plankton of the Caspian Sea (Kurashova, Abdullaeva, 1984).
Habitat. Water column, from the surface up to 200 m depth line.
Migrations. Acartia tonsa performs both horizontal and daily vertical migrations. During the nighttime it goes up to the surface layers while in the morning and daytime mature males and females sink to deeper layers of water. Young individuals undertake migrations only within surface layers (Brodsky, 1950).

Relation to abiotic environmental factors

Relation to salinity. Brackishwater euryhaline species
Acartia tonsa occurs in the Caspian waters at a salinity 1-13^o/_oo. The maximum density in the Northern Caspian (13,500 ind./m³) was recorded at a salinity 7-8^o/_oo).
Relation to temperature. Eurythermic species, which occurs all the year round at a temperature from 0 to 29.5⁰C. Maximum densities (252 000 ind./m³) were recorded in the Caspian Sea near Makhachkala City at a temperature 23.0-29.5⁰C (Kurasheva, Tinenkova, 1988).
Vertical distribution. Eurybathic species
Acartia tonsa colonizes mainly the coastal zone of the Caspian Sea up to 50 m. The largest densities are confined to the depths above 20 m. Its abundance decreases by a factor of 10-20 at depths 50-200 m. Occurs sporadically (2 ind./m³) in the halistatic zone at depths ranging from 200 to 600 m (Tinenkova, Tarassova, Petrenko, 2000).
Relation to oxygen conditions. Oxyphilic species
Low concentrations of dissolved oxygen retard the development of eggs and result in death of all copepodite stages of A. tonsa.
Relation to fluctuations of the sea level. Information is not available

Feeding

Feeding type. Heterotrophic
Feeding behavior. Acartia tonsa and Acartia clausi are prehensile organisms. Raking movements of maxillipaeds I and mandibles are the most important for food capture.. Food is captured at frequent small jumps or smooth sliding. The former type is used when capturing large food organisms (30-100 �), the latter when seizing small organisms (5-30 �) (Petipa, 1959).
Food spectrum.Euryphagous species
Acartia tonsa consumes both vegetable and animal food. It prefers the largest single, spherical or elliptic, algae. Acartia tonsa feeds actively on the algae Chaetoceros socialis, Exuviaella cordata (16x12 �), Gymnodinium sp. (56x39 �), Prorocentrum micans (40x30 �), Sceletonema costatum (1-200�; h � 6-10 �), Nitzschia closterium (30x40 �), Cyclotella caspia (1-15-36 �; h �10 �). It can also consume actively minute
  Flagellata (6-8 �) (Petipa, 1959).
Supply of food. Phytoplankton, zooplankton
Quantitative characteristics of feeding. Digestion lasts for 1-3 hours at 20-25⁰C. Digestion rate is 75-80%, 20-30% in conditions of excessive feeding. Daily food rations in different age groups vary from 6 to 20% of body weight. When using animal food or excess feeding, daily diet increases up to 26% of body weight. Average daily indices of stomach fullness range from 59 to 2, 620⁰/₀₀₀.

Reproduction

Reproduction type. Gamogenesis
Fertilization is internal. A male can fertilize several females. The number of spermatozoa attached to females varies from 1 to 3, and more. After spermatophore has attached, fertilization takes place. Acartia tonsa lays eggs into water. Eggs are heavier than water, therefore they descend to the bottom. Eggs are covered with 2 membranes: the outer is thick, and the inner is extremely elastic. A nauplius larva develops.
Reproduction areas. There are no specific areas of reproduction.
Terms of reproduction. Acartia tonsa reproduces throughout the year. The duration of reproduction period is 75 days. The spawning interval is 5-6 days.  The maximum duration of sexual activity in females is 2.5 months (Sazhina, 1971). 
Fecundity. Acartia tonsa can lay up to 20 eggs in each of 13 batches, fecundity is 260 eggs (Sazhina, 1971).
Limiting factors. Temperature, light factor, feeding, salinity, oxygen concentration (Sazhina, 1987)

Life history and development

Life history stages. The life cycle of Acartia tonsa has a long metamorphosis period (from egg to adult stage). There are twelve stages of development, of which six nauplial and 6 copepodite stages, their sequence is determined by molts. When the embryonic development is concluded, larvae hatch from eggs through the cracks in the thick egg membrane. The whole process of hatching lasts for 0.5-1.5 min. Copepodite stages supercede nauplial period. Duration of the full developmental cycle is 30-33 days.
Relation to envirtonmental factors. The most vigorousbreeding activity occurs at a water temperature 20-22⁰C (Sazhina, 1971).
Age of maturation. Acartia tonsa matures in 33 days after hatching. The life span of females is 70-80 days, 14 days - in males (Sazhina, 1987).
Thermal conditions of development. Latent eggs hatch rapidly at 5⁰C. When the temperature increases from 5 to 23⁰C, 70-100% of the nauplii hatch in 20 days, and 40% - at 25-40⁰C (Sazhina, 1987).
Quantitative characteristics of growth. Information is not available

Structural and functional population characteristics

Sex ratio. The population of Acartia tonsa is represented in the Caspian Sea by all the stages throughout the year. Females and males are distinguished at stages V and VI.
Age-size structure. The sizes  Acartia tonsa in the Caspian, at the first and subsequent copepodite stages are: I - 0.350-0.400, II � 0.425-0.450, III � 0.475-0.525, IV � 0.575-0.600, female V � 0.700-0.800, male V � 0.700-0.725, female VI � 0.900-0.975, male VI � 0.750-0.850 mm (Kurashova, Abdullayeva, 1984).
Quantitative characteristics. During the 1980s-1990s this species was distributed throughout the Caspian Sea. In the Northern Caspian high densities of 1, 400 ind./m³ occurred at the banks Tbilisi and Bolshaya Zhemchuzhnaya. The densities of Acartia tonsa at the Mangyshlak Peninsular and Kulali Island reached 4, 700 ind./m³, while its biomass was 47.4 mg/m³.
In the Middle and Southern Caspian Acartia tonsa occurred in mass quantities at the coastal areas from Agrakhan Kossa to Zhiloy Island. Maximum densities varied from 10, 800 to 15, 200 ind./m³, while the biomass was 0.73mg/m³. In the Southern Caspian the species occurred along the eastern coastline amounting to 4, 700 ind. and 47.4 mg/m³ at the Cheleken Peninsular.
Population trends. During the first years in the Caspian Sea , its population expanded rapidly and became rather abundant. By the late 1980-s the species abundance in the Northern Caspian increased 1.8 times, while in the Southern Caspian its abundance was recorded to be the highest. In 1991-1995, the number of Acartia tonsa in the Northern Caspian decreased by a factor of 2.7 and proved to be lower than the values recorded for the period 1981-1985. In the Middle Caspian, the number of Acartia tonsa was assessed to increase, and by 1991-1995 it reached 3, 900 ind./m³ (Kurashova, Tinenkova, Tarasova, unpubl.)

Abundance trends of A. tonsa in the Caspian Sea, ind./m³
(Kurashova, Tinenkova, Tarasova, origin data)

Interspecific relations

Acartia tonsa is a food competitor of Calanipeda aquae-dulcis. As a food item, Acartia tonsa is included in the diet of young and adult kilka and accounts for 77% of the stomach content of Clupeonella cultriventris caspia and 90% of the stomach content of Cl. engrauliformis (Yelizarenko, 1992).

Impact on the ecosystem

Invasion of Acartia tonsa into the Caspian Sea had a positive impact regarding its position as a fish food item. On the other hand, increase in its abundance was responsible for the stock decline of Calanipeda aquae- dulcis. Similarity of habitat requirements of these two species strained their competition.

Importance of species to bioresources production of the Caspian Sea

Acartia tonsa contributed to the increase in food supply of planktivorous fish.
Economic significance of species. None
Commercial characteristics of species, catches. None
Fishing gears and fishing zones. None

Impact of fisheries on the population status

Human impact/Threats. In view of the invasion of the Caspian Sea by the North American predatory species Mnemiopsis leidyi in 2000, the biomass in the zooplankton including Acartia tonsa has drastically decreased (Polyaninova, Ardabiyeva, Belova et al., 2001)
Conservation measures. Control over Mnemiopsis leidyi.

References

Brodsky, K.A. 1950. Copepods Calanoida in the Far East seas of the USSR and Arctic basin. In: Inventory on the USSR Fauna. USSR AS ZIN. Moacow-Leningrad. Pp. 418-420. (in Russian)
Dolgopolskaya, M.A. 1969. Order Colonoida. In: Inventory on the Fauna of the Black and Azov Seas Vol. 2 pp.34-48 (in Russian) 
Kurashova, E.K. and N.M. Abdullayeva, 1984. Acartia tonsa (Calanoida, Acartiidae)in the Caspian Sea. Zool.J., 63, 6 (in Russian ).
Kurashova, E.K. and V.I. Kuzmicheva, 1991. Relation of individual species and zooplankton comlpexes of the Caspian Sea to water salinity. In: Caspian Sea. Fisheries studies of plankton. P.II. Moscow. Pp. 19-27. (in Russian)
Kurashova, E.K. and D.H. Tinenkova, 1988. Abundance, biomass and distribution of the colonizer Acartia clausi Giesbrecht (Calanoida, Acartiidae) in the Northern and Middle Caspian. Hydrobiol J., 24, 2: 23-27 (in Russian).
Petipa, T.S., 1957. On mean weight of major forms of the zooplankton in the Black Sea. In: Proceedings of the USSR AS Sevastopol Biological Station. Vol.9. Pp. 39-57 (in Russian).
Petipa, T.S., 1959. Feeding of the copepod Acartia clausi Giesbrecht. Proceedings of the USSR AS Sevastopol Biological Station Vol. 11. Pp.72-99 (in Russian)
Polyaninova, A.A., A.G. Ardabiyeva, L.N. Belova et al. 2001. Hydrological characteristics of feeding conditions for commercially important fish in the Caspian Sea. In: Fisheries research in the Caspian. Results of Research Work in 2000 Astrakhan. Pp. 110-125 (in Russian)
Sazhina, L.I., 1987. Reproduction, growth, production of marine copepods. Kiev. Pp. 156 (in Russian)
Sazhina, L.I., 1971. Fecundity of mass pelagic Copepoda of the Black Sea. Zool. J. 4: 586-589 (in Russia)
Sterner A. 1923. Bausteine zur einer Monographie der Copopoden-Gattung Acartia .Zool. J. Bd.I, 5. Innsbruck
Tinenkova, D.H., L.I. Tarassova and E.L Petrenko, 2000. Distribution of the colonizer Acartia tonsa in the Caspian Sea. In: Species-invaders in the European seas of Russia. Proceedings of the Workshop. Murmansk Pp. 89-91. (in Russian)
Yelizarenko, M.M. 1992. Feeding of kilka in the Middle Caspian during the summer season In: Biological resources of the Caspian Sea. Book of Abstracts of the Ist International Conference, September 1992. BIVTS Kaspryba. Astrakhan. Pp.111-113 (in Russian)

Compiled by:

E.K. Kurashova (CaspNIRKH, Astrakhan, Russia)

Acknowledgements:

The author is grateful to Mrs. D.H. Tinenkova and Mrs. L.I. Tarasova for supplying unpublished data on A. tonsa abundance over the period 1990-1995.