Eurytemora grimmi, (Sars, 1897)

Taxonomic description of species

The body of Eurytemora grimmi like in all Calanoida consists of three parts: cephalon, thorax and abdomen or cauda. The cephalon is formed by the fusion of six segments, the thorax consists of five segments. The cephalon and thorax form a cephalothorax. The abdomen is much more narrow than the thorax and consists of segments (three in females, five in males except for a caudal branch) the first of which ends in a fork or caudal branches that carry five densely plumose caudal setae (Field Inventory of Plankton. USSR AS ZIN. 1979).
The cephalothorax carries 11 pairs of articulated appendages on the abdomen: I antennae, II antennae, mandibles, I maxillae, II maxillae, maxillopods and five pairs of swimming legs arranged according to five thoracic segments. The fifth pair of legs is somewhat modified. The structure of swimming legs is a very important taxonomic feature for identification not only of genera (pairs I-IV), but also of species (pair V). The distinctive feature of the genus Eurytemora is a monoarticulate structure of the endopodite of pair I of swimming legs, the other three pairs are biarticulate. Both legs of the fifth pair of the male lack endopodites. The second segment of the endopodite of the left leg is markedly widened at its end. Furcal branches are significantly longer than the last segment. The abdomen has 5 setae (Rylov, 1930; Atlas of invertebrates of the Caspian Sea, 1968). Heart is present.. Organs of sight are represented by an unpaired nauplial eye.
The species identification should be based on adults. Cephalothorax of the female is thick, shortened. Its length is about 2-2.5 times its maximum breadth in the area of the first segment. The posterior segment of cephalothorax has no palms, with outer rounded posterior angles. The length of the abdomen is about 5 times the breadth of the posterior abdominal segment. The genital segment is symmetrical, widened in the anterior part.
Caudal branches are long and narrow with a row of hairs on inner margins. The length of branches is equal to the total length of two posterior abdominal segments. Caudal setae are 5 in number, they are long, plumose.
Anterior antennae (1) are somewhat shorter than cephalothorax. They consist of 25 joints.
The swimming legs are slender, somewhat lengthened (especially Exp.III). Unlike other species, E. grimmi has no seta on the inner margin of the first basal segment of all the swimming legs. Endopodite of the first pair of legs is armed with six setae, pairs II-IV with 8 or 9 setae. At that, the first segment of the endopodite of three pairs of legs carries three setae while the second (distal) one has 5 or 6 setae.
The fifth pair of legs. The second basal segment is lengthened. Its length is 1.25-1.3 times its breadth. The first segment of the expodite is also somewhat lengthened with the inner thin, long, sharpened projection turned backward. The posterior margin of the projection is armed with a number of very small spines (in the distal part of the margin). The first segment of exopodite is armed with rather a short thin spine located nearly in the middle of the outer margin. The second segment of exopodite is small, about half as long as the previous one. It is armed at the end with two spines common to the genus: a short one on the outer side of the segment and a long thin one on it inner side.
Like in females, the first basal segment of all the swimming legs of males have no setae.
The fifth pair of males� legs is asymmetrical. The second basal segment of the right leg is elongated with evenly convex inner margin. The first articulation of the exopodite is narrow, slender, unarmed. The second articulation of the exopodite is longer than the first one. It is incorrectly curved at an obtuse angle, gradually constricted and armed with a pair of setae � in the anterior part of the inner margin and in the middle of the outer one.
The second basal segment of the left leg is almost right-angled somewhat shorter than the same segment of the right leg. The first articulation of the exopodite is longer than the second articulation of the basals which expand in the posterior part with almost a straight inner and curved outer margin carrying a short straight spine. The paliform expanded end of the second segment of the exopodite has two acuminate processes armed with small spines and hairs. Its outer margin carries rather a thick spine, the inner margin has a shorter and thinner spine (Rylov, 1930). The female is 1.4-1.7 mm in length (without caudal setae), males range in length from 1.33 to 1.4 mm (Sars, 1897; Rylov, 1930; Bening, 1938; Kuzmicheva et al., 1980).
Intraspecific forms. None
Related forms. Eurytemora minor (G.O. Sars) endemic of the Caspian Sea (1.10-1.50 mm in length) occurs everywhere. Adult individuals differ from E. grimmi females in the proportion of the first articulation of the exopodite of the fifth pair of legs and from E. grimmi males in the second segment of the right leg. When analyzing large samples, they are not distinguished and treated together with E. velox (Lilljeborg), E. affinis (Poppe), E. lacustris (Poppe) that also inhabit the Caspian Sea, E. lacustris occurs only in freshened areas, E. velox and E. affinis are found both in freshened and brackish waters. All three species occur almost everywhere from Scandinavian countries to southern seas � the Caspian, Azov (Rylov, 1930).

Distribution of species within the Caspian Sea

The abundant species of the Middle Caspian occurs in the southern part of the Northern Caspian and deep water zone of the Southern Caspian (Bening, 1936; Kun, 1965; Kuzmicheva, 1991).
Status as per International Red Data Book. Not defined
Status as per National Red Data Books. Not defined
First record for the Caspian Sea. The late 18th century, Sars, 1897
Redescription of species. Rylov, 1930; Kun, 1968. Atlas of Caspian Invertebrates.

General characteristics of species

Ecological-taxonomic group. Zooplankton
Origin. Caspian autochthon
World distribution. Ponto-Caspian endemic (Sovinsky, 1904).
Habitat. Water column of the Caspian Sea
Migrations. Eurytemora grimmi performs vertical migrations during all the seasons of the year (Bogorov, 1939; Kudelina, 1952, 1958; Badalov, 1965, 1967, 1974; Lipker, 1972). It goes to upper layers of water (mainly 25-10; 10-0 m) from the depth of 200 m at night. The speed of vertical migrations depends on the size of individuals and may be calculated based on the formulae:

where V is the speed of migration, mm/s
L is the total length of specimens, mm (Kuzmicheva, 2000)

Relation to abiotic environmental factors

Relation to salinity. Euryhaline marine species
The number and biomass increase as water salinity increases.
Relation to temperature. Eurythermic, cold-loving
The species occurs in all areas of the Middle and Southern Caspian throughout the year.
Vertical distribution. Eurybathic species
It occurs from upper layers to maximum depths.

Eurytemora grimmi vertical distribution in the Middle Caspian (Divichi-Kenderli transect)
in August 1991 (thou. ind./m²) (original data by D.H. Tinenkova)

Relation to oxygen conditons. The direct relationship between the biomass of Eurytemora grimmi and oxygen content was recorded in the Middle and Southern Caspian at the depth more than 200m (Caspian Sea, Vol.VI, 1996)
Relation to fluctuations of the sea level. The biomass of E.grimmi decreases as sea water level rises due to desalination of sea water.

Feeding

Feeding type. Heterotrophic
Feeding behavior. Filter feeders
Food spectrum. The major food organism is Exuviaella cordata of the genus Peridinium, second in importance are species of the genera Gymnodinium, Peridinium, Prorocentrum and Glenodinium and diatoms Cyclotella sp. Sometimes the finest detritus, probably of organic origin, may be found in the intestine (Bondarenko, 1974; Bening, 1938; Kuzmicheva, 1973; Yablonskaya, 1971).
Supply of food. Phytoplankton 5-30 � in size (Bondarenko, 1974).
Quantitative characteristics of feeding. The degree of intestine filling varies during a day and may depend on the size of the specimen and density of phytoplankton. Females and copepodites at stage V feed most actively. The males� intestine is filled only half or even ¼ as greatly as females� one. The amount of food in intestines diminishes as the size of specimens decreases (Kuzmicheva, 1973)..

Reproduction

Reproduction type. Gamogenesis
Reproduction areas. There are no specific areas of reproduction
Terms of reproduction. The reproduction of Eurytemora grimmi takes place all the year round, most actively during the winter season. Reproduction begins at a water temperature 6-6.3⁰C and reaches the peak at 8-8.3⁰C (Bening, 1938; Kun, 1968. Atlas of Caspian Invertebrates, 1968; Kuzmicheva, 1991).
Fecundity. The number of eggs in each laying may reach 85 at an average diameter of the egg 70-74� (Bening, 1938).
Limiting factors. Salinity, water temperature particularly in winter, trophic factor (Kuzmicheva, 1991).

Life history and development

Life-history stages. Eurytemora grimmi is dioecious. Females carry an unpaired sack with eggs. Males have spermatophores that become attached to the abdomen of females. They undergo twelve stages of development: six nauplial and six copepodite ones. The alternation of stages occurs due to molts following each stage and contributing to the growth of individuals.
Relation to environmental factors. It reproduces all round the year, but most intensively during the winter season.
Age of maturity. Information is not available
Thermal conditions of development. The summer biomass of E.grimmi in the Middle Caspian increases about 1.5-fold after severe winters as compared to temperate and mild winters (Caspian Sea. Fauna and biological productivity. 1985).
Quantitative characteristics of growth. During all the seasons, an increase in the relative size of furca is recorded from stage I (38-41%) to stage VI (56-62% of the females, 60-67% of the males). Simultaneously as copepodites grow, the relative breadth of the cephalophorax changes: it decreases from stage I to stage IV and increases again from stage V to VI. The following relationships exist between the weight of E. grimmi and its size:
W = aL³ (Kuzmicheva, 1979, 1980, 1985)
a = (0.295 � 0.194L/l + 0.398c/l)²
where
a is a condition factor;
L is the total length to the end of furca branches;
l is the length of cephalothorax;
C is the breadth of cephalothorax, the mean value of dorsoventral and lateral measurements (Kuzmicheva, 1979, 1980, 1985).

Structural and functional population characteristics

Sex ratio. E. grimmi is dioecious. It is represented in the Caspian Sea by 12 stages of development throughout the year. The sex of the individual may be differentiated at stage V. In winter during intensive breeding the number of sexually mature females exceeds that of males. Egg-bearing females account for 50%. In summer the female-male ratio is equal to 1:1.

Relations between Eurytemora grimmi individuals at various stages of development in the Middle Caspian (Divichi-Kenderli transect) in February and August 1990 (original data by D.H. Tinenkova).

Age-size structure. The variation coefficient of mean sizes of younger copepodites is equal to 5-7%, that of specimens at stage V is 7-8%, that of males at stage VI is 7-9%, of females is 7-11%. During all seasons an increase in the size of Eurytemora grimmi is recorded from shallow to deep water zones of the sea. E. grimmi specimens are larger in the western part of the sea than in the eastern part.

Average size of Eurytemora grimmi in the Middle Caspian during various seasons, mm
(Kuzmicheva, 1980)

Quantitative characteristics. The biomass of E. grimmi in the Caspian Sea varies widely from 0.1 to more than 200mg/m³. Eurytemora is not abundant in the shallow water zone (less than 50 m) along the western and eastern coastlines, its biomass here rarely exceeds 5 mg/m³. In the zone of circular current the biomass reaches a maximum (up to 247 mg/m³ in 1991). The areas of the largest density of E.grimmi are steep slopes near Divichi City and Derbent Depression (Original data by D.Kh.Tinenkova).

Population trends. E. grimmi abundance trends in the Northern Caspian Sea are not quite clear. In the Middle Caspian the biomass decreased between 1971 and 1993 while in the Southern Caspian Eurytemora population has expanded since 1988.

Interspecific relations

E.grimmi is a food competitor of other phytophagous species, mainly Calanipeda aquaedulcis, Eurytemora minor, Acartia clausi, Limnocalanus grimaldii. E. grimmi is a favorable food organism of young fish during the period of their conversion to active feeding and adult planktivorous fish due to its high nutritive value and availability (Gadjieva, 1978). 

Impact on the ecosystem

None

Importance of species to bioresources production of the Caspian Sea

Economic significance of species. None
Commercial characteristics of species, catches. None

Impact of fisheries on the population status

None
Human impact/Threats. In 2000 because of Mnemiopsis leidyi invasion of the Caspian Sea, the number of E. grimmi in the Middle Caspian was reduced to 1/10 and it was not found in the Southern Caspian (original data by Tinenkova, Petrenko).
Conservation measures. Different measures for Mnemiopsis leidyi control including biological control over its population.

References

Badalov, F.G. 1978. Daily vertical migrations of Eurytemora grimmi G.O. Sars in the Southern Caspian. P.p. 38-55. In: Biology of the Caspian Sea under new environmental conditions. Baku. AzSSR AS (in Russian).
Bening, A.L. 1938. On winter plankton of the Caspian Sea. In: Proceedings of the broad studies of the Caspian Sea. Moscow-Leningrad. USSR AS, 5:7-97. (in Russian).
Bening, A.L. 1938. The major feeds in the pelagic zone of the Caspian Sea. J. Nature, 9:33-36 (in Russian).
Bogorov, V.G. 1939. . Daily vertical migrations of Eurytemora grimmi in the Caspian Sea. P.p. 383-394. In: Collection of articles devoted to the research career of the honorary member of the Academy N.M. Knipovich. Moscow. USSR AS (in Russian).
Bondarenko, M.V. 1974. Feeding of mass copepods of the Caspian Sea. In: Broad investigations of the Caspian Sea. 4:197-201. Moscow University (in Russian).
Caspian Sea. Hydrochemical conditions and oceanographic principles of the development of biological productivity. 1996. Vol.VI, 2:254-263. St.Petersburg. Hydrometeoizdat
Kudelina, E.N. 1952. Daily vertical migrations of zooplanktonin the Middle Caspian. In: VNIRO Proceedings. 1:100-103. (in Russian).
Kudelina, E.N. 1958. Biomass distribution and daily vertical migrations of zooplankton in the Middle and Southern Caspian based on the data obtained in 1954. P.p. 12-13. In: Annotations of research papers. Baku (in Russian)
Kuzmicheva, V.I. 2000. The speed of vertical migrations of Eurytemora grimmi in the Caspian Sea. P.p. 54-61. In: Marine hydrobiological studies. Moscow. VNIRO (in Russian).
Kuzmicheva, V.I., T.A. Kortunova, E.Yu. Rostokina. 1980. Seasonal changes in the size and weight of Eurytemora grimmi in the Caspian Sea. In: VNIRO Proceedings. 133: 48-64.. Moscow (in Russian).
Kuzmicheva, V.I. 1973. Diet of copepods in the Caspian Sea. P.p. 92-94. In: Biological resources of the Caspian Sea. Book of Abstracts (in Russian)
Kuzmicheva, V.I. 1991. Ecology of the main species of zooplankton of the Caspian Sea. In: Fisheries studies of the plankton. Part II: 129-147 Caspian Sea. Moscow (in Russian).
Kuzmicheva, V.I. 1979. Methods for determination of individual weight of copepods. Oceanography, Vol. 19, 6:34-45 (in Russian).
Kuzmicheva, V.I. Determination of individual weight of copepods based on the body proportions. Oceanography .Vol. XXV, 5:867-871 (in Russian).
Sars, G.O. 1897. Pelagic Entomostraca of the Caspian Sea. In: Year Book of the Zoological Museum of the Imperial Academy of Sciences, II:1-73.
Sovinsky, V.K. 1904. Introduction into the study of the fauna of the Ponto-Caspian-Aral Sea basin considered from the point of view of an independent zoogeographical province. In: Notes of the Kievsky Society of Naturalists. Vol. XVIII, 6:1-487 (in Russian).

Compiled by:

D.H. Tinenkova (Caspian Fisheries Research Institute, Astrakhan, Russia)

Acknowledgements:

The author is grateful to E.K. Kurashova and L.I.Tarassova for furnishing unpublished data on E. grimmi abundance in the Northern Caspian, T.A. Kortunova for supplying unpublished data on the Middle Caspian during the period 1973-1976, to E.L. Petrenko for data on the Southern Caspian for 1990-1992 and assistance in preparing illustrations.