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## Abstract

In this study we examined the morphological anomalies of 2023 ovigerous females of the copepod Eurytemora affinis from laboratory and field samples. Field copepod collections were collected from 10 sites in Europe and North America. Laboratory cultured copepods were reared at 6 experimental conditions during several generations. The observation under the microscope of the ovigerous females’ bodies revealed the presence of a ‘Tumor Like Anomaly’ (TLA) on a few individuals, which were manifested on the urosome. The occurrence of TLA was only noticed on copepods from the field samples. This result shows that under controlled laboratory conditions, E. affinis do not manifest any abnormal morphology, even when temperature and salinity conditions are stressful, which also demonstrates the high plasticity of this copepod species and its capacity to live at a wide range of environmental parameters. In this study, we also confirmed that E. affinis is capable of producing eggs with a wide range of sizes, ranging from very small and abnormal eggs of 75.79 μm only, to very large eggs, reaching a maximal diameter of 128.71 μm. The observed large eggs are abnormal compared to the average size of the other eggs produced within the same clutch. Other abnormalities on eggs, such as deformed eggs or eggs infested by fungi, are only observed from field samples at a few sampling sites. In the Seine estuary, several abnormalities in eggs and adult females were observed during February 2010. The critical period for E. affinis’ development is sampling site-specific and needs to be accurately studied in the future to better explain the causes of the abnormalities described in this work, and their effects at population level.

In: Studies on Eurytemora

## Abstract

We studied the morphology of Eurytemora from inland waters at the shores of the White and Pechora seas and from the Lena River delta, and revealed a ubiquitous presence of Eurytemora gracilicauda Akatova, which results we confirmed with genetic data. We found this species for the first time in the Pechora Sea basin. In the White Sea basin, this species was previously described as E. brodskyi Kos, with the name that we suggest is a junior synonym of E. gracilicauda. E. gracilicauda differs from the co-living species: E. lacustris (Poppe), E. arctica Wilson M. S. & Tash, and E. raboti Richard, by the structure and armament of the caudal rami and the fifth legs (P5) of males and females. The caudal rami of both males and females were elongated. The female caudal rami showed a surface covered by spines. The male caudal rami were bare, or with rare spines on the sides. The appendages of the female P5 were also elongated: the length of the inner spine on the distal exopod segment was 2.27 ± 0.12 times as long as the outer spine; the exopod of the male right P5 had a specific trigonal ledge with a short spinule, and the coxopods (both or at least one) had groups of spinules. The morphometric parameters of the females were quite stable, while those of the male showed high variability within and between populations (CV = 11.5-43.5%). Similarities and differences of E. gracilicauda and three allochoric Eurytemora species were analysed, and the results presented herein.

In: Studies on Eurytemora

## Abstract

Anthropogenic activities can cause important changes in aquatic ecosystems, such as warming due to climate change, nutrient loading from agricultural runoff and urban areas, and decreased concentrations of oxygen in bottom waters. These changes may lead to impacts on both organism performance and ecosystem functionality. Studying planktonic species that form an aquatic ecosystem’s foundation is an important step towards understanding the entire food web and predicting how it may respond to a changing environment. One important planktonic species in the Laurentian Great Lakes is the invasive calanoid copepod Eurytemora carolleeae (formerly considered part of the Eurytemora affinis species complex). This study analyzes the metabolic activity of E. carolleeae from Green Bay, Lake Michigan, U.S.A. using two different methods, over a range of temperatures from 9 to 26°C. Total oxygen consumption was measured directly using a micropulse oxygen probe system, and the activity of aerobic metabolic enzymes in the electron transport system (ETS) was quantified using in vitro reduction of iodonitrotetrazolium chloride (INT). Respiration rate of E. carolleeae increases approximately linearly from 9 to 26°C. Measurements of ETS activity indicate that the copepod’s metabolic enzymes have an Arrhenius activation energy of 46.5 ± 15.6 kJ/mol with a thermal maximum between 22 and 26°C. Overall, E. carolleeae ETS rates increased by approximately 7% per °C over the range 9 to 22°C. This thermal limit has implications for future performance of this species, as the combination of higher temperatures and disappearance of oxygenated colder-water refuges may limit E. carolleeae’s success in the Green Bay system following warmer climate and increased nutrient conditions.

In: Studies on Eurytemora

## Abstract

The goal of this study was to evaluate and compare the short-term performances and the physiological plasticity of two cryptic species Eurytemora carolleeae and Eurytemora affinis (North-Atlantic clade) by simulating rapid advection from freshwater to brackish water conditions and reciprocally. To do so, two reciprocal transplant experiments without acclimation and under non-limited food condition were performed in the St. Lawrence estuarine transition zone during summer 2011. Results revealed that both species diverged in their short-term acclimation response when facing acute salinity changes that they might encounter when advected through the highly dynamic estuarine transition zone. We show that E. carolleeae could use the brackish environment without loss of performance and energy, while E. affinis needed to reallocate energy from other processes (i.e., reproduction) and required food intake to maintain itself in the freshwater environment. In addition, the transplant experiment highlighted that only 40% of the E. affinis showed short term capacity to acclimate to freshwater conditions, indicating that in situ advection by currents from brackish water to fresh water could be dramatic even for a short time period. Furthermore, the survivors of E. affinis in fresh water might not be able to reproduce, which limits establishment of a sustainable population of E. affinis (North-Atlantic clade) in the tidal freshwater part of the estuarine transition zone. Finally, we highlighted for the first time that both species of this pseudocryptic complex could use lipid remodelling to overcome temperature effects on membrane structure, but further studies are needed to determine the influence of membrane lipid remodelling on salinity tolerance.

In: Studies on Eurytemora
Author: V. I. Lazareva

## Abstract

The zooplankton composition of the Volga, Kama and Don rivers was studied in the summers of 2015-2018. The copepod Eurytemora caspica Sukhikh & Alekseev, 2013, recently isolated from the group of species contained in Eurytemora affinis, represents a widespread (50-100% of samples) and relatively abundant (maximum abundance up to 18 × 103 ind./m3) species. It is found in the Lower and Middle Volga from the city of Astrakhan to the mouth of the Kama River, in the Kama River to the upper reaches of the Kama reservoir, in the lower and middle sections of the Don River within the Tsimlyansk reservoir, as well as in the reservoirs of the Volga-Don shipping canal. In the Kama River, E. caspica was discovered by the author for the first time in 2016. In other habitats, it was previously identified as Eurytemora affinis (Poppe, 1880), although this particular species was not found in any of the analysed samples. The distribution features and habitat conditions of Eurytemora caspica in the rivers of the basins of the Caspian and Azov seas are discussed.

In: Studies on Eurytemora
Author: Elena G. Krupa

## Abstract

The purpose of this work is to analyse the influence of external factors on quantitative variables of Eurytemora affinis in water bodies of Kazakhstan. In 1997-2018, more than 150 water bodies located in various regions of the country were surveyed. The results obtained and analysis of the published literature showed that E. affinis primarily inhabits small shallow water bodies of Central and Eastern Kazakhstan. Populations from the North-Eastern Caspian Sea, originally identified as “Eurytemora affinis”, could be attributed to the recently described species Eurytemora caspica. In the water bodies examined, E. affinis was found at water temperatures from 12 to 25°C and at salinities from 0.2 to 7.1 g/dm3. The highest abundance of Eurytemora was recorded at 21-22°C and a salinity of 1.0-3.0 g/dm3. The optimum temperature range in the water bodies examined in an extreme continental climate is higher than the values given for E. affinis in water bodies in a moderately temperate climate. The optimal TDS [Total Dissolved Solids] value (1.0-3.0 g/dm3) has shifted towards more truly fresh waters than was stated for E. affinis collected from the sea (6.2-8.2‰), and this was confirmed in experiments (from 3.0-10.0‰ to 10.0-15.0‰). In the majority of the examined water bodies of Kazakhstan, males dominated the Eurytemora populations. Male dominance in E. affinis populations might be due to the adverse effect of elevated temperature on the lifespan of females, and it was also connected with the level of toxic pollution of the water bodies. It seems that E. affinis endures a relatively high content of heavy metals, but it does not inhabit water bodies with very high toxic pollution. The revealed features of E. affinis’ biology in Kazakhstan water bodies might be associated with both the heterogeneity of its populations from different parts of the range, and the existence of a complex of closely related species that at present is collectively named “Eurytemora affinis”.

In: Studies on Eurytemora

## Abstract

The Taiwan Strait, located between Taiwan Island and the southeast of the mainland of China, is the main passageway connecting the East China Sea and the South China Sea. The particular coastline of the mainland created several semi-enclosed embayments along the west coast of the Taiwan Strait. Runoffs from land bring large amounts of nutrients into the bays, which made these bays important natural spawning and breeding grounds for several economically important marine organisms. In order to reveal the effects of the China Coastal Current (CCC) on the zooplankton communities in Fuqing Bay in northeastern Fujian, zooplankton samples were collected at 12 stations in early March 2014. The average surface seawater temperature was 12.53 ± 0.14°C, and salinity was 28.33 ± 0.21 PSU in the investigation area during the research period. In total, 23 identified copepod species and in addition several unidentified benthic harpacticoid copepods were recorded with an average abundance of 77.44 ± 60.07 ind. m−3. In the present study, the most dominant group consisted of juveniles (copepodites) with an average density of 59.97 ± 51.49 ind. m−3, which was followed by Calanus sinicus Brodsky, 1965 with an average density of 5.04 ± 4.95 ind. m−3. The occurrence rate of Calanus sinicus was 91.67% in our study, which indicates that the research area was controlled by the CCC water mass. So, we concluded that the CCC played an important role in transporting cold water copepod species from the Bohai Sea and the Yellow Sea to the western Taiwan Strait. A noteworthy discovery in our samples was Eurytemora pacifica Sato, 1913, which was recorded for the first time in waters of the western Taiwan Strait with an occurrence rate of 33.33% and an average density of 0.81 ± 1.91 ind. m−3. Eurytemora pacifica was first reported at Yantai harbour and is widely distributed in waters of the northern Pacific Ocean. The co-occurrence of Calanus sinicus and Eurytemora pacifica in our research area possibly indicates that E. pacifica could be used as a bioindicator for the directional movement of the CCC. The high occurrence of this species in our research area indicated that the CCC affects the community structure of copepods in the western Taiwan Strait in early spring.

In: Studies on Eurytemora

## Abstract

The present paper reports the results of two surveys conducted in the Gulf of Riga (Baltic Sea) during 1997. Elemental composition (carbon and nitrogen) and diurnal vertical migration (DVM) throughout development of the calanoid copepod Eurytemora affinis (Poppe, 1880) were studied. This is a species with a high genetic and morphological heterogeneity, inhabiting a seasonally stratified, herring (Clupea harengus Linnaeus, 1758) rich ecosystem. The DVM of copepods is a widespread advantageous behaviour, although with several trade-offs. At the expense of time spent in the food-richer surface layer, copepods avoid visual predators (planktivorous fish) by descending to greater depths, thereby decreasing their mortality. Being a part of the food web, copepods themselves serve as a source of nutrients with specific elemental contents. In the present study, we observed differences both in DVM and elemental content between the developmental stages of the studied species. The majority (57-89%) of nauplii and 1st to 3rd copepodids were found near the thermocline during daytime, whereas at night they were detected above the thermocline. The 5th stage copepodids and adults displayed evident nocturnal ascent, when most (>90%) individuals moved above 15 m depth. During daytime, they tended to disperse throughout the water column. Carbon (C%) and nitrogen (N%) in dry weight of E. affinis increased with ontogenetic development, reaching the highest values in 4th to 5th copepodids and adults. Adult males showed higher values of C% and N%, as well as an increased C : N molar ratio, compared to adult non-ovigerous females. The amount of time spent at the food-richer surface layer appeared to be related to N% in E. affinis.

In: Studies on Eurytemora

## Abstract

Epischura baikalensis Sars is a dominant pelagic species of the Lake Baikal zooplankton. We empirically determined the duration of the development of Epischura from field data, as it is not possible to determine this experimentally. Based on our data, we conclude that the central pelagic food web of metazoan species in Lake Baikal has not two, as was previously accepted, but three generations during a year and each lasts not 6 but instead 3 to 4 months. The life cycle of the species also possibly includes a 3-months long winter embryonic diapause, starting in November and ending in late February with the start of the under-ice algae bloom. The high nauplii density in spring is not the result of high adult density, suggesting the presence of resting eggs in the E. baikalensis life cycle. We also found a large morphological difference between the pelagic eggs of the species in the winter and in the summertime. The possible participation of diatom algal toxins in the regulation of the seasonal dynamics of their main consumer in Lake Baikal, similar to those known in marine ecosystems, is discussed. The population dynamics in a year with relatively low diatom algae density show four peaks of reproduction, i.e., in February, June, September, and December. In years with significant (more than 15 000 cells/l) diatom blooming we found mass mortality of E. baikalensis juveniles in the plankton, with only two clear peaks of nauplii density. A negative correlation was found between the number of population density peaks for E. baikalensis and the spring-blooming of diatom algae.

In: Studies on Eurytemora

## Abstract

The South China Sea is the largest marginal sea in the world. The northern South China Sea has a complex hydrographic system affected by interplay waters of the Kuroshio Branch Current and the South China Sea waters. To understand the changes in the community structure of the planktonic calanoid copepod family Temoridae in relation to the interplay waters, we collected zooplankton samples in the northern South China Sea from surface waters (0 to 2 m) from November 2001 to January 2007 during 24 research cruises using a standard North Pacific zooplankton net (with a mesh size of 200 μm, a length of 1.8 m, and a mouth diameter of 0.45 m). Among a total of 253 samples, 5 species belonging to 3 genera of Temoridae were identified: Eurytemora pacifica Sato, 1913, Temora discaudata Giesbrecht, 1889, Temora stylifera (Dana, 1849), Temora turbinata (Dana, 1849), and Temoropia mayumbaensis Scott T., 1894. The 3 most abundant species were: T. turbinata (relative abundance, RA: 53.28%; mean density ± standard deviation, MD: 29.33 ± 89.53 ind. m−3), T. stylifera (RA: 36.02%, MD: 19.83 ± 145.22 ind. m−3), and T. discaudata (RA: 7.70%, MD: 4.24 ± 11.82 ind. m−3). The 3 most frequently occurring species were: T. turbinata (occurrence ratio, OR: 52.17%), T. discaudata (OR: 35.18%), and T. stylifera (OR: 9.88%). A noteworthy discovery in our samples was E. pacifica, a marine and brackish water species, collected from 3 stations located near the Kuroshio Branch Current in April 2003. This is the only month when this species was recorded during the investigation period. Variations of dominant species of Temoridae show a clear pattern of seasonal succession. The total abundance was significantly higher in the third quarter (September-November, 116.98 ± 314.49 ind. m−3) than in the fourth quarter (December-February, 24.26 ± 47.72 ind. m−3) ($\mathit{p}=0.017$, one-way ANOVA). In general, the present results demonstrate that the assemblages of the Temoridae are very much structured by the water masses of the Kuroshio Current and the South China Sea.

In: Studies on Eurytemora