The First Detection of Parasite Ellobiopsis sp. on Calanoids (Crustacea: Copepoda) Inhabiting the Caspian Sea (Central Asia: West Kazakhstan)

Abstract

The data on hosts of Ellobiopsis in Central Asia waterbodies are nearly non-existent. All research in this direction was conducted in other regions (Europe and Brazil). Parasitological studies were carried out in different seasons in the North and Middle Caspian Sea. Twenty-one taxa were registered in zooplankton, and only dominants of community calanoids Acartia (Acanthacartia) tonsa and Calanipeda aquaedulcis were infected with Ellobiopsis sp. Calanoida C. aquaedulcis was reported for the first time as a host for Ellobiopsis. The number of parasites per host was equal to one. The body length of parasites varied from 0.10 to 0.80 mm. The highest infection degree was recorded in C. aquaedulcis (5.71%), and it varied from 2.61% to 3.35% in Acartia. The individuals in the juvenile developmental stages were infected in Calanipeda, while in Acartia, individuals in all developmental stages were vulnerable to infection. The infected calanids had reduced body lengths. The findings suggest the possible influence of Ellobiopsis sp. on quantitative variables of hosts in the Middle Caspian, especially on biomass, by reducing the body sizes of hosts. However, no effect on the abundance and biomass of the host and the structure of the zooplankton of the North Caspian Sea has been detected.

1. Introduction

Calanoida are a group of aquatic invertebrates. They are key components in transferring energy from lower to higher trophic levels [1]. As the main consumers of phytoplankton, they significantly contribute to the control of algal blooms [2]. Furthermore, representatives of the Calanoida family are good biological indicators and can be used to assess water quality [3].

The same as in some marine ecosystems of other regions, Calanoids are the dominant component of zooplankton in the Caspian Sea [4,5,6]. It includes Acartia (Acanthacartia) tonsa Dana and Calanipedia aquaedulcis Krichagin. These latter invaded the Caspian Sea fifty years ago [7] and increased the diversity of calanoids. Calanids are likewise a vital food source for sturgeon larvae and other fish species [8]. Therefore, the appearance of these calanids is of great economic importance since over 90% of sturgeon reserves are concentrated in the Caspian Sea, and the Sea is the habitat of 28 commercial fish species [5]. Considering the enormous importance of these calanids in the Caspian Sea, the spread of any kind of negative influence on them is noteworthy.

Parasites play a significant role in controlling biodiversity and provide a large amount of information about the environmental conditions of their hosts [9]. Representatives of the genus Ellobiopsis are parasites of many planktonic crustaceans but often infect copepods in the marine environment [10,11,12,13,14]. At a high level of infection intensity, these parasites destroy the stability of the species population and lead to their extinction. Various authors conducted studies to identify the negative effects of Ellobiopsis, resulting in the mortality of the host [15,16,17]. In the Indian Ocean, Ellobiopsis formed a mucous mass that completely covered the body of the host calanids, which led to the ceasing of one’s lifecycle [15]. Another case is also known when, due to the attachment of Ellobiopsis to the thoracic segments with the swimming legs of the host, they decreased their swimming speed. Slowly swimming individuals became easy prey for predators and thus reduced their abundance [16,17]. The latter particularly highlights the danger of infection by Ellobiopsis of the calanids of the Caspian Sea, where predators such as Mnemiopsis leidyi A. Agassiz, 1865 [18] and Evadne nordmanni Lovén, 1836 [5] have recently become widespread.

Ellobiopsis are parasitic forms of the group Alveolata [19]. The differential feature of Ellobiopsis is the presence of trophomeres and gonomers, i.e., the body of the parasite consists of one trophomere and several gonomers. The function of the trophomere is to attach to the host body and suck out the nutrient liquid from it. The gonomer is responsible for the process of spore formation. The body shape of the parasites changes during the life cycle. The immature stages of the parasite are pear-shaped, and the mature stages are spherical [20] and cylindrical [14].

The data on hosts of Ellobiopsis in Central Asia waterbodies are nearly non-existent [14]. All research in this direction was conducted in other regions, such as Europe and Brazil [19,21,22,23,24,25,26]. This work partially eliminates this gap and aims to record new hosts for Ellobiopsis in the largest inland waterbody of the world—the Caspian Sea, the prevalence of the parasite in hosts, and the influence of the Ellobiopsis sp. on the hosts.

2. Materials and Methods

2.1. Description of Study Area

The Caspian Sea is the world’s largest inland waterbody (Figure 1). It lies between Europe and Asia. The Sea is divided into the North (25% of the total area), Middle (36% of the total area), and South Caspian (39% of the total area) according to the physical and geographical conditions. The largest tributary, Volga, flows into the sea from the north. The water salinity in the Caspian Sea varies from 0.05‰ near the estuary of the Volga to 11–13‰ in the middle and southeast parts [27]. The smallest depth in the north part of the sea is 8 m, and the greatest one in the middle part of the sea is 788 m. In spring, the water temperature reaches 16–17 °C in the northern part, 13–15 °C in the middle part, and 17–18 °C in the southern part. The water throughout the sea warms up by summer to 24–26 °C, and in the southern regions, it rises to 28 °C. With the cold weather by autumn, the water temperature decreases to 12–13 °C in the middle part and to 16–17 °C in the southern part. In winter, the water temperature throughout the sea diminishes to 0–5 °C.

2.2. Field Sampling

Studies of zooplankton organisms were carried out in the North and Middle parts of the Caspian Sea in different periods of 2024 (spring, summer, autumn, winter) (

Table 1. The station coordinates.

The North Caspian Sea			The Middle Caspian Sea
Station	Latitude	Longitude	Latitude	Longitude
1	45°50′01.407′′ N	50°47′11.214′′ E	43°1′49.19″ N	50°51′13.97″ E
2	45°50′00.946′′ N	50°55′31.655′′ E	42°51′1.19″ N	50°36′19.4″ E
3	45°49′59.699′′ N	51°04′54.617′′ E	42°51′3.1″ N	50°21′38.59″ E
4	45°44′07.502′′ N	50°47′05.269′′ E	43°1′51.2″ N	50°21′46.4″ E
5	45°44′06.480′′ N	51°00′25.979′′ E	43°1′49.19″ N	50°36′29.81″ E
6	45°44′04.990′′ N	51°09′03.852′′ E	43°12′37.3″ N	50°36′40.28″ E
7	45°44′02.776′′ N	51°17′56.462′′ E	43°23′25.3″ N	50°36′49.61″ E
8	45°43′59.852′′ N	51°26′52.625′′ E	43°12′39.31″ N	50°21′54.29″ E
9	45°38′48.391′′ N	51°04′35.280′′ E	43°12′39.31″ N	50°7′8.33″ E
10	45°38′46.497′′ N	51°13′34.383′′ E	43°12′37.51″ N	49°52′22.33″ E
11	45°38′43.673′′ N	51°23′13.737′′ E	42°40′13.12″ N	50°36′9.11″ E
12	45°35′50.774′′ N	50°36′45.688′′ E	42°17′33.61″ N	52°22′55.2″ E
13	45°35′50.666′′ N	50°50′25.038′′ E	42°17′33.61″ N	52°8′20.4″ E
14	45°35′51.039′′ N	50°57′22.716′′ E	42°17′33.61″ N	51°53′45.6″ E
15	45°33′13.299′′ N	50°31′56.199′′ E	42°8′6.36″ N	52°8′20.4″ E
16	45°31′06.398′′ N	50°47′59.492′′ E	41°57′16.99″ N	52°8′20.4″ E
17	45°31′13.683′′ N	50°57′15.175′′ E	42°8′6.36″ N	52°19′42.49″ E
18	45°44′44.640′′ N	50°54′09.960′′ E	41°56′55.97″ N	52°19′42.49″ E
19	45°44′55.657′′ N	50°54′10.780′′ E	41°57′16.99″ N	51°53′45.6″ E
20	45°45′11.858′′ N	50°54′10.752′′ E	41°49′35.83″ N	51°39′14.4″ E
21	45°45′28.059′′ N	50°54′10.724′′ E	41°57′16.99″ N	51°39′14.4″ E

). Twenty-one stations were selected in each part of the sea. The station coordinates were determined using a GPS navigator (Garmin, Ltd., Olate, KS, USA). A total of 105 samples were collected, among them, 63 samples from the Middle part of the Sea (21 samples per season: summer, autumn, winter), and 42 samples from the North part of the Sea (21 samples per season: spring, summer). Zooplankton was sampled using the Juday plankton net (net with an inlet diameter of 37 cm and mesh size of 65 µm), by stretching it from the bottom to the surface. Filtered water was poured into 250 mL plastic bottles and fixed with 40% formalin to a final concentration of 4% [28]. Further processing of the collected samples was carried out in the laboratory. The sixteen samples collected in different seasons were examined immediately after collection without fixation.

2.3. Laboratory Processing

A total of 105 samples were analyzed. The individuals with parasites Ellobiopsis were counted. The attachment points of Ellobiopsis sp. were noted and their sizes were recorded.

An important morphological characteristic of genus Ellobiopsis that distinguished it from other taxa was the pear-shaped, spherical, and cylindrical forms of the body consisting of trophomers and several gonomers (Figure 2E). The parasites with the same morphological characteristics were found in the Sea of Japan and identified at the genus level [14]. Therefore, the main identification key to confirming the genus Ellobiopsis sp. was Konovalova’s work [14], and other publications containing morphological data of this genus were used [11,21]. Parasitological calculations were carried out using Lakin’s method [29].

The prevalence is the percentage of infested individuals in the population. It was calculated by the formula:

$$\mathrm{P}={\displaystyle \frac{\mathrm{m}}{\mathrm{n}}}\ast 100\%$$

(1)

where P—proportion of infested individuals; m—number of infested individuals; n—number of sample.

The intensity is mean number of parasitic individuals per infested copepod. It was calculated by the formula:

$$\mathrm{I}={\displaystyle \frac{\mathrm{p}}{\mathrm{H}}}$$

(2)

where p—the total number of parasitic specimens found in infested individuals; H—number of infested host individuals.

In order to identify the hosts of the Ellobiopsis sp. the keys to Calanoida species identification and article containing descriptions of morphological characteristics of individuals of these species in different developmental stages were used [30,31]. Other representatives of zooplankton were identified at the species level using the identification keys of the respective groups and genera [32,33,34].

A crucial morphological feature of adult Acartia (A.) tonsa is the structure of the fifth pair of legs [30]. Differences between the individuals in the different developmental stages are the number of metasome segments and the quantity and structure of swimming legs [31]. The body of nauplii consists of prosomes which bear antennules, antennae, and mandibles [30]. The body of copepodid (stage I) consists of a cephalosome with a length more than half the length of the prosome, a three-segmented metasome, and a two-segmented urosome. The number of swimming legs is two, consisting of exopodite and endopodite. The exopodite of the first leg is one-jointed. There are setae along the edges of the joint, and one of the setae is serrated. The endopodite is two-jointed, and there are six setae along the edges of the joints. Caudal rami has five setae. Metosome of copepodid stage II is four-segmented and the number of swimming legs is three. Endopodite and exopodite have setae. One of the exopodite setae is serrated. Each caudal rami has six setae. Metasome of copepodid stage III is five-segmented. There are four pairs of swimming legs. Sexes can be distinguished in the penultimate (IV) and last (V) copepodid stages. The female urosome is two-segmented and as for males, it is three-segmented. There are five pairs of swimming legs. The individuals of both sexes have similar structures of the first four pairs of legs. Males and females at these stages have a uniramous fifth pair of legs. The fifth legs of females are symmetrical, whereas males have asymmetrical ones. The appendages of the fifth legs that are characteristic of adults are easily noticeable in these stages in both sexes [31].

Genus Calanipeda has a single species—Calanipeda aquaedulcis. Individuals of these species are noticeable due to divergent elongated caudal rami, the length of which is about four times their width. This feature is noticed in all individuals at all developmental stages, except nauplii. An important morphological characteristic of adult males and females is the structure of the fifth pair of legs. Therefore, identification of copepodites at developmental stages IV–V from adult individuals is not difficult. Copepodites at developmental stages IV–V have noticeable abdomen joints, and the body length of individuals is one and a half times less than that of adults. It is impossible to distinguish surely the sexes in copepodid in the last two stages (IV–V) since the structure of the fifth pair of legs is not fully developed. The body length of copepodites at developmental stages I–III is approximately two times less than that of individuals in the upcoming stages. The abdomen consists of one or two barely noticeable joints and caudal rami. The latter is longer than the joints. Nauplii is distinguished by the presence of a large unpaired spine at the posterior side of the body [30].

The standard methods were used to process samples quantitatively [28]. Zooplankton taxa were counted in a particular part of the sample. The sample concentration at 300 cm³ was the initial part of counting representatives of zooplankton. After careful mixing, three portions of the sample were taken. A stempel pipette with a volume of 1 mL was used to take the sample and the Bogorov counting chamber for counting. The most numerous taxa were counted in this sample. Following, the sample was concentrated to the volume of 150 cm³ to count not numerous taxa. Three sub-samples were taken from it. We concentrated the sample to 25 cm³ and the described above process repeated. The entire sample was viewed to count the individuals of rare taxa. The body sizes of all encountered taxa were measured separately. Adult females, females with eggs, males, copepodites at development stages 1–3 and 4–5, and nauplii were counted and measured separately in Calanoida family. For each species, the total abundance and biomass were calculated. The formula of the dependence between mass and body length was used to calculate the individual mass of zooplankton taxa [28,35].

The obtained results of counting were recalculated per 1 m³ to obtain the abundance of species.

$$\mathrm{N}={\displaystyle \frac{\mathrm{n}\times \left(\frac{{\mathrm{V}}_1}{{\mathrm{V}}_2}\right)}{{\mathrm{V}}_3}}$$

(3)

where N—abundance (individual/m³), n—number of individuals per portion (individual), V₁—concentration volume (cm³), V₂—subsample volume (cm³), V₃—volume of filtered water (m³).

The following formula was used to calculate the volume of filtered water:

$${\mathrm{V}}_3=\mathrm{h}\times \mathsf{\pi }{\mathrm{r}}^2$$

(4)

where V₃—volume of filtered water, h—depth of caught water column, π—mathematical constant (π ≈ 3.14), r—internal radius of the inlet hole of the Juday plankton net. The total abundance and biomass of zooplankton were obtained by summarizing the abundance and biomass of all species registered in the sample.

Identification of dominant species was carried out according to Lyubarsky’s scale [36].

2.4. Statistical Analysis and Comparisons with Previous Studies

The similarity of zooplankton species composition in different years was determined by the Bray–Curtis index in the Primer 5 program [37]. We used statistical methods (non-parametric) to find significant statistical differences in zooplankton quantitative variables over different years. The Kruskal–Wallis test was used to determine whether there were any statistically significant differences between quantitative zooplankton variables in different years of study. Spearman’s correlation analysis was used to identify the effects of parasites on crustacean body size. The Kruskal–Wallis test and Spearman’s correlation analysis were performed in R studio software (R—4.1.1) [38]. Comparative analysis was carried out to determine whether there are alterations in the structural variables of zooplankton (i.e., in the species composition, in quantitative variables, and the complex of dominants) due to the infection with Ellobiopsis sp. Similar data from 2008 [6], 2016 [39], 2020, and 2021 [4] were used for the comparative analysis. Methods (stations, seasons) of the 2008 studies in the North Caspian [6], except for sample collection, were the same as the sampling and methods of the current research [6]. Field sampling (stations) and laboratory processing (species identification and quantification methods) of the studies in 2016 and, 2020, 2021 in Middle Caspian [4] were consistent with those of the current research. An exception was the sampling period; investigations were conducted in the spring of 2016 and 2020, 2021 [4,39], and in the current work, the considered seasons were summer, autumn, and winter of 2024.

3. Results

3.1. Prevalence of Infestation and the Intensity of Infestation of Ellobiopsis sp. in Acartia (Acanthacartia) tonsa and Calanipeda aquaedulcis Populations

During the study period, of the 21 taxa registered in the Middle and Northern Caspian, only the calanoids Acartia (Acanthacartia) tonsa and Calanipeda aquaedulcis were infected with parasites genus Ellobiopsis sp.

A. (Acanthacartia) tonsa was found in both water areas. C. aquaedulcis was registered only in the North Caspian.

Acartia individuals in all development stages were infected in the Middle Caspian Sea (

Table 2. Seasonal dynamics of infestation of the Acartia (Acanthacartia) tonsa population with genus Ellobiopsis sp. in the Middle Caspian Sea, 2024.

Stage	Season
	Winter			Summer				Autumn
	Total Examined Individuals	The Infected Individuals	*P, %	*I	Total Examined Individuals	The Infected Individuals	*P, %	*I	Total Examined Individuals	The Infected Individuals	*P, %	*I
Male	262	15	5.72	1	271	15	5.53	1	306	4	1.30	1
Female	386	7	1.81	1	445	18	4.04	1	1059	16	1.51	1
Juvenile copepodid, stage IV–V	570	27	4.73	1	437	44	10.06	1	1414	42	2.97	1
Juvenile copepodid, stage I–III	796	52	6.53	1	655	40	6.10	1	1011	37	3.65	1
Nauplii	186	2	1.07	1	1	0	0	0	0	0	0	0
Total	2200	103	4.68	1	1809	117	6.46	1	3790	99	2.61	1

Note. *P—prevalence, *I—intensity.

). The prevalence of the parasite was high in winter and summer, and by autumn, the rate had decreased fivefold. The most frequently infected developmental stages were immature individuals and males in all seasons. The intensity of infestation of all individuals was equal to one in summer, autumn, and winter, which indicated a low level of parasite load.

Acartia individuals in all developmental stages, except nauplii, were infected in the North Caspian (

Table 3. Seasonal dynamics of infestation of the Acartia (Acanthacartia) tonsa and Calanipeda aquaedulcis populations with genus Ellobiopsis sp. in the Middle Caspian Sea, 2024.

Species and Developmental Stages	Season
	Spring				Summer
	Total Examined Individuals	The Infected Individuals	*P, %	*I	Total Examined Individuals	The Infected Individuals	*P, %	*I
A. (Acanthacartia) tonsa	326	13	3.98	1	865	29	3.35	1
Male	13	1	7.69	1	53	3	5.66	1
Female	27	1	3.70	1	345	16	4.63	1
Juvenile copepodid, stage IV–V	59	1	1.69	1	268	13	4.85	1
Juvenile copepodid, stage I–III	217	10	4.60	1	194	7	3.60	1
Nauplii	10	0	0	0	5	0	0	0
C. aquaedulcis	403	3	0.74	1	35	2	5.71	1
Male	7	0	0	0	1	0	0	0
Female	5	0	0	0	7	0	0	0
Juvenile copepodid, stage IV–V	44	0	0	0	9	2	22.22	1
Juvenile copepodid, stage I–III	174	1	0.57	1	8	0	0	0
Nauplii	173	2	1.15	1	10	0	0	0

Note. *P—prevalence, *I—intensity.

). The highest prevalence of the parasite was recorded in males of Acartia in both study periods. The same level of prevalence was observed in the population in spring and summer.

The prevalence of the parasite in Calanipeda aquaedulcis of the Northern Caspian was low in spring (Table 3). The rate varied from 0.74% to 5.71%. During this period, copepodids were the most commonly infected with Ellobiopsis sp. The prevalence rate increased to 22.22% by summer, and only individuals in juvenile developmental stages (stage IV–V) were infected. During the research, each host was infested with a single parasite in this water area.

3.2. Description and a Possible Influence of Parasites Ellobiopsis sp. on Body Length of Acartia (Acanthacartia) tonsa and Calanipeda aquaedulcis

Ellobiopsis sp. was attached to different parts of the host body, including prosoma, metasoma, and urosome (Figure 2). It was found in the dorsal part of the body of adults of Acartia (Figure 2A,B). The ventral side was the preferred attachment point of Ellobiopsis sp. in juvenile copepodids of Acartia (Figure 2C,D,F), and as for the nauplii, parasites chose mainly the mouth part for the attachment (Figure 2G). The points of attachment of the parasite to the body of the Calanipeda differed from that of the Acartia. Ellobiopsids were noticed in the posterior side of the body of nauplii (Figure 2H) and the mouth part of juvenile copepodids (Figure 2I).

The most commonly registered forms of parasites were spherical, elongated, spindle-shaped, and pear-shaped, with dark granular structure contents. Ellobiopsids cells with elongated spindle-shaped forms were found in juvenile copepodids of both species (Figure 2F,I,J). Other individuals of Acartia (A.) tonsa and C. aquaedulcis were infected with Ellobiopsids, which had the pear-shaped forms of the body (Figure 2A–D,G,H). The parasites had an elongated body. The values varied from 0.10 to 0.80 mm.

Infected individuals of Acartia and Calanipeda had a reduced body length than the uninfected individuals (

Table 4. The body length (mean values and standard error) of infected and uninfected populations of Acartia (Acanthacartia) tonsa and Calanipeda aquaedulcis in the Caspian Sea, 2024.

Species	Development Stages	Infected, mm	Not Infected, mm
The North Caspian Sea
C. aquaedulcis	Juvenile copepodid, stage IV–V	0.90 ± 0.10	0.95 ± 0.10
	Juvenile copepodid, stage I–III	0.55 ± 0.10	0.58 ± 0.10
	Nauplii	0.60 ± 0.10	0.65 ± 0.10
Acartia (A.) tonsa	Female	1.45 ± 0.10	1.85 ± 0.10
The Middle Caspian Sea
Acartia (A.) tonsa	Male	1.08 ± 0.14	1.60 ± 0.09
	Female	1.70 ± 0.03	2.0 ± 0.06
	Juvenile copepodid, stage IV–V	1.40 ± 0.03	1.40 ± 0.01
	Juvenile copepodid, stage I–III	0.55 ± 0.03	0.68 ± 0.01

). Parasites influenced only the body size of Acartia females and immature individuals of C. aquaedulcis in Northern Caspian. Infected Acartia individuals, except for the nauplii, had a lessened body size in the Middle Caspian.

3.3. Assessment of the Current State of Zooplankton in the Middle and Northern Parts of the Caspian Sea After the Presence of Parasite Ellobiopsis sp. Was Established

During the research period, 21 taxa were registered in the zooplankton of the Caspian Sea, including 14 taxa in the Northern Caspian and 11 taxa in the Middle Caspian. The following species were spread throughout the Middle part of the Sea: cladocerans Evadne anonyx (G. Sars), Pleopis polyphemoides (Leukart), copepods Acartia (Acanthacartia) tonsa (Dana), facultative inhabitants of the water column Cirripedia gen.sp. Cladocerans Evadne nordmanni and Podon intermedius (Lilljeborg) were widespread.

Rotifers Brachionus plicatilis (Muller), Brachionus quadridentatus (Hermann), cladocerans Podonevadne camptonyx (G.O. Sars), Podonevadne trigona (G.O. Sars), the copepods A. (Acanthacartia) tonsa, Calanipeda aquaedulcis (Kritschagin), and the facultative inhabitants of the water column Bivalvia gen.sp. were present across all investigated sites of North Caspian. The facultative inhabitants of the water column Hediste diversicolor (O.F. Müller) were widespread.

The highest abundance and biomass of zooplankton were recorded in the Northern Caspian Sea in summer (

Table 5. Quantitative variables of zooplankton in the Caspian Sea, 2024.

Water Area and Season	Abundance, Individuals/m3	The Dominant Species	%	Biomass, mg/m3	The Dominant Species	%
North Spring	7672 ± 1.10	A. (Acanthacartia) tonsa	86.74	68.43 ± 1.40	C aquaedulcis	46.03
					Acartia (A.) tonsa	48.96
North Summer	16,694 ± 1.60	A. (Acanthacartia) tonsa	41.26	889.50 ± 1.35	A. (Acanthacartia) tonsa	43.95
		C. aquaedulcis	46.47		C. aquaedulcis	48.17
Middle Winter	5532 ± 0.55	A. (Acanthacartia) tonsa	90.56	106.57 ± 1.50	A. (Acanthacartia) tonsa	59.71
Middle Summer	4291 ± 0.89	A. (Acanthacartia) tonsa	86.78	131.24 ± 1.93	E. anonyx	45.72
					A. (Acanthacartia) tonsa	49.38
Middle Autumn	4312 ± 1.54	A. (Acanthacartia) tonsa	86.74	132.56 ± 1.45	E. anonyx	46.03
					A. (Acanthacartia) tonsa	48.96

). Copepods A. (Acanthacartia) tonsa and C. aquaedulcis were the most abundant species. As for the Middle Caspian, quantitative variables of zooplankton remained unchanged from winter to summer. The dominant species in zooplankton quantitative variables of the Middle Caspian were Cladocera Evadne anonyx and copepoda A. (Acanthacartia) tonsa.

3.4. Statistical Analysis

The body length between infected and uninfected calanids differed. Spearman’s correlation analysis (p < 0.05) was conducted to determine the possible connections between two variables—body lengths of hosts and infestation intensity. The strong negative correlations (Spearman’s correlation coefficient = −0.86) between these two variables were found in females and males of Acartia of Middle Caspian. The same negative correlations (Spearman’s correlation coefficient = −0.57) were recorded in individuals of Acartia in juvenile developmental stages (stage I–III). The relationship between body lengths of females of Acartia and individuals in juvenile developmental stages (nauplii and copepodid stage IV–V) of Calanipeda in North Caspian and index of infestation intensity was also negative (Spearman’s correlation coefficient = −0.94).

The analysis of the similarity of zooplankton species composition revealed two clusters at 50% level (Figure 3). The first cluster included the species composition of zooplankton in the spring of 2021 and in the summer of 2024, the second—the species composition of zooplankton of North Caspian in spring and summer of 2024. The clearest division at the 10% level occurred between zooplankton of the middle Caspian in the spring of 2020, spring of 2021, and summer of 2024. The similarity of the taxonomic composition of zooplankton of the North Caspian in 2024 with that of 2008 was only 40%, the same insignificant similarity in the taxonomic composition was found between the data of the Middle Caspian in 2016 and 2024.

The Kruskal–Wallis test evidenced that zooplankton abundance (chi-squared = 7, df = 7, p-value = 0.4289) of the North and Middle Caspian did not vary significantly from year to year since there were no statistical differences between the data of 2008, 2016, 2020, 2021, and 2024. According to the test, the difference between the zooplankton abundance of North Caspian in 2008 and 2024 was insignificant (chi-squared = 3, df = 3, p-value = 0.3916) even if the abundance of zooplankton in the spring of 2024 reached 7.67 thousand individuals/m³ which was much lower than that of 2008 spring data (60.2 thousand ind./m³). However, zooplankton abundance in the summer of 2024 (16.6 thousand ind./m³) was almost equal to data from the summer of 2008 (14.2 thousand ind./m³).

There were also no statistical differences (chi-squared = 7, df = 7, p-value = 0.4289) between the abundance of zooplankton in Middle Caspian with similar data from 2008, 2016, 2020, 2021, and 2024. The recorded values of zooplankton abundance were 4.2 thousand ind./m³ in 2024; 215 thousand ind./m³ in 2021; 1.9 thousand ind./m³ in 2020; 30 thousand ind./m³ in 2016; and 3.8 thousand ind./m³ in 2008 [4,5,6,39].

The same test evidenced the presence of a statistically significant difference (chi-squared = 3, df = 3, p-value = 0.01173) between zooplankton biomass of 2008 and 2024 in North Caspian; between zooplankton biomass of 2008, 2016, 2020, 2021, and 2024 data in Middle Caspian (chi-squared = 7, df = 7, p-value = 0.00118).

Zooplankton biomass of Middle Caspian was equal to 131.24 mg/m³ in 2024; 160.54 mg/m³ in 2021; 159.71 mg/m³ in 2020; 488 mg/m³ in 2016; and 33.4 mg/m³ in 2008 [4,5,6,39]. The following results were obtained on zooplankton biomass of North Caspian in 2024: spring—68.4 mg/m³, summer—889.50 mg/m³, and in 2008: spring—208.4 mg/m³, summer—53.2 mg/m³ [6].

4. Discussion

4.1. Prevalence of Infestation and the Intensity of Infestation of Ellobiopsis sp. in Acartia (Acanthacartia) tonsa and Calanipeda aquaedulcis Populations

Copepods Acartia (Acanthacartia) tonsa and Calanipeda aquaedulcis were infected with Ellobiopsids out of 21 zooplankton taxa identified in this study. The specificity of this parasite to representatives of the Calanoida family was noted earlier [11]. In previous studies, representatives of Calanoida inhabiting waterbodies of Crimea [22] and Brazil [23], the Mediterranean [19], the Sea of Marmara [24], the Arabian Sea [21], and the Baltic Sea [25] were infected. Acartia (Acanthacartia) clausi Giesbrecht, 1889, the dominant zooplankton in the water bodies of Crimea, was infected. In the Baltic Sea, along with the representatives of the genus Acartia sp., all recorded taxa were infected, including cladocerans and representatives of facultative inhabitants of the water column Cirripedia gen. sp. [25]. In the waterbodies of Brazil, in the Mediterranean, Marmara, and Arabian seas, various species of calanoids dominated the zooplankton community, and only they were infected. Thus, it follows from this that Ellobiopsis mainly attacks species that are prevailing in plankton.

As in other regions [14,19,21,22,23,24,25,26], in the Caspian Sea, all developmental stages of calanids were infected with Ellobiopsis, and infection with parasites did not occur in a specific season of the year. Parasites were encountered in all seasons. However, a high level of prevalence of parasites in calanoids in the Caspian Sea, as well as in calanids of the Clyde Sea, was recorded in the summer [25]. In the Middle Caspian, the level of A. (Acanthacartia) tonsa infestation is higher than in the Northern part, but the prevalence of parasites is low, less than 10%. The prevalence of the parasite in calanoids of other regions was much higher and reached 15.5% in Crimea [22], 0.05% in Brazil [23], 54% in the Baltic Sea [25], 37.8% in the Sea of Marmara [25], and 54% in the Arabian Sea [21]. There is no information on C. aquaedulcis infestation with Ellobiopsis sp. in water bodies of other regions. The prevalence of the parasites in Calanipeda aquaedulcis of the Northern Caspian is high. The individuals at the juvenile stages of development were infected.

The number of parasites per host was equal to one in the North and Middle Caspian Sea. The latter aligned with the results of research conducted in Brazil [23] and the Arabian Sea [21].

4.2. Description and a Possible Influence of Parasites Ellobiopsis sp. on Body Length of Acartia (Acanthacartia) tonsa and Calanipeda aquaedulcis

Ellobiopsis sp. was attached to different parts of the host body—prosoma, metasome, and urosome in the Caspian Sea (Figure 2). The dorsal and ventral parts (metasome) of the body and mouth appendages of individuals of Acartia were preferred attachment points of Ellobiopsis sp. Individuals of Calanipeda carried the parasites in the mouth part and posterior side of the body. The listed preferable locations of attachments of Ellobiopsis sp. in the calanids of the Caspian Sea are also characteristic of Ellobiopsis sp. found in the calanids of the Baltic Sea [25]. The body shape of the parasite changes during the life cycle [14]. The most commonly registered forms of parasites in calanids of the Caspian Sea were spherical, elongated, spindle-shaped, and pear-shaped. The lengths of the parasite body varied from 0.10 to 0.80 mm. Similar length values and body shapes of the parasites were previously recorded in the Sea of Japan [14].

Infected individuals of Acartia and Calanipeda had a reduced body length than the uninfected individuals, as evidenced by Spearman’s correlation analysis. In C. aquaedulcis, all immature individuals had reduced body sizes; in Acartia, all infected individuals, except for individuals in the nauplii stage, had a lessened body size, especially in the middle part of the sea. The difference in body length between infected representatives of Calanoida compared to uninfected ones was also found in Calanids from water bodies in Crimea [22] and Brazil [23].

4.3. Assessment of the Current State of Zooplankton in the Middle and Northern Parts of the Caspian Sea After the Presence of Parasite Ellobiopsis sp. Was Established

During the research period, 21 taxa were registered in the Caspian Sea zooplankton, which is almost two times less than that identified in previous years [4,6,39]. Despite the almost twofold decrease in the number of taxa and the low level of similarity of the taxonomic composition (Figure 3), the main set of species characteristic of previous years was preserved [4,6,39]. It included the following species in Middle Caspian: cladocerans Evadne anonyx (G. Sars), Pleopis polyphemoides (Leukart), copepods Acartia (Acanthacartia) tonsa (Dana), facultative inhabitants of the water column Cirripedia gen.sp. Cladocerans Evadne nordmanni and Podon intermedius (Lilljeborg); and in the North Caspian: rotifers Brachionus plicatilis (Muller), Brachionus quadridentatus (Hermann), cladocerans Podonevadne camptonyx (G.O. Sars), Podonevadne trigona (G.O. Sars), the copepods A. (Acanthacartia) tonsa, Calanipeda aquaedulcis (Kritschagin), and the facultative inhabitants of the water column Bivalvia gen.sp., Hediste diversicolor (O.F. Müller).

Zooplankton abundance in the North and Middle Caspian did not vary significantly from year to year since there were no statistical differences between the data of 2008, 2016, 2020, 2021, and 2024 [4,6,39].

According to the Kruskal–Wallis test, a statistically significant difference was found only between the biomass of zooplankton of different years. During the dominance of Calanoida representatives, total zooplankton biomass in 2024 was higher than during the period of rotifers dominance in 2008 in the North Caspian [6]. The dominants of previous years of Middle Caspian retained their leading positions [4,6,39]. Minor differences in the biomass of zooplankton of the Middle Caspian with the data of previous years were noted earlier as a result of the introduction of non-indigenous cladocerans and predatory species of jellyfish [5,18]. A slight decrease in biomass in 2024 might occur as a result of infection of dominant Acartia (A.) tonsa with Ellobiopsis sp. since strong negative correlations were found between body lengths of Acartia (A.) tonsa and infestation intensity. The previous studies in other regions also mentioned the influence of Ellobiopsis sp. on the body length of hosts but did not provide data on the effects of Ellobiopsis sp. on quantitative variables of hosts [14,19,21,22,23,24,25,26]. The fact that the body length is also used to calculate the individual mass of zooplankton taxa [28,35] suggests the inevitability of the influence of Ellobiopsis sp. on the biomass of hosts.

5. Conclusions

For the first time, our studies revealed the infection of the calanid Acartia (Acanthacartia) tonsa and Calanipeda aquaedulcis with Ellobiopsis parasites in the Caspian Sea. The number of parasites per host was equal to one in the North and Middle Caspian Sea. The infection rate of calanids varied from 2.61% to 5.71%. The highest degree of infection was found in C. aquaedulcis, and it was reported for the first time as a host for Ellobiopsis sp. The infected calanids had reduced body lengths. The individuals in the juvenile developmental stages were infected in Calanipeda, while in Acartia, individuals in all developmental stages were vulnerable to infection. The findings suggest the possible influence of Ellobiopsis sp. on quantitative variables of hosts in the Middle Caspian, especially on biomass, by reducing the body sizes of hosts. However, no effect on the abundance and biomass of the host and the structure of the zooplankton of the North Caspian Sea has been detected. Further investigation is necessary, and it should be devoted to the genetic identification of species of the genus Ellobiopsis as well as its influence on quantitative variables of hosts.