Understanding the life history of species and their populations is a fundamental requirement, since life history traits are adaptations which have a significant impact on population dynamics and the survival of a species (San Vicente 2018; Braendle et al. 2011). Life history traits, i.e. population structure and reproductive parameters, are among the main preconditions for successful invasion by an alien species, including amphipods. For example, juveniles’ dominance indicates rapid growth of the population (Dobrzycka-Krahel et al. 2019). The reproductive potential of amphipods is related to body size, the number and size of eggs and embryos, the length of the reproductive period, the time of maturation and voltinism (San Vicente 2018; Bacela et al. 2009; Grabowski et al. 2007a; Bacela, Konopacka 2005; Sainte-Marie 1991). Life history traits are also dependent on other biological (competition and predation) and environmental factors (temperature, food, habitat, water quality and geographical location) (Poznańska-Kakareko et al. 2021; Grabowski et al. 2007a; 2007b; Pöckl 2007; Kley, Maier 2006; Panov, McQueen 1998; Sutcliffe 1993; Sainte-Marie 1991; Welton, Clarke 1980; Nilsson 1977; Hynes 1955).
Until recently
The Ponto-Caspian
The life history of the Ponto-Caspian
The species is characterised by having one or two generations per year. Females form two to four or more broods, which under optimal thermal conditions can breed almost throughout the year, with peaks of juveniles in spring and summer (Maazouzi et al. 2011; Graça et al. 1994; Sutcliffe 1993; Ward 1986; Sutcliffe et al. 1981; Welton, Clarke 1980; Karaman, Pinkster 1977; Nilsson 1977; Hynes 1955).
The population structure and reproductive traits of the Ponto-Caspian amphipods
The Daugava River is one of the largest rivers in Eastern Europe. Starting in the Valday Highlands in Russia, the river flows through the East European Plain, crosses Belarus and Latvia and flows into the Gulf of Riga. The catchment area of the Daugava River is around 87 900 km2 and its total length is 1005 km, of which 342 km are located in Latvia. The cascade of three large hydroelectric power plants along the Lower Daugava at Pļaviņas, Ķegums and Riga forms the largest artificial reservoirs in Latvia (Anosova et al. 2006). Among the Daugava reservoirs, the Pļaviņas Reservoir is the deepest (average depth: 14.5 m, maximum depth: 47 m) and the largest (volume: 509.5 million m3, area: 35 km2) with an average annual discharge of 567 m3 s−1 (Tidriķis 1997, Environmental impact assessment of wastewater of new projected cellulose factory 1998).
The physicochemical parameters of water were measured and amphipods were sampled within the Pļaviņas Reservoir at Gostiņi and upstream of the Pļaviņas Reservoir at Veczeļki, Jēkabpils and Daugavpils. This was done once or twice a month from April/May to September/October from 2017 through 2019 (except Veczeļki in 2018) (Figure 1, Table 1). Qualitative samples of amphipods were collected in the wadeable depths (up to 0.5 m) using a Hidrobios hand net with a mouth opening of 25 × 25 cm (500 μm mesh size). Two replicates of samples were collected by hitting and sweeping the net along the substrate. The final sample consisted of 10 to 12 sweep units; it was preserved in 75% ethanol. The substratum of the study sites consisted of sand, silty sand, detritus, pebbles, some boulders and macrophytes (Table 1). Simultaneously, the physicochemical parameters of the water (temperature, pH, conductivity and dissolved oxygen and chlorophyll
Background characteristics of the sampling sites during the study
Characteristics | “Daugavpils” | “Jēkabpils” | “Veczeļki” | “Gostiņi” |
---|---|---|---|---|
Position | 55°52′04″N |
56°29′52″N |
56°31′50″N |
56°36′56″N |
Gammarids | ||||
2018 no sampled | ||||
Substrate | sand, silty sand covered by detritus, some pebble and boulders, emergent and submerged macrophytes | sand, silty sand covered by detritus, emergent and submerged macrophytes | sand, fine gravel, pebble and some boulders covered by detritus, emergent macrophytes | sand, silty sand covered by detritus, some pebble and boulders, emergent and submerged macrophytes |
2017, month | V–X | V–IX | V–IX | V–IX |
average (range) | ||||
T (°C) | 16.03 (9.76–20.45) | 18.14 (15.75–20.47) | 17.86 (15.38–20.25) | 17.42 (14.14–20.52) |
Cond. (μS cm−1) | 306 (251–335) | 313 (280–337) | 311 (274–334) | 317 (274–375) |
DO (mg l−1) | 7.67 (6.09–9.19) | 7.45 (6.31–8.83) | 8.20 (9.57–7.19) | 7.74 (5.11–9.94) |
pH | 7.96 (7.72–8.24) | 8.05 (7.84–8.14) | 8.16 (7.98–8.47) | 8.12 (7.51–8.60) |
CHL (μg l−1) | 3.89 (2.84–5.45) | 4.98 (3.38–8.30) | 3.23 (2.97–3.65) | 4.68 (3.20–5.61) |
2018, month | V–X | V–X | V–X | |
average (range) | ||||
T (°C) | 18.62 (11.79–21.97) | 17.16 (6.42–23.01) | 2018 |
17.86 (6.56–23.62) |
Cond. (μS cm−1) | 382 (233–467) | 367 (280–404) | 404 (390–427) | |
DO (mg l−1) | 9.60 (7.51–13.75) | 8.48 (6.19–11.16) | 9.63 (6.62–11.71) | |
pH | 8.73 (8.16–9.22) | 8.81 (8.45–9.06) | 9.00 (8.92–9.41) | |
CHL (μg l−1) | 3.26 (1.77–6.69) | 3.99 (3.06–5.19) | 4.56 (3.96–5.30) | |
2019, month | IV–IX | IV–IX | VI–IX | VI–IX |
average (range) | ||||
T (°C) | 18.13 (11.42–22.18) | 18.28 (13.05–22.68) | 19.68 (13.27–23.22) | 20.79 (14.61–23.12) |
Cond. (μS cm−1) | 354 (216–512) | 316 (227–378) | 341 (289–374) | 342 (288–377) |
DO (mg l−1) | 8.13 (6.39–10.64) | 7.83 (5.91–9.94) | 8.20 (6.33–10.19) | 10.18 (8.00–13.67) |
pH | 7.73 (7.38–8.20) | 7.80 (7.41–8.19) | 8.09 (7.77–8.29) | 8.40 (8.06–8.95) |
CHL (μg l−1) | 4.39 (3.03–8.26) | 4.39 (2.16–6.89) | 2.96 (2.11–4.29) | 4.21 (2.95–6.60) |
Abbreviations: T – temperature, Cond. – conductivity, DO – dissolved oxygen, CHL – chlorophyll
In general, the average annual air temperature in Latvia gradually increased during the study period (Table 2). In 2017, the study season from May to October was more characteristic of Latvian climatic conditions. The duration of meteorological summer was typical of Latvian summer (60 days, June–August). In contrast, the duration of meteorological summer in 2018 and 2019 was twice as long (May–September). The spring (April and May) and the latter half of summer (the end of July and August) were hot in 2018; in fact, it was one of the warmest years. 2019 was the warmest year during the study period. The temperatures in June and August were +18.6°C and +17.0°C. The precipitation also varied within the study period. The summer and autumn of 2017 were rich in rainfall (Table 2), raising the water level in the Daugava River (Latvian Environment, Geology and Meteorology Centre 2017, 2018, 2019).
Air temperature and precipitation in Latvia during the study*
2017 | 2018 | 2019 | Latvian norm | |
---|---|---|---|---|
Annual mean air temperature, °C | +6.9 | +7.6 | +8.2 | +6.4 |
Annual total precipitation (mean in Latvia), mm | 809.8 | 472.7 | 629.2 | 685.6 |
Spring | ||||
Average air temperature, °C | +5.6 | +6.9 | +7.2 | +5.6 |
Precipitation, mm | 116.3 | 80.7 | 100.5 | 122.7 |
Summer | ||||
Average air temperature, °C | +15.4 | +18.1 | +17.2 | +16.2 |
Precipitation, mm | 237.0 | 162.4 | 176.7 | 225.7 |
Autumn | ||||
Average air temperature, °C | +7.6 | +8.6 | +8.3 | +6.7 |
Precipitation, mm | 313.5 | 124.3 | 236.7 | 201.0 |
Latvian Environment, Geology and Meteorology Centre, 2017, 2018, 2019
The specimens were identified using sources from the literature (Eggers, Martens 2004; Eggers, Martens 2001; Karaman, Pinkster 1977; Jażdżewski 1975; Pinkster 1970; Guide for Identification of the Fauna of the Black and Azov Seas 1969). A ZEISS Stemi 508doc stereomicroscope fitted with an ocular micrometer (10:100) was used to identify and measure the length of the specimens, taken as the distance from the anterior margin of the head to the telson base (Bacela, Konopacka 2005).
Based on the literature on the subject (Berezina 2016; Copilaş-Ciocianu, Boroş 2016; Bacela, Konopacka 2005; Sainte-Marie 1991), the population structure was divided into small specimens or juveniles (<5 mm), medium specimens or subadults (5–8 mm) and large specimens or adults (>8 mm). Adult specimens were further divided into males, females without eggs and ovigerous females. The following life history traits were recorded: body size, number of eggs per brood, length of reproductive period (indicated by the presence of ovigerous females in a population, in months) and generations per year.
The differences in the breakdown of population by size for each species population were compared with the Kruskal–Wallis test among all years at each site separately. The Mann–Whitney test was used to estimate differences in the adult size of amphipod species between seasons in all years and sites of the study, as well as to investigate differences in the size and number of eggs of ovigerous females between species in all years and sites. Spearman's rank correlation was used to determine the relationships between the size of ovigerous females and the number of eggs per brood in each species population in all years and sites, as well as relationships between water temperature and the size of ovigerous females and the number of eggs per brood in each species population in each study year for all study sites. Data manipulation and analysis were done using IBM SPSS Statistics 20.
The occurrence of the three species –
The breakdown of the population of all amphipods by size varied seasonally and differed from year to year at the study sites (Figure 2, 3) (Kruskal–Wallis test for
The breeding period of
The adult specimens of
The breeding period of
Adult specimens of
In the middle flow of the Daugava River, ovigerous females of native
Adult specimens of
The population structure of
A shorter life span, faster juvenile development and a tendency to produce more broods are usually associated with a higher average water temperature (Pöckl et al. 2003; Panov, McQueen 1998). For example, in the Eastern Gulf of Finland, the development of
Males and females of the alien species occurred among the population during all study seasons. Females prevailed, especially in the reproductive period (Figure 5, 7, 9), thus indicating a high potential to increase the population in a short time, as has been observed in studies on alien species. For example, in the aggressive alien species
In order to measure the reproductive potential of an amphipod, the average number of eggs per brood and the average size of ovigerous females were used. In the
Reproductive parameters of amphipod species during the study
2017 | 2018 | 2019 | |
---|---|---|---|
n | 34 | 68 | 32 |
Average size of ovigerous females, mm | 11.2 ± 1.1 SD |
11.3 ± 1.2 SD |
11.4 ± 1.3 SD |
Average number of eggs per brood | 26 ± 11 SD |
30 ± 13 SD |
37 ± 14 SD |
n | ovigerous females were not obtained | 22 | 17 |
Average size of ovigerous females, mm | - | 11.6 ± 1.3 SD |
10.9 ± 1.5 SD |
Average number of eggs per brood | - | 25 ± 16 SD |
31 ± 22 SD |
n | 8 | only two ovigerous females were obtained | only one ovigerous female was obtained |
Average size of ovigerous females, mm | 10.7 ± 0.5 SD |
- | - |
Average number of eggs per brood | 27 ± 5 SD |
- | - |
In the population of
These results are no higher than those from the Central European (in the Wloclawek Reservoir of the Middle Vistula, Poland) and the Eastern European freshwaters (Kuybyshev Reservoir and Saratov reservoirs in the middle and lower flows of the Volga, Russia), but were closer to the
In the native
For all species, fecundity is also associated with female size. The number of eggs significantly and positively correlated with the size of ovigerous females (for
If the environmental conditions are favourable, alien gammarids in the Daugava River have a successful reproductive period. They produce several consecutive broods in a shorter time within the breeding season, as evidenced by the bivoltine and multivoltine life cycle. The length of the favourable seasons in 2018 and 2019 influenced the size of adult specimens and the number of eggs, as particularly observed in the case of