Size structure and body condition of Ponto-Caspian gammarids in the Vistula estuary (Poland)

Human activities have greatly facilitated the range expansion of aquatic organisms (Carlton 1985; Carlton & Geller 1993; Konopacka 2004), for example Ponto-Caspian gammarids (Crustacea: Pontogammaridae, Amphipoda): Pontogammarus robustoides (G. O. Sars, 1894), Obesogammarus crassus (G. O. Sars, 1894) and Dikerogammarus haemobaphes (Eichwald, 1841). Over the past few decades, they have colonized the European inland and coastal brackish waters (e.g. Bij de Vaate et al. 2002; Grabowski et al. 2009).

At present, the issues related to migrations of gammarids are well recognized (Bij de Vaate et al. 2002; Panov et al. 2009). Although the gammarid species examined in this study come from habitats with a wide temporal or spatial salinity range, they are mainly known as invaders of freshwater ecosystems in non-native areas (Dodd et al. 2014; Guerlet et al. 2008; Kinzler et al. 2009; Dobrzycka-Krahel et al. 2013; Jermacz et al. 2015), where they are successful colonizers (Grabowski et al. 2007; Jermacz et al. 2015). There are numerous publications on the distribution, biology and ecology of invasive gammarid species in freshwater reservoirs (e.g. Dodd et al. 2014; Grabowski et al. 2009; Kinzler et al. 2009), but their status in coastal brackish waters is less known. Based on the previous laboratory studies (Dobrzycka-Krahel & Surowiec 2011; Dobrzycka-Krahel & Graca 2014; Dobrzycka-Krahel & Graca 2018), we can conclude that low salinities below 5 PSU are suboptimal for these species. Results of these experimental studies on P. robustoides, O. crassus and D. haemobaphes indicated that the salinity optimum for these species is about 7 PSU (in the salinity range from freshwater to 7 PSU) and the condition of these species in the Gulf of Gdansk (salinity above 5 PSU) is good (Dobrzycka-Krahel et al. 2016). Due to these facts, the objective of the presented work was to determine whether population parameters (size structure) and biological indicators (condition) of Ponto-Caspian gammarid species, considered as their status (Jakob et al. 1996; Nalepa et al. 2000), are good in salinities below 5 PSU. The status of alien gammarids in a suboptimal habitat may show phenotypic plasticity and invasive potential of the species.

Similarities in body length and weight of the analyzed invasive gammarid species were observed. The results also show a high proportion of juveniles (characterized by a fast growth rate) in the case of P. robustoides and O. crassus, and therefore good condition of these species, as the growth rate of animals is higher in younger individuals than in the older ones (Pauly 1979; 2010). In the case of D. haemobaphes, the lowest value of slope “b” was determined due to a smaller proportion of immature individuals. If rapid growth provides a selective advantage and growth varies as a function of habitat, we should also expect individuals to choose habitats with the optimal growth potential (Sogard 1992).

In general, our results show that the environment of the Vistula estuary is favorable for the examined species and the relationships demonstrate good fitness of gammarids and the high body weight growth rate in relation to the length growth rate.

If we consider the importance of the body condition of organisms, we can mention examples of previous research. Many authors have examined the relationships between length and weight (e.g. Siegel 1982; 1989; Morris et al. 1988), because species differ in body weight growth in relation to length growth (Lindqvist & Lahti 1983). Factors such as resource availability (Basset & Glazier 1995) and competitive interactions among species (Basset & Rossi 1990) are known to affect the body size and body condition of individuals. The relationships between the total body length and wet weight of species may be used to assess the size and mass of species consumed by predators (Janas 2005). The Ponto-Caspian gammarid species in question may be important food components for such fishes as Neogobius gymnotrachelus (Kessler, 1857), Perca fluviatilis (Linnaeus, 1758) (Grabowska & Grabowski 2005) or N. fluviatilis (Pallas, 1811) (Grabowska et al. 2009).

The length–weight relationships may also indicate morphological differences in various areas (Färber-Lorda 1994). To our knowledge, data on the condition of the examined gammarid species from their original area have not yet been published. However, we have calculated intercepts (a) and slopes (b) of Ponto-Caspian gammarids from the Gulf of Gdansk (salinity above 5 PSU) (Dobrzycka-Krahel et al. 2016). The values of the intercepts (a) and slopes (b) calculated for P. robustoides, O. crassus and D. haemobaphes in the present study (salinity below 5 PSU) were compared with the corresponding values obtained for these species from the Gulf of Gdansk (salinity above 5 PSU). There were no statistical differences between these values. The results of the analysis show good condition of gammarids. These results contradict the scientific hypothesis of the study that salinity below 5 PSU is suboptimal for Ponto-Caspian gammarids. Field studies (Dobrzycka-Krahel & Graca 2018) also indicated that salinity within the investigated range, i.e. below 5 PSU (0.3, 3.4) and above 5 PSU (7.3 PSU), was not a key factor governing the abundance of gammarids and, consequently, their distribution pattern. We can conclude that most likely the plasticity of Ponto-Caspian gammarids (Dobrzycka-Krahel & Surowiec 2011, Dobrzycka-Krahel & Graca 2014, Dobrzycka-Krahel et al. 2015) causes that salinity in the examined range is not a key factor for Ponto-Caspian gammarids. Comparing the intercepts (a) and slopes (b) of Gammaridae living in North America of Gammarus minus and of G. pseudolimnaeus (Basset & Glazier 1995) and from the Black Sea (Rosati et al. 2012) with our gammarids, we can conclude that the species examined in our study are in similar condition (Table 2).

On the one hand, the good condition of Ponto-Caspian gammarids (P. robustoides, O. crassus and D. haemobaphes), as demonstrated by the results of this study, indicate that they grow well in low salinity environments (Table 2). It is likely that the increasing salinity of European rivers facilitated the intensive spread of many alien species (Bäthe & Coring 2011; Dobrzycka-Krahel & Graca 2014; Petruck & Stöffler 2011; Piscart et al. 2011), so we can conclude that low salinity (below 5 PSU) is not a limiting factor for Ponto-Caspian gammarids in a non-native area. According to Grabowski et al. (2009), the increased number of alien species and their abundance in Polish rivers is associated with higher water conductivity (higher salinity). Furthermore, increased salinity of the Vistula Lagoon in the 20th century (Chubarenko & Tchepikova 2001) was probably an important factor facilitating the expansion of alien species in this area. The invasive alien gammarids are considerably more tolerant to elevated ionic levels compared to native species (Piscart et al. 2011). Forecast of desalinization of the Baltic Sea (Vuarinen et al. 1998; Rajasilta et al. 2014) caused by the absence of a saline water pulse since the late 1970s and the simultaneous increase in the run-off (Malmberg & Svansson 1982; Launiainen & Vilhma 1990; Matthäus & Schinke 1994) is unlikely to affect the future dispersal of the examined gammarids in the Baltic Sea, because they are tolerant to salinity (Grabowski et al. 2009; Dobrzycka-Krahel & Surowiec 2011; Dobrzycka-Krahel & Graca 2014; Dobrzycka-Krahel et al. 2015). Also other factors apart from salinity could affect the condition of gammarids, e.g. temperature measured during this study, but it is a varying factor (even in the daytime) in the Vistula estuary and all gammarids are eurythermic species (e.g. Wijnhoven et al. 2003).

According to Calow & Forbes (1998), fitness traits help not only to define stress, but more generally to understand how environmental variables affect the distribution and abundance of organisms. Good fitness corresponds to normal responses (Jacobsen & Forbes 1997). On the other hand, good condition of alien species affects the process of invasion or colonization. Fitness of invaders increases its ability to displace residents (Peacor et al. 2006). When invasive species strays from its optimal strategy, it can no longer compete with native species (Gross 2006). The optimal strategy of invasive alien gammarids may help them maintain strong competitive position in the environment as well as affect the colonization process of the Vistula Lagoon and the Vistula Delta. Such studies may be used in aquatic habitat management as they indicate the status of invasive alien species.