First record of Dikerogammarus bispinosus Martynov, 1925 in Kazakhstan: invasive or overlooked native in the Caspian Sea basin?

The Ponto-Caspian amphipod Dikerogammarus bispinosus is regarded as a native species throughout the lower stretches of rivers that drain into the Black Sea. Its occurrence in the Caspian Sea basin was uncertain due to conflicting reports. Here, we provide the first conclusive evidence for its presence in this basin. Individuals of both sexes, including ovigerous females, were collected in May 2000 from the Ural River in Kazakhstan, suggesting full establishment. If it was a recent invasion, the most probable dispersal pathway into the Caspian basin would have been via the Volga-Don canal as D. bispinosus was reported in the early 2000s from the lower Don River and the Saratov reservoir on the Volga River. However, given that until relatively recently D. bispinosus was considered a subspecies of D. villosus , we cannot rule out that it has been overlooked in earlier reports from the Caspian Sea basin by being mentioned as D. villosus or even D. haemobaphes . We also provide new data on the distribution of Gammarus lacustris, Obesogammarus platycheir , Pontogammarus abbreviatus , P. robustoides , Turcogammarus aralensis and Wolgagammarus dzjubani in western Kazakhstan and southwestern Russia

Here, we report D. bispinosus for the first time from Kazakhstan and thus confirm its presence in the Caspian basin. This is the second locality where this species is mentioned in this basin and the easternmost point of its entire distribution range.
One male (14.3 mm) and four ovigerous female (10.7-13.7 mm, 46-114 eggs) D. bispinosus were collected from the Ural River in the vicinity of the settlement Zelenoe, Kazakhstan. The identified specimens exhibited the usual morphological characteristics of this species. Diagnostic features are presented in Figure 2.
The compilation of own and literature data indicated that D. bispinosus has a broad geographical distribution (> 3000 km), being encountered from the Rhine estuary, throughout the lower Rhine (North Sea basin), Danube, Dniester, lower Dnieper and lower Don rivers (Black Sea basin) and reaches the middle Volga and lower Ural rivers (Caspian Sea basin) (Figure 3, Table 2). The record from this study represents the easternmost point of the species range (Figure 3).

Discussion
Dikerogammarus bispinosus was described by Martynov (1925) from the lower Dnieper and appears to be native to the Black Sea basin (Cărăuşu et al. 1955;Jażdżewski and Konopacka 1988). In Western Europe it has spread throughout the southern invasion corridor reaching the Rhine estuary via the Rhine-Main-Danube canal (Bij de Vaate et al. 2002). This wide-ranging dispersal is contrasted by its considerable decline during recent decades in its native region in the lower Danube and also in Lake   Table 2 for references.  (Grigorovich et al. 2002 and references therein). However, undocumented or unintentional introductions cannot be completely excluded either (Grigorovich et al. 2002). This further suggests that the species could have dispersed naturally via the Volga-Don canal or was passively introduced through shipping activity.
On the other hand, it is also possible that D. bispinosus reached the Caspian basin earlier than the 1990s given that the Volga-Don canal was opened in 1952. Moreover, D. bispinosus was considered for a long time as a subspecies of D. villosus and only relatively recently was elevated to specific status based on mitochondrial and nuclear genetic markers (Müller and Schramm 2001;Müller et al. 2002). In addition, Pjatakova and Tarasov (1996) considered D. villosus (and consequently D. bispinosus) as a synonym of D. haemobaphes, so they may have overlooked D. bispinosus in the Caspian basin (Tarasov 1995). Similarly, it is likely that other authors did not distinguish D. bispinosus from D. villosus due to its subspecific status until 2002. Nevertheless, it appears that neither D. villosus is native to the Caspian basin (Mordukhai-Boltovskoi 1979), where it has been reported at least since 1964 (Mordukhai-Boltovskoi 1964), suggesting a similar dispersal route as for D. bispinosus. It is important to keep in mind that Dikerogammarus species are some of the most successful Ponto-Caspian invaders, being highly capable of dispersal in anthropogenic landscapes (Rewicz et al. 2014(Rewicz et al. , 2015Šidagytė et al. 2017). The only Dikerogammarus species that is most likely native to both basins is D. haemobaphes since it was described from the Black Sea but has been reported from the Caspian Sea since 1880 (Sars 1894), well before the construction of the Volga-Don canal. In contrast, D. caspius, a native Caspian species, has spread into the Black Sea basin in recent times (Sayapin 2003). Thus, according to the available data, we tentatively conclude that even if D. bispinosus has been overlooked, it appears that it is not a native species in the Caspian basin and that it reached it between 1952 and late 1990s. Of course, at present, we also cannot completely rule out the possibility that it might be a native Caspian species. Phylogeography could prove invaluable in illuminating its origin and dispersal pathways. So far, D. bispinosus, D. villosus and Shablogammarus shablensis appear to be the only Black Sea native amphipod species that have spread into the Caspian basin (Grigorovich et al. 2002). Further upstream dispersal of D. bispinosus along the Volga and Ural rivers may be expected given its rheophilous affinity (Borza et al. 2017).