First record of the North American amphipod Melita nitida Smith, 1873 in Polish coastal waters

The human-mediated introductions of non-indigenous marine species to regions outside their natural occurrence range are nowadays recognized as a problem of global concern because of the unexpected and usually negative environmental consequences caused by these species (Olenin & Leppäkoski 1999; Chandra & Gerhardt 2008). Ecosystems most vulnerable to biological invasions are coastal seas and estuaries which offer newcomers favorable living conditions (Wolff 1999; Leppäkoski & Olenin 2000; Preisler et al. 2009). The Baltic Sea – also called “the sea of invaders” (Leppäkoski et al. 2002) is a typical example, where up to now 132 introduced and cryptogenic species have been recorded (AquaNIS 2015). More than 20% of them are Malacostraca, mostly represented by amphipods native to various regions, e.g. Ponto-Caspian, Atlantic or Pacific (HELCOM 2013; AquaNIS 2015). In addition to the species-rich family Gammaridae, amphipods introduced to the Baltic Sea encompass also representatives of the families Corophiidae, Talitridae and Melitidae. The latter family includes the species Melita nitida Smith, 1873 native to the Atlantic coast of North America. In 2010, this amphipod was found in the westernmost part of the Baltic Sea (Kiel Canal, Germany), but re-identification of the material from the previous surveys showed that M. nitida was present in these waters at least since 2008 (Reichert & Beermann 2011). So far, this species was reported by Lackschewitz et al. (2014) from the south-western Baltic Sea (Mecklenburg Bay), but without any details. In this paper we report, for the first time, the presence of M. nitida in the southern Baltic Sea (the Gulf of Gdańsk, Poland).

Sampling took place in the Port of Gdynia (54°32’107” N, 18°31’781” E) located in the eastern part of the Gulf of Gdańsk (the southern Baltic Sea). Traps (n = 3) consisting of a plastic crate (30 × 30 × 30 cm) filled with empty oyster shells (Fowler et al. 2013) were used to collect mobile epifauna. They were deployed in 2014 from June to July (8 weeks) at a depth of less than 10 m. The water temperature varied between 10.5 and 19.1°C, whereas salinity was 7 PSU. Sampled specimens were fixed in 4% formaldehyde, transferred to the laboratory and determined to the species level based on the characteristics given by Bousfield (1973), Chapman (1988) and Jarrett & Bousfield (1996). Specimens were deposited in the RBINS Marine Taxonomic Reference Centre in Ostend.

Among 79 amphipods collected in 2014, 13 were identified as M. nitida (Fig. 1), a species hitherto not recorded in the Polish waters. The sample contained 3 adult males. Species identification was done based on the following features: (1) absence of the dorsal teeth on the first urosome segment and the presence of a group of dorsolateral spines on either side of the second urosome segment (Fig. 2A) and (2) the shape of the male gnathopod 2 (Fig. 2B).

Figure 1

Figure 2

Melita nitida Smith, 1873 is native to the Atlantic coast of North America (Bousfield 1973). The species was introduced through human activities, first to the northeast Pacific (Chapman, 1988) and then to European waters (Faasse & van Moorsel 2003). The first documented finding in Europe dates back to 1996 when Melita specimens were collected at Doel (the Belgian part of the Western Scheldt) on October 15^th, but at that time the taxon was identified as native Melita palmata (Ysebaert et al. 2000). However, re-identification of the material showed that the specimens were actually M. nitida (VLIZ Alien Species Consortium 2011). Up to now, the species has been recorded from estuaries and ports of the North and Baltic Seas, in Belgium, the Netherlands, Germany and Poland as well as of the Atlantic Ocean (Bay of Biscay), in France (Fig. 3). Regarding vectors responsible for the introduction of M. nitida to European waters, the most likely ones are fouling on ship hulls or international oyster transports (Heiman et al. 2008; Reichert & Beermann 2011; Gouillieux et al. 2016). The identification of M. nitida may pose some difficulties due to the intraspecific morphological variations as well as nuances in taxonomic characters which allow to distinguish this species from the other American amphipods in the so-called “Melita nitida complex” (Chapman 1988; Jarrett & Bousfield 1996; Faasse & van Moorsel 2003; Reichert & Beermann 2011). In its introduced range in Europe, the species may be misidentified when using European identification literature, e.g. Lincoln (1979), and indeed in the past it has been identified as native Melita palmata (see Ysebaert et al. 2000) and Melita pellucida, now Allomelita pellucida (see BioConsult 2009). However, the presence of the group of dorsolateral spines on the second urosome segment is characteristic of M. nitida (Faasse & van Moorsel 2003).

Figure 3

Due to the fact that the majority of new introductions to marine waters are vessel-mediated, ports and adjacent regions are often the first regions where introduced species occur (Gollasch et al. 2002; Ojaveer et al. 2014; Lehtiniemi et al. 2015). In Poland, M. nitida was found in the Port of Gdynia where numerous vessels, arriving from other ports of the Baltic Sea or the North Sea, discharge ballast water (information on the ship traffic was provided by the Environmental Protection Department, Port of Gdynia Authority SA). These findings allow to conclude that the species was presumably introduced to the Polish coastal waters with commercial maritime transport from the Netherlands/Belgium or/and Germany, but only detailed molecular analyses could provide more reliable information on the source population of M. nitida. Unfortunately, there is no regular, nationwide monitoring program for introduced species, either in Polish ports or Polish coastal waters. Implementation of such a program could have led to earlier detection of the new species (Lehtiniemi et al. 2015). Therefore, it is highly likely that M. nitidia was introduced to Polish waters already before 2014.

The use of different sampling techniques during biological surveys increases the efficiency of finding the introduced species. Therefore, different traps have been developed to attract specific organisms (e.g. Hewitt & McDonald 2013). One of them was a self-made non-baited trap filled with empty oyster shells developed by Fowler et al. (2013). Melita nitida was caught in such a trap, previously used in the Polish coastal waters to catch Harris mud crab Rhithropanopeus harrisii. Although oysters do not occur in the Polish waters, they were used to provide shelter for small mobile fauna. Previous studies demonstrated that M. nitida is commonly associated with oysters in both its native and non-native region (Watling & Maurer 1972; Faasse & van Moorsel 2003; Heiman et al. 2008; Gouillieux et al. 2016). It seems that the species prefers hard substrates, both natural and artificial, which offer crevices, such as reefs of the Pacific oyster Crassostrea gigas, boulders, rock fills or asphalt fragments in loose gravel (Chapman 1988; Faasse & van Moorsel 2003; Reichert & Beermann 2011). In the invaded regions of the Pacific Ocean, the species lives between macrophytes, such as Enteromorpha spp. or colonial invertebrates, like hydroids or ectoprocts (Chapman 1988). In ports, wharfs and other underwater constructions provide a hard substrate suitable for the development of fouling organisms (both flora and fauna), which in turn provide habitat for many mobile species. In the Port of Gdynia, M. nitida may find a shelter in colonies of Mytilus edulis trossulus or Amphibalanus improvisus or between macrophytes, such as Cladophora glomerata, Ulva spp. or Pylaiella littoralis (Normant-Saremba, unpublished data).

Melita nitida usually co-occurs with other amphipods (Chapman 1988; Faasse & van Moorsel 2003; Reichert & Beermann 2011). In the port of Gdynia, five species belonging to the families Gammaridae (n = 3) and Corophiidae (n = 2) were identified together with M. nitida. Only two of them, Gammarus salinus Spooner, 1947 and Leptocheirus pilosus Zaddach, 1844, co-occurred earlier with M. nitida in Dutch and German waters, respectively (Faasse & van Moorsel 2003; Reichert & Beermann 2011). The native species, Melita palmata (Montagu, 1804) has not been found in the traps despite the fact that, according to Faasse & van Moorsel (2003), the species frequently co-occurs with M. nitida in Dutch waters. Both species might be distinguished based on the taxonomic characters presented in Table 1. It is worth mentioning that the distribution range of native M. palmata in the Gulf of Gdańsk significantly decreased over the last decades. By the end of the 1960s, M. palmata was common in the phytal zone of the inner part (Klekot 1980), but in the mid-1970s it was recorded at only one site (Legeżyńska & Wiktor 1981). At that time, a change in the bottom environment caused by the ongoing eutrophication has been observed (Kruk-Dowgiałło 1991; Kruk-Dowgiałło & Szaniawska 2008). Consequently, this led to significant changes in macrozoobenthos communities (Wiktor & Pliński 1992; Szaniawska et al. 1999). During a biological survey in 2007, M. palmata was not recorded in the inner part of the Gulf of Gdańsk (Janas & Kendzierska 2014). This species was found only at the Polish open sea coast (Gic-Grusza et al. 2009). It seems that M. palmata is sensitive to anthropogenic changes in the environment – it was classified as a sensitive taxon in the assessment of the ecological quality status of the Polish waters according to the EU Water Framework (Osowiecki et al. 2012). This may also be a reason why it was not found in the Port of Gdynia. In general, ports being under strong anthropogenic pressure (e.g. vessel traffic, noise, and pollutants) create specific living conditions which may disrupt functioning of some native species, at the same time favoring exotic species which are known to be more tolerant of adverse conditions (Stachowicz et al. 1999; Piola & Johnston 2008; Crooks et al. 2011).

Table 1

Taxonomic characters to distinguish M. nitida from M. palmata (Bousfield 1973; Chapman 1988; Jarrett & Bousfield 1996)

Melita nitida is an eurythermal and euryhaline species (Table 2), more tolerant of lower salinities than native M. palmata (Jażdżewski & Konopacka 1995; Faasse & van Moorsel 2003). Moreover, contrary to M. palmata which rather prefers sheltered, sandy bottoms where silt is deposited (Żmudziński 1982; Lincoln 1979; Jażdżewski & Konopacka 1995; Gic-Grusza et al. 2009), the North American species occurs frequently also on muddy substrates (Table 2) and it may live deeper (Faasse & van Moorsel 2003). The Polish location is now the most eastern locality for M. nitida in Europe and can be a stepping stone to further colonization of the brackish Baltic Sea by this species. Based on the ecological characteristics of M. nitida (Table 2), it can be assumed that this amphipod has a high potential for successful invasion of the Baltic Sea and adjacent mesohaline waters, such as lagoons and estuaries. Moreover, it may also replace native M. palmata, especially in habitats being under anthropogenic pressure. The introduction of non-native species may lead to changes in local biodiversity as well as to modification of the ecosystem structure and functioning (Ojaveer & Kotta 2015). It is known that M. nitida is a herbivorous and detritivorous species that may form the base of fish food chains (Zimmerman et al. 1979; Borowsky et al. 1997). The available literature does not provide evidence for negative impacts of this amphipod, but of course this may vary depending on the invaded regions. For this reason and in the context of the EU Marine Strategy Framework Directive (Ojaveer et al. 2014), further research on this species is required to monitor its spatial distribution and trends in abundance as well as to evaluate the impacts on the colonized ecosystem.

Table 2

Basic ecological characteristic of M. nitida Smith, 1873