The family Sciaenidae comprises approximately 70 genera and 270 species of demersal fishes that are commonly known as drums and croakers, found mainly over muddy or sandy bottoms of tropical, subtropical and warm temperate regions of the Atlantic, Indian and Pacific oceans, from the shoreline up to about 600 m depth (McEachran and Fechhelm 2005, Nelson 2006). Most species are marine, a few are euryhaline and support a wide range of salinities, and 25 species, mostly from South America, are strictly freshwater (Casatti 2002).

The eastern Atlantic, with 19 sciaenid species, is the least diverse geographical area, compared to the eastern Pacific, with 82 species, the western Atlantic, with 82, and the Indo-West Pacific, with 92 (Lo et al. 2015). Only five native species occur in Iberian and Atlantic European waters: Argyrosomus regius (Asso, 1801), Sciaena umbra Linnaeus, 1758, Umbrina canariensis Valenciennes, 1843, Umbrina cirrosa (Linnaeus, 1758) and Umbrina ronchus Valenciennes, 1843 (Quéro et al. 2003, Lloris 2015).

Two introduced sciaenids species have also been reported in Atlantic European waters. Two living specimens of the Atlantic croaker, Micropogonias undulatus (Linnaeus, 1766), were reported in Belgian waters in 1998 and 2001 (Stevens et al. 2004) and one fresh specimen of about 28 cm total length, tentatively identified as M. undulatus, which was found displayed for sale at Plymouth Fish Market in September 2011 (http://www.glaucus.org.uk/), was probably captured in the southwestern approaches (SW Cornwall). One specimen of the spotted seatrout, Cynoscion nebulosus (Cuvier, 1830), was caught in 2011 by a fishing vessel on a sandy bottom near the Guadalquivir River estuary, southern Spain (Acosta et al. 2013).

The genus Cynoscion Gill, 1861 is a large, ecologically and economically important genus of marine fishes in the family Sciaenidae that is found throughout tropical and subtropical coastal waters of the New World (Vergara-Chen 2009). Currently, there are 24 recognized species, 12 from the eastern Pacific and 12 from the western Atlantic, inhabiting estuarine and inshore waters, mainly in the tropical and subtropical regions of Central and South America (Chao 2002a).

The Gulf of Cádiz is the sub-basin of the North Atlantic nearest to the Strait of Gibraltar. It is bordered to the north by the southwest coasts of the Iberian Peninsula, to the south by the Atlantic coast of Morocco, to the east by the Strait of Gibraltar, and to the west by the 9°W meridian (Criado-Aldeanueva et al. 2006). There is an important artisanal fishery and shellfishing sector in the area, with a fleet composed of about 1000 vessels that capture more than 50 commercial species (Silva et al. 2002), although current official data reduce the number of vessels by nearly half. Human impacts on this marine ecosystem through exploitation of marine resources, fishing and the invasion of non-indigenous species have also been reported (Chicharo et al. 2009).

There are three general mechanisms through which non-native species may enter a new region: importation as or with a commodity, arrival through a transport vector and dispersal by the species themselves, either along infrastructure corridors (e.g. roads, canals) or unaided (Hulme et al. 2008). For fishes, the most widely reported pathways of introduction are ballast water transport, marine culture, aquarium trade, oil drilling platform transfer, fisheries development, species released for scientific research, and movement through canals (Liao et al. 2010).

Molecular techniques have been successfully integrated with traditional morphological analysis in the identification of fishes, reinforcing the resulting taxonomic identification and reducing the possibility of identification mistakes (Bañón et al. 2013). In addition, the use of DNA barcodes efficiently helps to detect and understand marine invasions through the identification of exotic species and their geographic origin (Bariche et al. 2015).

The present work discusses the current taxonomic status of the Cynoscion species found in the Gulf of Cádiz based on morphological and molecular features within an integrative context. The distribution, abundance, possible mechanisms of introduction and ecological consequences of this introduced species are also discussed.

The morphological measurements and meristic counts of the examined specimens identify the species as C. regalis, a finding which is also strongly supported by the molecular identification.

The presence of scales at the base of the soft dorsal and anal fins, the colouration (mainly the colour and arrangement of the spots; Fig. 1), biometrics and meristic counts distinguish this species from other sciaenid species. Slight differences found in some biometrics (Table 1) could be due to the lower number of individuals or incomplete size range reported in the literature.

Acosta et al. (2013) reported one specimen of the congeneric C. nebulosus in the same area, near the Guadalquivir River estuary (southern Spain). However, a detailed revision of this manuscript showed an incomplete description of the specimen and, finally, an erroneous identification. In their description, the authors omitted an important distinctive character of the species. The presence/absence of scales at the base of soft dorsal and anal fins is a diagnostic character which distinguishes C. nebulosus (absence) from the similar C. regalis (presence) (Chao 2002a, b, Lassuy 1983). Although Acosta et al. (2013) reported the presence of spots on the caudal and dorsal fins characteristic of C. nebulosus, this aspect is not visible in the attached figure. C. nebulosus has distinct spots, comparatively fewer and bigger than those in C. regalis, and randomly scattered, whereas in C. regalis they form oblique and undulating lines (Chao 2002a, b). However, the figure of Acosta et al. (2013) shows a distribution pattern of spots typical of C. regalis. Moreover, the reported ten gill rakers count on the lower limb of the gill arch and the pectoral fin slightly longer than the pelvic fin also agree with the description of C. regalis, whereas C. nebulosus has 6-9 gill rakers on the lower limb of the gill arch and its pectoral fin is shorter than the pelvic fin (Lassuy 1983, Chao 2002a, b).

Molecular identification (16S and Cox1 genes) confirms the previous morphological identification of these specimens as C. regalis. The 16S sequences obtained for the three specimens (Fig. 3) present the same haplotype, which fits 100% with the sequence of C. regalis deposited in GenBank under the accession number AY857947 and also 100% with a shorter sequence of C. regalis (390 bp) deposited in GenBank under the accession number EF095595, obtained in the context of a phylogenetic study of the percoid family Gerreidae (Chen et al. 2007).

In the case of the Cox1 sequences (Fig. 4), the three specimens also have the same haplotype, which fits 100% with four sequences of C. regalis deposited in GenBank under the accession numbers KT075323, KT075305, KF929807 and KP722717, belonging to different specimens collected in the US Atlantic waters, and also 100% with three sequences (KP111936, KP111685, KP111197) identified as Actinopterygii environmental samples of eggs attributed through barcoding to C. regalis in a study integrating DNA barcoding of fish eggs into ichthyoplankton monitoring programmes (Lewis et al. 2016). In GenBank there is only one more Cox1 sequence, identified as belonging to C. regalis (EU180145), which differs in just one mutation from the other seven (GenBank) and three (present study) sequences. This sequence belongs to specimens collected in Delaware (North Carolina) by Seyoum et al. (2013).

After the re-examination of the Acosta et al. (2013) manuscript, we conclude that C. nebulosus reported by these authors is a misidentification of C. regalis. Thus, this is the only introduced sciaenid species reported until now in the Gulf of Cádiz. The presence of C. regalis in the Gulf of Cádiz confirms its presence in Atlantic European waters.

Fishes are one of the largest contributors to the total number of non-indigenous species documented in European seas, accounting for 16.8% of the 879 multicellular non-indigenous species reported by Galil et al. (2014). However, most of these fish species are found in the Mediterranean, introduced via the Strait of Gibraltar (Atlantic species) and the Suez Canal (Lessepsian species) (Oral 2010). In the Atlantic European waters, including the Gulf of Cádiz, the most frequent case is the arrival of southern species by natural displacement due to global warming and changes in the oceanic conditions (Farias et al. 2012, Bañón et al. 2014). In both the Mediterranean (excluding Lessepsian species) and the Atlantic, only a few cases of rare fish species introduced from large distances are documented, such as the South American fish Pinguipes brasilianus Cuvier, 1829 in the Mediterranean (Orsi-Relini 2002) and the aforementioned M. undulatus in the Atlantic.

As in most cases of marine alien species, the introduction vector cannot be clearly identified. C. regalis is highly dependent on estuaries for food, shelter and spawning (Mercer 1989), so it is very unlikely that this species would have swam across the Atlantic Ocean to Spain. However, we cannot rule out this possibility.

Among the anthropogenic pathways of introduction, fishes have been undersampled in ballast water studies and the role of ballast transport in promoting fish invasions has probably been underestimated (Wonham et al. 2000). There are two major ports near the Guadalquivir estuary: the American naval station in Rota, located 20 km from Chipiona, and Sanlúcar de Barrameda, where most of the weakfish is caught. The station has a seaport and provides support to US and NATO ships, and not only military ships. In addition, the port of Cádiz, located some 25 km from Chipiona, features four commercial docks plus a shipyard that repairs ships from all over the world. To support this theory on the introduction of weakfish, some marine organisms are believed to have been introduced through ship ballast water in the Gulf of Cádiz (Cuesta et al. 2004, Chicharo et al. 2009) and the occurrence of the other European alien sciaenid M. undulatus, reported in Belgian waters, is likely to be due to transportation in ship ballast water (Stevens et al. 2004) as well.

Aquaculture is the other main vector of introduction of alien species and is the reported pathway of the Atlantic American scianid Sciaenops ocellatus (Linnaeus, 1766) on the western Taiwanese coast (Liao et al. 2010). However, as far as we know, the culture of weakfish is only in initial phases at global level, and there is no culture of this species in Spanish waters. Other possibilities of its introduction are very unlikely and were not taken into account. Therefore, as there is no culture data available, introduction by ballast water seems to be the most plausible introduction pathway.

All data suggest that weakfish has survived well and has successfully established populations in the Guadalquivir estuary. The species has been present in the area for at least the last five years. It is very abundant and now forms part of the bycatch of coastal fishers, with declared catches of more than one tonne during three months (Table 2). The population is composed of different year classes and most records are larger than the estimated mean length at first maturity, established at age1 and 164 mm TL for males and 170 mm TL for females (Lowerre-Barbieri et al. 1996), so they would be able to spawn.

Introduction and spreading of alien species are considered one of the main threats to biodiversity. Invasive species alter ecosystems, displace native species, change community structures and food webs, and alter fundamental processes, such as nutrient cycling and sedimentation (Molnar et al. 2008). In the case of fish species, decline of commercial stocks due to direct predation or competition for resources (food or space) is the presumed mechanism of negative impact (Katsanevakis et al. 2014). Therefore, the impact of C. regalis in the local ecosystem should be studied in detail as soon as possible.