New extension range and complementary description of Eurytemora americana (Calanoida: Temoridae) in northern Korea

The brackish calanoid, Eurytemora americana Williams, 1906, is reported in the northern part of Korea. It is found to be an important copepod in the mesozooplankton community structure from the estuarine and shallow waters. The range extension and of E. americana is reported additionally to previously known distributional range from brackish waters of the South Atlantic Ocean, United Kingdom, Canada, English Channel, Iceland, the North Atlantic Ocean, the North Pacific Ocean (Japan, Korea, and Russia), and the North Sea. In this study, both sexes of E. americana are redescribed and illustrated including their undescribed mouthparts. The most important characters that have been overlooked in the previous descriptions are: shape of the last pedigerous somite in female, structure of the genital double-somite, and ornamentation of the fifth leg in both sexes. The distributional range of E. americana is now given with its northern and southern hemisphere limit as 70 to 37°N in the North Pacific and 38°S in the Southwest Atlantic.


Background
Most of the species belonging to the family Eurytemora Giesbrecht, 1881 are distributed in both, northern and southern hemispheres and about 30 species have so far been described (Boxshall and Halsey 2004;Alekseev and Souissi 2011;Sukhikh and Alekseev 2013). The genus has hitherto been recorded exclusively from subtropical to subarctic, but with the increasing species diversity toward northern latitudes (Dodson et al. 2010). Most of the Eurytemora are characterized by a wide habitat distribution ranging from the open ocean waters, coastal marine, hypersaline salt marshes, and brackish estuaries, to completely freshwaters (Heron and Damkaer 1976;Lee 2000;Dodson et al. 2010).
Two of Eurytemora species E. composita Keiser, 1929 andE. pacifica Sato, 1913 have so far been recorded from Korea (Lee et al. 2007;Soh et al. 2013) and E. pacifica has known to be a dominant species in estuarine to shallow waters of Korea (Park et al. 2002;Moon et al. 2006Moon et al. , 2012.
We have conducted the faunistic studies on the calanoids in eastern estuarine and shallow waters of Korea from May to June 2011. In the materials collected of the studied area, the predominant species was E. americana Williams, 1906, it co-occurred with E. pacifica Sato, 1913 In the current re-examination of the obtained materials, we have found that E. americana was erroneously identified as E. pacifica, while E. pacifica was actually rare in these waters. The complete redescription of E. americana from the eastern Korea estuarine and shallow waters is presented herein to clarify the identification of the northern Korean calanoid reported as E. pacifica to avoid possible misunderstanding of the identity of the species.

Materials and methods
The specimens were collected from the estuarine and coastal waters of eastern Korea (Table 1, Fig. 1) using a plankton net (100 and 200 μm mesh). The samples were preserved immediately in 5 % buffered formalin/seawater solution. Individuals of the species were sorted out from the samples. Before dissection and microscopic observation, specimen was immersed in 70 % lactic acid for about an hour. Dissection of specimen was done using reverse slide method (Humes and Gooding 1964). The descriptive terminology follows Huys and Boxshall (1991). Abbreviations used in the text and figures: A1, antennule; ae, aesthetasc. All voucher specimens are deposited at the National Institute of Biological Resources (NIBR), Incheon, Korea.

Discussion
The original description of E. americana was brief with inadequate figures for the identification (Williams 1906). Some morphological features, that remain overlooked or undescribed still (Williams 1906;Willey 1923;Gurney 1933;Heron 1964;Kos 1977;Brylinski 2009) are redescribed in details in the present study for the both sexes of E. americana.
The following characters are proposed as diagnostic for female: (1) a symmetrical, lateral margins slightly constricted at midlength of the genital double-somite and (2) presence of patched spinules on the dorsal surface of the anal somite and caudal rami; and for the male: (1) a swelling on the inner margin of the right P5 basis; (2) the right distal segment of left P5 with falcate and double broad-ended apical portion; (3) presence of two setae on the dorsal surface of the coxa in right P5; (4) presence of two setae on the basis of right P5; (5) presence of five setae on the first exopodal segment of male right P5; (6) presence of two setae on the basis of left P5; and (7) presence of six setae on the first exopodal segment of left P5. The specimens herein identified as E. americana correspond well to the description of Heron (1964) and their undescribed mouth parts are illustrated in details for the first time.
Eurytemora americana usually co-occur with the other neritic calanoids, Acartia omorii Bradford, 1976, Centropages abdominalis Sato, 1913, E. pacifica, and Paracalanus intermedius Shen and Bay, 1956 as it was revealed during the biodiversity survey aimed at populating a distributional database on copepods in the eastern estuarine and shallow waters of Korea. A dense population of E. americana have been found in the estuary of Jeoncheon Stream, near by the Donghae harbor in the eastern Korea (salinity 25 psu, 5 May 2011).
Eurytemora americana has so far been known as a species widely distributed in the northern hemisphere, in the cold and temperate latitudes of North America (Williams 1906;Heron 1964), northern Europe (Gurney  (Kos 1977;Borutzky et al. 1991), and of Argentina (Hoffmeyer et al. 2000) in the southern hemisphere. This record extends to the known latitudinal distribution of this species southward into the temperate zone of the northern Korea (37°39'N), which was the previous southern border of its known distributional range (see Fig. 6). This species is a brackish copepod native to the northern hemisphere (Kos 1977), but was accidentally introduced into the Argentina estuary with ballast water of ships coming to the harbour zone from the northern hemisphere locations (Hoffmeyer et al. 2000) and ultimately contributed to the establishment of invasion corridors (Ricciardi and MacIssac 2000;Drake and Lodge 2004). This species also has been reported several times as an invader species in Europe (Brylinski 2009;Noël 2011) through the ballast water (Brylinski 2009;Berasategui et al. 2009).
We report this particular species from the estuarine and coastal waters in the northern part of Korea for the first time. There are three possible explanations of this discovery: (1) insufficient studies of the northern Korean estuaries; (2) probable misidentification in previous studies; and (3) recent invasion of the species in the northern Korean waters. Since, it is a first record of occurrence in northern Korean waters, a detailed knowledge on its taxonomy and molecular variability imperative.

Conclusions
The present paper described for the complementary redescription of E. americana based on adult specimens collected from the estuarine and coastal waters of eastern Korea. When compared with descriptions and illustrations of earlier authors, they overlooked the shape of the last pedigerous somite in female, structure of the genital double-somite, and ornamentation of the fifth leg in both sexes. The expansion of the geographical distribution of E. americana is now given with its northern and southern hemisphere limit as 70 to 37°N in the North Pacific and 38°S in the Southwest Atlantic.