Journal 
INTRODUCTION
The genus Navicula Bory s.l. is traditionally a single genus comprising over 10,000 species. It is the largest group in the diatom classification system. The majority of the genus is composed of many groups with a variety of valve morphologies (Round et al. 1990; Lange-Bertalot 2001; Kociolek and Spaulding 2003). Many genera have been separated from Navicula and now comprise new genera.
The genus Navicula s.s. is still a diverse group. It inhabits a broad range of fresh and inland waters, and also coastal brackish waters. The genus Craticula features elongate and typical naviculoid valve morphology. It has a peculiar morphology consisting of a series of parallel transverse bars in internal valves that visually resemble a small gridiron (a ‘craticular’ structure). The Craticula was named by A. Grunow in 1867, while being included in the genus Navicula, section Orthostichae. The genus name was resurrected by Mann and Stickle (1991).
Navicula and Craticula are representative naviculoid diatoms in epipelic and periphytic habitats in fresh- and brackish water (Round et al. 1990). This study was undertaken to discover and characterize previously unrecorded species in the coastal regions on the west coast of Korea, with the aim of enhancing biodiversity. There are an estimated 1,390 diatom species in Korean waters, according to a biodiversity summary compiled over 20 years ago (Kim 1994). Many floristic studies of diatoms have been conducted in fresh-, brackish and marine waters, with an abundant number of diatoms identified. However, taxonomic studies have been relatively insufficient.
The National Institute of Biological Resources (NIBR) under the Ministry of Environment of Korea was established in 2006. Investigation of native diatom species diversity commenced at that time (The Project on Survey and Excavation of Korean Indigenous Species), and related research has burgeoned since then. These national projects have focused on discovering species not yet described or recorded to Korea, and publishing morphological descriptions of diatom species native to Korea.
In the 10 years since the start of the NIBR research, the number of diatom species has reached approximately 2,000 (NIBR 2015). More than 600 species have been added, likely because of the national research initiatives. Among the species already reported, many taxa are simply recorded on the species list of diatoms, with no taxonomic description or authorization. Marine diatoms comprise about 800 of the 2,000 species (NIBR 2015). Marine diatoms have been studied extensively for plankton, with relatively little research on benthic diatoms attached to sediment or substrates.
On the other hand, there has been considerable change in diatom taxonomy and classification at the species and genus levels (Mann 1999). The broad morphological variation between and within species have been modified into the narrow species concept, and a number of species have been separated and revised from existing species. The approximately 20,000 known species of diatoms worldwide is likely only about ten percent of less of the actual number of diatom species (Mann and Droop 1996). Thus, a survey of diatoms in coastal waters of Korea would be expected to yield more species.
The aim of this study is to identify and describe species not yet recorded in Korea. The study was conducted in the waters near three islands off the western coast of mainland Korea. The diatom taxa were confined to the naviculoid group. Their morphological characteristics were compared with neighboring species.
MATERIALS AND METHODS
Diatoms were collected during May and September, 2015, from the waters of Wido Island in Buan, Wonsando Island in Boryeong and Dekjeokdo Island in Incheon. Various attached substrates were chosen. They included bottom sediment, stones, macrophytes and even bryophytes from streams and rice paddies located in coastal areas. These areas are affected by sea water, such as high tides. Epiphytic diatoms were scraped off of macrophytes, diatoms attached to stones were dislodged using a tooth-brush and bottom sediment was collected using a small (3.0 cm diameter) tube. Planktonic and benthic diatoms were collected from the suspended material in swirled rice paddy water.
After sampling, organic matter was oxidized with nitric acid and potassium dichromate on a hot sand bath (APHA 1995) and rinsed with distilled water until a neutral condition was attained. Five permanent slides for each sample were mounted using Pleurax as the resin medium.
All species names and authorities were revised. In certain cases, nomenclatural updates were made, confirming all of the currently accepted taxonomic names. To ascertain taxonomic identities and their synonymies, many references are consulted including Krammer and Lange-Bertalot (1986), Witkowski et al. (2000) and Lange-Bertalot (2001) and three databases were used: Algaebase (www.algaebase.org, Guiry and Guiry 2017), Catalogue of Diatom Names of California Academy of Science (http://researcharchive.calacademy. org/research/diatoms/names/index.asp) (Foutainer and Kociolek 2017) and Diatoms of the United States (Spaulding et al. 2010).
RESULTS AND DISCUSSION
Many diatom species belonging to the genera Navicula and Craticula were found in the survey from three islands in the western coast of Korea, fifteen diatom taxa are presented in this monograph. The morphological features of each species are described with other ecological information. We present features that include the basic morphology of frustules for local specimens, valve outlines and sizes, the forms of raphe, axial area and central area and the strial numbers in 10 μm. The dimensions of diatom valves are given for local specimens. The taxonomic authorities of diatom species are added to each description as well. The figures of plates are presented at ×2000 magnification.
Genus Navicula Bory 1822
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1. Navicula alineae Lange-Bertalot in Tsarenko, Lange-Bertalot, Stupia and Wasser 2000 (Pl. 2, Figs. 7-11) (Tsarenko et al. 2000, p. 268, Pl. 22, Figs. 175- 178; Lange-Bertalot 2001, p. 82, Pl. 33, Figs. 28-32, Pl. 70, Fig. 6)
Valves lanceolate in outline, ends of the valve weakly produced and finally obtusely rounded, 29.5-51 μm long and 7.5-9.8 μm broad. Raphe filiform, central ends of the raphe curved to the primary side of the valve, axial area very narrow, central area large and longitudinally asymmetrical. Transapical striae strongly radiate in the middle, but abruptly parallel towards the ends, with 11-12 rows in 10 μm.
Habitats include oligohalobious, mesotrabentic to eutraphentic localities, and alkaliphilous in high electrolyte waters (Lange-Bertalot 2001). The species occurred rarely in a rice paddy of Wido Island in Buan, and is newly recorded to Korea. Navicula alineae has more slender valves and denser striae than N. viridula (Kützing) Ehrenberg (Lange-Bertalot 2001).
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2. Navicula ammophilaGrunow 1882 (Pl. 1, Figs. 21-29) (Krammer and Lange-Bertalot 1991, p. 392, Pl. 76, Figs. 14, 15)
Valves lanceolate to linear-lanceolate in outline, ends of the valve weakly produced and finally acute, 26.5-42 μm long and 5.5-7.5 μm broad. Raphe straight, central ends of the raphe close together, axial area very narrow, central area moderately elliptical. Transapical striae radiate in the middle, but parallel and convergent towards the ends, 14- 15 rows in 10 μm.
This was reported as an important benthic diatom in in tertidal and estuarine areas (Cognie et al. 2001). The species occurred in brackish waters of Wonsan Island in Boryeong, and is newly recorded to Korea.
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3. Navicula cariocincta Lange-Bertalot in Tsarenko, Lange-Bertalot, Stupina and Wasser 2000 (Pl. 1, Figs. 17-20) (Tsarenko et al. 2000, p. 271, Pl. 23, Figs. 203-207; Lange-Bertalot 2001, p. 23, Pl. 11, Figs. 21-30)
Synonym:Navicula cari auct. partim non Ehrenberg.
Valves linear-lanceolate in outline, ends of the valve cuneate, 32-44 μm long and 6-7 μm broad. Raphe filiform, central ends of the raphe curved in one direction, axial area narrow and more or less linear, central area moderately rectangular to elliptical with irregular shortened striae. Transapical striae strongly radiate in the middle, but parallel or convergent towards the ends, with 11-12 rows in 10 μm.
This is widespread in temperate and tropical regions, and is mesohalobious (Lange-Bertalot 2001). The species occurred frequently in streams, near the coast of Wido Island in Buan, and is newly recorded to Korea.
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4. Navicula elegantoidesHustedt 1942a (Pl. 2, Fig. 16) (Hustedt 1942a, p. 76, Fig. 142; Simonsen 1987, p. 277, Pl. 411, Figs. 1-4, Pl. 412, Figs. 1-4)
Valves broadly elliptical in outline, ends of the valve obtusely rounded, 80 μm long and 24 μm broad. Raphe straight, the central ends of the raphe largely expanded, axial area broadly lanceolate, central area largely circular to elliptical. Transapical striae broad and radiate in the middle, but parallel or convergent towards the ends, with 6 rows in 10 μm.
The type locality is Laguna de Bay, a large freshwater lake, on Luzon Island in the Philippines. The species is mainly found in Asia, Philippine, Malaysia, China and Japan, and in Australia (John 1983). This was found in streams in Wido Island in Buan and in Wosando in Boryeong, and is newly recorded to Korea.
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5. Navicula longicephalaHustedt 1944 (Pl. 1, Figs. 1-4) (Hustedt 1944, p. 277, Fig. 17; Simonsen 1987, p. 316, Pl. 474, Figs. 6-10; Krammer and Lange-Bertalot 1991, p. 384, Pl. 68, Figs. 31-33; Lange-Bertalot 2001, p. 45, Pl. 32, Figs. 41-47)
Valves narrow linear-lanceolate in outline, ends of the valve produced and rostrate to capitate, 18-20 μm long and 3.5-4.2 μm broad. Raphe filiform, central ends of the raphe close together, axial area very narrow, central area small, but variable from lanceolate to rectangular. Transapical striae strongly radiate in the middle, but parallel and convergent towards the ends, with 18-20 rows in 10 μm.
The species is widespread from fresh to brackish waters. It is eutraphentic and prefers a moderate electrolyte condition (Lange-Bertalot 2001). The species occurred in streams and rice paddies near the coasts of Wido Island in Buan, and is newly reported to Korea.
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6. Navicula normaloidesCholnoky 1968 (Pl. 1, Figs. 13-16) (Cholnoky 1968, p. 55, Figs. 76-86; Lange- Bertalot 2001, p. 322, Pl. 43, Figs. 16-21)
Valves lanceolate to linear-lanceolate in outline, ends of the valve not protracted and obtusely rounded, 20-32 μm long and 6-6.5 μm broad. Raphe filiform and straight, central ends of the raphe expanded and not curved, axial area narrow and linear, central area small and narrow lanceolate, not distinct. Transapical striae slightly radiate in the middle, but parallel to convergent towards the ends, with 12-14 rows in 10 μm.
Type locality is Santa Lucia, an estuarine lake, in Natal, South Africa. The habitat is the tidal area (Cholnoky 1968). The species occurred rarely as epilithons and epiphytons in streams of the coast in Wido Island in Buan, and is newly reported to Korea.
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7. Navicula notha Wallace 1960 (Pl. 1, Figs. 5, 6) (Krammer and Lange-Bertalot 1991, p. 388, Pl. 70, Figs. 15-24; Lange-Bertalot 2001, p. 80, Pl. 40, Figs. 16-28, Pl. 65, Fig. 7)
Valves narrowly lanceolate in outline, ends of the valve weakly protracted and acute to obtusely rounded, 30 μm long and 5 μm broad. Raphe filiform and straight, central ends of the raphe slightly curved, axial area very narrow and linear, central area small and inconspicuous. Transapical striae strongly radiate in the middle, but parallel to convergent towards the ends, 14 rows in 10 μm.Plate 1
This is widely distributed throughout various continents, preferring oligotrophic waters with poor electrolyte (Lange- Bertalot 2001). The species occurred rarely in streams of the coast in Wido Island in Buan, and is newly reported to Korea.
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8. Navicula novaesibericaLange-Bertalot 1993 (Pl. 2, Figs. 1-4) (Krammer and Lange-Bertalot 1991, p. 380, Pl. 66, Figs. 5-8; Lange-Bertalot 1993, p. 126, Pl. 61, Figs. 1-8, Pl. 66, Figs. 5-8; Lange-Bertalot 2001, p. 89, Pl. 36, Figs. 10-14)
Valves elliptic-lanceolate in outline, ends of the valve abruptly tapering and obtusely rounded, 29-34 μm long and 8 μm broad. Raphe filiform, central ends of the raphe curved to the primary side of the valve, axial area very narrow, central area large, circular to rectangular and longitudinally asymmetrical. Transapical striae strongly radiate in the middle, but abruptly parallel towards the ends, with 10- 12 rows in 10 μm.
Habitats is freshwaters of Siberia, South Germany and France. It is eutrophentic with a preference for average electrolyte condition (Lange-Bertalot 2001). The species occurred rarely as epiphytic diatoms in Wido Island of Buan, and is newly reported to Korea.
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9. Navicula pseudosalinarioidesGiffen 1975 (Pl. 1, Figs. 7-12) (Witkowski et al. 2000, p. 300, Pl. 130, Figs. 20-22)
This is probably known only from the type locality of South Africa (Giffen 1975). The species was already reported from the estuarine sediments of the Nakdong River in Korea (Joh 2013). Presently, it was moderate abundant in fresh and brackish waters of Wido Island, Wonsando Island, Deojeokdo Island. The cellular dimensions of the local specimens were measured. Valves are 13-24 μm long, 5- 6.3 μm broad and transapical striae 15-20 rows in 10 μm.
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10. Navicula riediana Lange-Bertalot & U Rumrich in Rumrich, Lange-Bertalot and Rumrich 2000 (Pl. 2, Figs. 5, 6) (Rumrich et al. 2000, p. 169, Pl. 43, Figs. 9-14; Lange-Bertalot 2001, p. 90, Pl. 34, Figs. 1-7, Pl. 70, Fig. 5)
Valves moderately narrow linear-lanceolate in outline, ends of the valve shortly protracted and finally obtusely rounded, 31-36.5 μm long and 7 μm broad. Raphe filiform, central ends of the raphe distinctly curved to the primary side of the valve, axial area very narrow, central area large, rectangular or elliptical. Transapical striae strongly radiate in the middle, but abruptly parallel towards the ends, with 12-13 rows in 10 μm.
This was known from South America and New Caldonia. It has not been found in European waters. It is eutrophentic, preferring an average electrolyte environment (Lange-Bertalot 2001). The species occurred rarely in rice paddies of Wido Island, and is newly reported to Korea. Navicula riediana has more slender valves than N. novaesiberica, more denser striae and lineolae and distinctly deflected central pores (Lange-Bertalot 2001).
Genus CraticulaGrunow 1867
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11. Craticula ambigua (Ehrenberg) D.G. Mann in Round, Crawford and Mann 1990 (Pl. 3, Figs. 10, 11) (Lange-Bertalot 2001, p. 109, Pl. 82, Figs. 4-8, Pl. 83, Figs. 3, 4, Pl. 84, Figs. 1-10, Pl. 86, Figs. 3, 4)
Basionym:Navicula ambigua Ehremberg 1843 (Ehrenberg 1843, p. 417, Pl. 2/2, Fig. 9).
Synonym:Navicula cuspidata var. ambigua (Ehrenberg) Cleve 1894 (Cleve 1894, p. 110) Navicula halophila f. robustaHustedt 1959 (Hustedt 1959, p. 401, Figs. 1-3).
This species occurred very rarely in rice paddies near the coasts of Wido Island. Morphological characters of specimens collected in local areas were measured. Valves are 71.5-81 μm long, 18-21 μm broad, and transapical striae throughout the valve comprise 16-17 rows in 10 μm.
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12. Craticula buderi (Hustedt) Lange-Bertalot in Rumrich, Lange-Bertalot and Rumrich 2000 (Pl. 2, Figs. 12-15) (Rumrich et al. 2000, p. 101; Lange- Bertalot 2001, p. 110, Pl. 90, Figs. 1-27, 91, Figs. 15-20)
Basionym:Navicula buderiHustedt 1954 (Hustedt 1954, p. 279, Figs. 11-15; Simonsen 1987, p. 401, Pl. 599, Figs. 4-6).
Synonym:Navicula simplex Krasske in Hustedt 1930 (Hustedt 1930, p. 296, Fig. 500); Navicula pseudohalophilaCholnoky 1960 (Cholnoky 1960, p. 74, Figs. 231-235); Navicula adsiduaArchibald 1971 (Archibald 1971, p. 34, Figs. 49-51).Plate 2
Valves broadly elliptical, elliptic-lanceolate in outline, ends of the valve broadly rounded or weakly protracted, 24 μm long and 6.5 μm broad. Raphe filiform and straight, central ends of the raphe distant, axial area very narrow and linear, central area small circular and inconspicous. Transapical striae parallel in the middle, slightly convergent towards the ends, with 19 rows in 10 μm.
This is widespread in freshwaters with average to higher electrolytes (Lange-Bertalot 2001). The species occurred in rice paddies near the coasts of Wido Island, and is newly recorded to Korea.
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13. Craticula halophila (Grunow) D.G. Mann in Round, Crawford and Mann 1990 (Pl. 3, Figs. 8, 9) (Lange-Bertalot 2001, p. 114, Pl. 89, Figs. 1-7)
Basionym:Navicula cuspidata var. halophila Grunow in Van Heurck 1885 (Van Heurck 1885, p. 100, Suppl. Pl. B, Fig. 30).
Synonym:Navicula halophila (Grunow) Cleve 1894 (Cleve 1894, p. 109) Navicula halophila f. robustaHustedt 1959 (Hustedt 1959, p. 401, Figs. 1-3).
Valves broadly elliptical, rhombic-lanceolate in outline, ends of the valve slightly protracted and acute to obtuse, 46.5-54 μm long and 13-14 μm broad. Raphe filiform and straight, central ends of the raphe distant, axial area distinct, very narrow and linear, central area not developed. Transapical striae more or less parallel throughout the valve, with 16 rows in 10 μm.
This is common in freshwaters with average to higher electrolytes, but less frequent in brackish waters (Lange- Bertalot 2001). The species occurred very rarely in rice paddies near the coasts of Wido Island in Buan, and is newly recorded to Korea.
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14. Craticula riparia (Hustedt) Lange-Bertalot 1993 (Pl. 3, Figs. 1-3) (Lange-Bertalot 1993, p. 14, Pl. 70, Figs. 1-8, Pl. 71, Figs. 1-5; Lange-Bertalot 2001, p. 117, Pl. 92, Figs. 1-8)
Basionym:Navicula ripariaHustedt 1942b (Hustedt 1942b, p. 52, Figs. 77, 78).
Valves lanceolate in outline, ends of the valve protracted and rostrate to subcapitate, 37.5-47 μm long, 8.5-10 μm broad. Raphe filiform and straight, central ends of the raphe somewhat distant, axial area very narrow and linear, central area small, but inconspicuous. Transapical striae parallel or weakly radiate in the middle, slightly convergent towards the ends, with 16-18 rows in 10 μm.
This is widespread in weakly to moderately acidic and rich electrolyte waters (Lange-Bertalot 2001). The species occurred very rarely in rice paddies near the coasts of Wido Island in Buan, and is newly recorded to Korea.
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15. Craticula riparia var. mollenhaueriLange-Bertalot 1993 (Pl. 3, Figs. 4-7) (Lange-Bertalot 1993, p. 14-15, Pl. 70, Figs. 10-13; Lange-Bertalot 2001, p. 117, Pl. 92, Figs. 9-12)
The morphology of the variety is same with the nominate variety. Valves 31 μm long, 6.5-7 μm broad, Transapical striae 18-19 rows in 10 μm.
The variety differs with the nominate variety in valve size and strial density, and less than 35 μm in the length of the valve. Distributed in more or less acidic and poor electrolyte waters (Lange-Bertalot 2001). The species occurred very rarely in rice paddies near the coasts of Wido Island in Buan, and is newly recorded to Korea.
This study presents the taxonomic description of 15 diatom species. Of these, 13 are recorded for the first time in Korea. These all species belong to the old genus Navicula (Navicula s.l.); Navicula is divided into 18 subgroups (Krammer and Lange-Bertalot 1986). The species containing in this document belong to two subgroups of the genus: Lineolata and Fusiformes. The genus Navicula is recognized historically as a heterogeneous group, and the subgroups nearly comprise new genera (Mann and Droop 1996). Fusiformes division of Navicula was resurrected to the genus Craticula, which has not been used for a long time.
The samples for this study were collected from freshwater joining the coast and tidal flats, and mainly periphytic and benthic materials in fresh and brackish waters. A majority of the presently described diatom taxa were infrequently observed, and likely are low in abundance.Plate 3
The present descriptions enhance the species diversity or richness of diatom in this country. Many other diatom taxa remained unrecorded or under-described, especially in the genus Nitzschia. This data highlights the need of more floristic studies to properly clarify the diatom diversity of the waters near islands off the west coast of Korea. Furthermore, this region features vast (up to 10 km wide) open intertidal mudflats from the southeast to the northwest. Approximately 3,000 small islands are scattered over the flats (Deppe 2000). The sediment flats show remarkable abundance and richness of benthic diatoms (Smith and Underwood 1998).
