The morphology, taxonomy, and distribution of species belonging to the diatom family Rhizosoleniaceae were stud-ied from the marine coastal waters of Korea. Rhizosolenid diatom taxa were collected at 30 sites from September 2008 to February 2010 and were analyzed by light and scanning electron microscopy. We identified 6 rhizosolenid genera, including Rhizosolenia, Proboscia, Pseudosolenia, Neocalyptrella, Guinardia , and Dactyliosolen . We describe 5 genera in this study, except Rhizosolenia . Five genera were compared in detail with congeneric species. Six genera within the fam-ily Rhizosoleniaceae were divided into two groups based on morphological diagnostic characters including valve shape, areolae pattern, the shape of external process, and girdle segments in the column. The first group had a conoidal valve and loculate areolae, which comprised Rhizosolenia, Proboscia, Pseudosolenia , and Neocalyptrella , and the second group of Guinardia and Dactyliosolen showed a flat or rounded valve and poroid areolae. Important key diagnostic characters were based on valve shape, areolae pattern on the segment, external process, position of the tube, and the valve margin. D. phuketensis was new to Korean coastal waters. Peragallo (1892) regarded the genera Dactyliosolen Castracane, Lauderia Cleve, Attheya T. West, Guinardia H. Peragallo, and Rhizosolenia Brightwell as members of Rhizosoléniées. Thereafter, Hustedt (1930) suggested that 23 marine and 4 freshwater species belonged to this family. He synonymized several species, as several ear-lier studies described variations of seemingly the same species. Sundström (1986) suggested that only those species with valves bearing an external process, otaria, claspers, and copulae perforated by loculate areolae should be included in the genus Rhizosolenia . According to Sund-ström (1986), the genus Proboscia Sundström and Pseu-dosolenia Sundström were separated from Rhizosolenia because they have external processes. The two species of R. calcar-avis Schultze and R. alata Brightwell were subsequently transferred to Pseudosolenia calcar-avis Sundström and Proboscia alata Sundström, respectively (Sundström 1986). R. robusta Norman was also trans-ferred to Neocalyptrella robusta Hernández-Becerril and Meave (Hernández-Becerril and Meave del Castillo 1996, 1997). More recently, the family Rhizosoleniaceae included Neocalyptrella, Pseudosolenia, Proboscia , and Urosolenia . Representatives of these genera are commonly found as solitary cells in marine environments, except Urosolenia , which is restricted to freshwater (Edlund and Stoermer 1993, Rott et al. 2006, Li et al. 2009). The genera Guinardia and Dactyliosolen have been allocated to the family Rhi-zosoleniaceae. In Korea, the genera Rhizosolenia, Guinardia , and Dac-tyliosolen were recorded by Shim (1994). Many authors (Moon and Choi 1991, Yoon et al. 1992, Chang and Shim 1993, Kim et al. 1993, Yoon and Koh 1994, 1995) and Lee (1995) added Pseudosolenia and Proboscia to his check-list. Additional studies on the family Rhizosoleniaceae in Korea have been conducted sporadically (Yun and Lee 2010, Yun et al. 2011), but species identification, synon-ymies, and the phylogeny the family Rhizosoleniaceae have been insufficiently investigated. The present study provides a detailed survey of marine diatoms belonging to the genera Proboscia, Pseudosolenia, Neocalyptrella, Guinardia , and Dactyliosolen from the coastal waters of Korea. This survey provides detailed light and scanning electron microscopy illustrations and a critical review of the taxonomical and distributional data. Field samples were collected in Korean coastal waters from September 2008 to February 2010 ( Table 1 ). Phyto-plankton was collected using a 20 μm mesh-sized net by vertical towing. Samples were immediately fixed in neu-tralized formalin (final concentration 4%), glutaraldehyde (final concentration 2%), and Lugol’s solution. Organic material in the samples was removed using the methods of Hasle and Fryxell (1970) and Simonsen (1974). The ma-terials were examined under a light microscope (Axios-kop 40; Carl Zeiss, Jena, Germany), photographed with a MRc5 camera (Carl Zeiss) and a scanning electron micro-scope (JSM-5600LV; Jeol, Tokyo, Japan). Sizes of cells were measured using image calculation software (AxioVision AC v. 4.5; Carl Zeiss). Terminology was from that recommended in the first report of the working Committee on Diatom Terminol-ogy (Anonymous 1975) from the third Symposium on Recent and Fossil Marine Diatoms, Kiel. Other terminol-ogy follows Ross et al. (1979), Sundström (1986), Round et al. (1990), Hernández-Becerril (1995), and Hasle and Syvertsen (1996). We identified 6 rhizosolenid genera, including Rhizo-solenia, Proboscia, Pseudosolenia, Neocalyptrella, Guinar-dia , and Dactyliosolen . We described five of these genera except Rhizosolenia . The morphological characters ob-served in the genera Proboscia, Neocalyptrella, Pseudoso-lenia, Dactyliosolen , and Guinardia species are shown in Table. 2 - 4 . According to the system suggested by Sund-ström (1986), 9 phytoplanktonic diatom taxa represent-ing 1 order, 1 suborder, 1 family, 5 genera, and 9 species were identified in this study. The systematic accounts are as follows: Class Bacillariophyceae Haeckel 1878 Brightwell 1858, p. 95, Pl. 5, Fig. 8; Peragallo 1892, p. 115, Pl. 18, Fig. 11-20; Hustedt 1920, Pl. 317; Hustedt 1930, p. 600, Fig. 345; Cupp 1943, p. 90, Fig. 52A & B; Oku-no 1952, p. 353, Pl. 2, Fig. 5 & 6; Okuno 1960, p. 310, Pl. 1, Fig. 1; Hendey 1964, p. 146, Pl. 2, Fig. 2; Drebes 1974, p. 57, Fig. 39c & d; Navarro 1981, p. 430, Fig. 33 & 34 as R. alata; Sundström 1986, p. 99, Fig. 258-266; Jordan et al. 1991, p. 65, Fig. 1-9; Takahashi et al. 1994, p. 413, Fig. 2-7; Hernández-Becerril 1995, p. 252, Fig. 2-4; Hasle and Syvertsen 1996, p. 159, Pl. 30; Sunesen and Sar 2007, p. 639, Fig. 82-88 & 98. Synonyms . Rhizosolenia alata Brightwell 1858, Rhizo-solenia alata f. gracillima (Cleve) Gran 1905. Cells are solitary or in pairs, narrow cylindrical, bilat-erally symmetrical, 3.3-13.3 μm in diameter, 270.0-485.7 μm long. Valve is sub-conoidal, the ventral part longer than the dorsal part and proboscis structure is slightly curved, tapering towards the apical part of the valve, cir-cular in cross section, 15.0-30.0 μm long. Apical surface of the proboscis is composed of variously sized spinules. Number of spinules is 7-16, 0.1-0.4 μm long. Contiguous area is convex towards the valve surface, distally limited by asymmetric claspers. The valve areolae are rounded, 52-90 in 10 μm, arranged in longitudinal striae, converg- ing towards the apex. Girdle segment areolae are loculate, arranged in columns, with the external velum perforated by central pores, and internal circular foramina, 25-62 in 10 μm. Interlocular pores are commonly surrounded by six loculi. Segment horizontal axis and perpendicular axis are 3.3-13.3 and 10.0-26.7 μm long, respectively. Distribution . Proboscia alata has frequently been re-ported in the Argentine Sea (Ferrario and Galávan 1989 as R. alata and R. alata f. gracillima ). During this study, P. alata was frequently observed in September 2008 and June 2009 at the Wolsung coast, Jeju Island, and the Korea Strait. Remarks . Sundström (1986) did not share the biogeo-graphical limits of P. alata because synonyms were used for probably all taxa included in the genus. The specific limits of the taxa including Proboscia have been deter-mined by many authors (Jordan and Priddle 1991, Jordan et al. 1991, Takahashi et al. 1994, Jordan and Saito 1999, Jordan and Ito 2002, Jordan and Ligowski 2004, 2006). Takahashi et al. (1994) reported that the genus contains five modern species distributed from polar to temperate regions. Jordan and Ligowski (2004) stated that P. alata is not cosmopolitan, because it appears to be a com-plex cryptic species. Some P. alata representatives are commonly found in polar waters. However, Hernández-Becerril (1995) found that P. alata is distributed from tropical to subtropical waters. Hustedt 1930, p. 602, Fig. 346; Cupp 1943, p. 93, Fig. 52C; Hendey 1964, p. 147, Pl. 2, Fig. 4; Hernández-Becerril 1995, p. 254, Fig. 5 & 6; Moreno et al. 1996, p. 15, Pl. 29, Fig. 6 & 7; Jordan and Ligowski 2004, p. 98, Pl. 4, Fig. 5-7; Gómez and Souissi 2007, p. 287, Fig. 4g-h; Sunesen and Sar 2007, p. 639, Fig. 89-97 & 99. Synonyms . Rhizosolenia indica H. Peragallo 1892, Rhi-zosolenia alata f. indica (Peragallo) Gran 1905. Cells are solitary or in pairs, cylindrical, bilaterally symmetrical, 25.0-125.0 μm in diameter, 193.4-764.2 μm long. Valve is sub-conoidal, the ventral part longer than the dorsal part. Proboscis structure is strongly curved, ta-pered towards the apical part in the valve, circular in cross section, 39.0-83.3 μm long. Apical surface of the proboscis is composed of varied sized spinules and the slit is pore shape situated below the apex. Spinule number is 8-13 and 0.2-0.5 μm long. Contiguous area is convex towards the valve surface, distally limited by asymmetric claspers. The valve areolae are rounded, 30-60 in 10 μm, arranged in longitudinal striae, converging towards the apex. Gir-dle segment areolae are loculate, arranged in columns, and the external velum is perforated by central pores and internal circular foramina. Interlocular pores are com-monly surrounded by four loculi. The horizontal axis of the segments is 25.0-125.0 μm and the perpendicular axis is 10.6-16.0 μm. Distribution . Hendey (1964) reported that P. indica is common in temperate and sub-tropical seas as R. alata var. indica . This species has been reported from Buenos Aires marine waters (Marques Da Cunha and Da Fonseca 1917, Balech 1964, 1971, 1979, Lange 1985 as R. alata var. indica ). During the present study, P. indica was rare but distributed widely at Geoje Island, Daebu Island, Dae-cheon Harbor, the Yangyang coast, and the Yellow Sea from September 2008 to October 2009. Remarks . Proboscia indica and P. alata are fairly simi-lar species. However, P. indica differs from P. alata by the larger diameter of the frustule and valve morphology. The valve shape of P. indica is sub-conical, round, and tapers into a strongly curved proboscis. Additionally, the pat-tern of the interlocular pores also differs between the two species, similar to a feature previously described by Hasle (1975) and Takano (1990). P. indica have interlocular pores surrounded by four loculae, but the P. alata inter-locular pores are surrounded by six loculae, as observed by Sundström (1986). Pritchard 1861, p. 866, Pl. 8, Fig. 42; Peragallo 1892, p. 109, Pl. 14, Fig. 1; Hustedt 1920, Pl. 320, Fig. 1-3; Hustedt 1930, p. 578, Fig. 330; Cupp 1943, p. 83, Fig. 46; Okuno 1957, p. 105, Pl. 2, Fig. 1; Okuno 1968, Fig. 1(6), 10A, 17 & 18; Navarro 1981, p. 430, Fig. 43 & 45; Sundström 1986, p. 104, Fig. 289 & 290 as R. robusta ; Hernández-Becerril and Meave del Castillo 1996, p. 199, Fig. 1-20 as Calyptrella robusta ; Hasle and Syvertsen 1996, p. 159, Pl. 30 as R. ro-busta ; Gómez and Souissi 2007, p. 287, Fig. 4i; Sunesen and Sar 2007, p. 637, Fig. 62-67. Synonyms . Rhizosolenia robusta Norman in Pritchard 1861, Calyptrella robusta (Norman) Hernández-Becerril and Meave 1996. Cells are solitary, large, bilaterally symmetrical, 108.3-190.6 μm in diameter, 413.3 μm long, elliptical in cross sec-tion, crescent shaped in lateral view and of sigmoid form in ventro-dorsal view. Valve is conoidal with a rounded or truncated apex and with longitudinal undulations. Pro-cess is a cylindrical external tube, straightened towards the distal part, merging with the calyptra structure and circular pore in the distal part of the tip, 6.7-7.3 μm long, 0.8-1.0 μm in diameter. Valve areolae, 13-16 in 10 μm, are arranged in regularly straight striations, with a secondary quincuncial pattern. Otaria, claspers, and contiguous ar-eas are absent. Girdle segments are oriented in a straight line and arranged in two dorsiventral columns. Segment areolae, 17-22 in 10 μm, are arranged in regular, straight striations, with a secondary quincuncial pattern, loculate areolae, with the velum perforated by slit-like pores and internal foramina, circular to subcircular. Horizontal axis and perpendicular axis of segments are 108.3-190.6 and 11.7-25.0 μm in length, respectively. Distribution . Neocalyptrella robusta is distributed from tropical to temperate waters (Hasle and Syvertsen 1996, Hernández-Becerril and Meave del Castillo 1996). It has been reported to occur in littoral Argentinean wa-ters (Ferrario and Galávan 1989, as Rhizosolenia robusta ). During this study, N. robusta was rare but observed in September 2008 and June 2009 at Geoje Island, the Korea Straight, the Wolsung coast, and the Yellow Sea. Schultze 1858, p. 339, Pl. 13, Fig. 5-10; Peragallo 1892, p. 113, Pl. 17, Fig. 9; Hustedt 1930, p. 592, Fig. 339 as R. cal-car-avis ; Cupp 1943, p. 89, Fig. 51 as R. calar-avis ; Navarro 1981, p. 430, Fig. 36 & 37 as R. calcar-avis ; Sundström 1986, p. 95, Fig. 40-46 & 247-257; Hernández-Becerril 1995, p. 254, Fig. 7-10; Hasle and Syvertsen 1996, p. 160, Pl. 30; Sunesen and Sar 2007, p. 637, Fig. 68-81. Synonym . Rhizosolenia calcar-avis Schultze 1858. Cells are usually solitary, elongated, of cylinder shape, bilaterally symmetrical, circular in cross section, 9.3-90.0 μm in diameter, 206.7-793.8 μm long. Valve is sub-conical, asymmetrical, with the ventral part slightly longer than the dorsal part. Contiguous area is a narrow groove, sig-moid, extended from the basal part of the process to the margin in the ventral part of the valve. Process is claw or screw shaped, slightly or strongly curved, and tapered towards the distal part, 10.7-51.4 μm long. Otaria and claspers are absent. Valve areolae are poroid, circular, 16-34 in 10 μm. Striations are regular and straight, with a secondary quincuncial pattern. Girdle segments are scale-shaped to rhomboidal, arranged in two or multiples of two columns, with a sub marginal seam-like structure close to the advalvar margin with entire hyaline edges. Horizontal axis and perpendicular axis of segments are 9.3-90.0 and 9.3-46.2 μm long, respectively. Segmented areolae are 21-38 in 10 μm in a secondary quincuncial pattern. Distribution . Pseudosolenia calcar-avis is a circum-globally distributed species (Sundström 1986) and occurs in warm waters and occasionally in temperate waters (Hasle and Syvertsen 1996). It has been reported several times in both oceanic and near-shore waters along the coastline of Argentina (Ferrario and Galávan 1989, as Rhizosolenia calcar-avis ). In the present study, this spe-cies was rarely observed in September 2008 to September 2009 in the oceanic waters of Jeju Island and the Yellow Sea. Cleve 1900, p. 28, Fig. 11; Hustedt 1930, p. 577, Fig. 328; Cupp 1943, p. 83, Fig. 44; Hendey 1964, p. 147, Pl. 4, Fig. 2; Drebes 1974, p. 49, Fig. 35a; Sundström 1986, p. 103, Fig. 272 & 273. Basionym . Rhizosolenia delicatula Cleve 1900. Cells form fairly straight chains and are bilaterally sym-metrical. Cells are 7.9-13.2 μm in diameter, 24.9-30.0 μm in length. Valve margins are round. External process is thin and short, and narrow, tube-shaped, and oblique to the pervalvar axis. External processes are 2.1-5.0 μm long. External process fits into a depression on the adjacent valve. Girdle segments are composed of open bands, with poroid areolae, and are not noticeable. Segment horizon-tal axes are 7.9-13.2 μm long. Distribution . Hasle and Syvertsen (1996) reported that G. delicatula is a cosmopolitan species in temperate and tropical waters. During the present study, this species was recorded in July 2009 and January 2010 in the coastal wa-ters of Sacheon, Incheon, and Mokpo. Castracane 1886, p. 74, Pl. 29, Fig. 4; Peragallo 1892, p. 107, Pl. 1, Fig. 3-5; Bergon 1903, p. 78, Pl. 2, Fig. 1-3; Hustedt 1930, p. 562, Fig. 322; Cupp 1943, p. 78, Fig. 40; Hendey 1964, p. 147, Pl. 5, Fig. 5; Drebes 1974, p. 58, Fig. 43a; Hasle 1975, p. 116, Fig. 64, 65 & 81-89; Navarro 1981, p. 430, Fig. 31 & 32; Takano 1990, pp. 260-261. Basionym . Rhizosolenia flaccida Castracane 1886. Cells are solitary or form fairly straight chains, and are bilaterally symmetrical. Cells are 14.0-42.5 μm in diame-ter, 50.0-125.0 μm in length. Valves are flat or slightly con-cave. External processes are short and tube-shaped. Short tube-shaped external processes are located on the exter-nal valve surface. External processes are 1.5-1.8 μm long. Girdle segments composed of open bands with poroid areolae. The segment horizontal axis and perpendicular axis are 14.0-42.5 and 1.3-5.0 μm in length, respectively. Distribution . Guinardia flaccida shows a cosmopoli-tan distribution except the two polar bodies of water (Hasle and Syvertsen 1996). During the present study, G. flaccida was frequently observed in September 2008 and August 2009 in the coastal waters of the Yellow Sea, Geoje Island, Namhae, Sacheon, Tongyeang, and Incheon. Stolterforth 1879, p. 836, Fig. a & b; Peragallo 1888, p. 82, Pl. 6, Fig. 44; Peragallo 1892, p. 108, Pl. 13, Fig. 17 & 18; Bergon 1903, p. 57, Pl. 1, Fig. 1-8; Hustedt 1920, Pl. 320, Fig. 4 & 5; Hustedt 1930, p. 578, Fig. 329; Cupp 1943, p. 83, Fig. 45; Hendey 1964, p. 148, Pl. 4, Fig. 5; Drebes 1974, p. 49, Fig. 35b; Hasle 1975, p. 113, Fig. 66-73; Sundström 1980, p. 580, Fig. 2-4; Navarro 1981, p. 430, Fig. 48 as R. stolterforthii ; Sundström 1986, p. 103, Fig. 274 & 275; Von Stosch 1986, p. 319, Fig. 13 & 14; Hernández-Becerril 1995, p. 262, Fig. 53-56. Basionym . Rhizosolenia stolterforthii (Stolterforth) H. Peragallo 1888. Cells form curved chains, rarely spiraling chains, and are bilaterally symmetrical. Cells are 10.0-20.0 μm in diameter, and 50.0-120.0 μm in length. Valve flat and rounded at margin. External processes are thin, and hook-shaped to the pervalvar axis, and the external pro-cesses are 4.3-6.7 μm long. External process fits into a de-pression on the adjacent valve. Girdle bands composed of open bands with poroid areolae. Segment horizontal axis and perpendicular axis are 10.0-20.0 and 4.3-4.6 μm long, respectively. Distribution . Guinardia striata is cosmopolitan, but it does not occur in polar bodies of water (Hasle and Sy-vertsen 1996). During the present study, G. striata was rarely observed in September 2008 and August 2009 in the coastal waters of Namhae, Daebu Island, Incheon, and Yeongdeok. Bergon 1903, p. 49, Pl. 1, Fig. 9 & 10; Hustedt 1930, p. 571, Fig. 324; Cupp 1943, p. 80, Fig. 41; Drebes 1974, p. 48, Fig. 34b & c; Hasle 1975, p. 114, Fig. 61, 62 & 74-78; Navarro 1981, p. 430, Fig. 38 as R. fragilissima ; Sundström 1986, p. 103, Fig. 268 & 269; Takano 1990, pp. 262-263. Basionym . Rhizosolenia fragilissima Bergon 1903. Cells are cylindrical with rounded marginal parts, forming straight chains. Cells are connected in loose fit-ting chains at the center of the valve surface. Cells are 8.3-20.0 μm in diameter, and 25.0-33.4 μm in length. Valves are flat or convex at the central part. External process is a thin, oblique tube in the central part of the valve. External processes are 1.1-4.3 μm long and fit into a depression on adjacent cells. Girdle bands composed of half bands with poroid areolae. Distribution . Dactyliosolen fragilissimus is probably cosmopolitan (Hasle and Syvertsen 1996) but was rarely observed in February 2010 in Goseong. Remarks . Cell length of Dactyliosolen fragilissimus varies from 30.0-80.0 μm (Gran and Angst 1931), 50.0-80.0 μm (Cupp 1943), 42.0-67.0 μm (Hendey 1964), and 42.0-300.0 μm (Hasle and Syvertsen 1996). During this study, D. fragilissimus was not observed with a length of pervalvar axis up to 100 μm. Sundström 1980, p. 579, Fig. 1-3; Sundström 1986, p. 103, Fig. 270 & 271; Von Stosch 1986, p. 323, Fig. 15-17; Hernández-Becerril 1995, p. 262, Fig. 50-52. Basionym . Rhizosolenia phuketensis Sundström 1980. Cells are cylindrical with a rounded marginal part, forming curved chains. Cells are connected in fairly fitting chains and are bilaterally symmetrical. Cells are 10.0-20.0 μm in diameter, and 31.3-129.2 μm long. Valves are flat or slightly convex. External process is usually an obtuse, short tube in the valve marginal part. External processes are 1.3-6.7 μm long and fit into a depression in adjacent cells. Girdle bands are composed of half bands with po-roid areolae. Segment horizontal axis and perpendicular axis are 10.0-20.0 and 1.3-6.6 μm in length, respectively. Distribution . Sundström (1986) reported that D. phuketensis occurs in warm water regions to temperate regions including the North Sea and Skagerrak. During this study, D. phuketensis was newly recorded in June 2009 and October 2009 from the coastal waters of Geoje Island, Sacheon, Tongyeang, Yeongduk, and the Yellow Sea. Remarks . Dactyliosolen phuketensis was recorded for the first time in Korean coastal waters, but this species was already observed a long time ago, resembling G. stri-ata . Although the two species belonged to different gen-era and were recorded in Korean coastal waters, they can be easily misidentified using the curved cell and colony shape as discriminating characters. The external process shapes are different between two taxa. The external pro-cesses of G. striata are hook shaped, whereas those of D. phuketensis are short external tubes. The position of the external process is not the same, and segment (band) shape also differs. Representatives of the genus Guinar-dia are composed of an open band, but the genus Dactyli-osolen is composed of a half band. The family Rhizosoleniaceae includes Rhizosolenia, Proboscia, Pseudosolenia, Neocalyptrella, Guinardia, Dactyliosolen , and Urosolenia . The key morphological characters of the family Rhizosoleniaceae are cylindrical cells in solitary or chain form, unipolar and symmetrical valves, numerous chloroplasts, and a few resting spores. The genera within Rhizosoleniaceae are very common in the marine ecosystem and sometimes dominate the phytoplankton biomass in highly productive oceanic re-gions (Sundström 1986, Hernández-Becerril and Meave del Castillo 1996, 1997). Some species of the family Rhi-zosoleniaceae are causative bloom organisms in various regions of the world (Jordan and Priddle 1991, Jordan et al. 1991, Takahashi et al. 1994). These species are very im-portant diatoms in marine environments. As shown in Table 2 of Yun and Lee (2010), Table 2 in Yun et al. (2011), and Table. 2 - 4 in the present study, we divided the 6 genera within the family Rhizosoleniaceae into two groups by morphological diagnostic characteris-tics including the shape of the external process and girdle segments in the column (Yun and Lee 2010, Yun et al. 2011). The first group had a conoidal valve and loculate areolae and was comprised of Proboscia, Pseudosolenia, Rhizosolenia, Neocalyptrella , and the second group had a flat or rounded valve and poroid areolae and was of Gui-nardia and Dactyliosolen . In the present study, 2 species belonged to Proboscia , 3 species to Guinardia , 2 species to Dactyliosolen , 1 species to Pseudosolenia , and 1 species belonged to Neocalyptrella . Cell diameters of Proboscia alata were 3.3-13.3 μm, but previous studies reported 7.0-18.0 μm (Cupp 1943 as Rhi-zosolenia alata ), 8.5-11.5 μm (Sundström 1986), 2.5-42.0 μm (Jordan et al. 1991), 7.0-24.0 μm (Hernández-Becerril 1995), 2.5-13.0 μm (Hasle and Syvertsen 1996), and 7.0-11.0 μm (Sunesen and Sar 2007). Cell diameters of P. indi-ca were 25.0-125.0 μm. Our specimens closely resembled those described previously (Cupp 1943, Hernández-Becerril 1995, Jordan and Ligowski 2004), but their cell diameters were smaller (16.0-73.0 μm) than those of our specimens. Cell diameters of N. robusta (108.3-190.6 μm) and P. calcar-avis (9.3-90.0 μm) were smaller than those of Hasle and Syvertsen (1996) and Sunesen and Sar (2007), respectively. Cell diameters of G. delicatula were 7.9-13.2 μm, G. flac-cida were 23.3-42.5 μm, and G. striata were 10.0-20.0 μm. No differences were observed in the 3 species cell diam-eters compared with those of many studies (Cupp 1943, Hernández-Becerril 1995, Hasle and Syvertsen 1996). Cell diameters of D. fragilissimus and D. phuketen-sis were 8.3-20.0 μm and 10.0-20.0 μm, respectively. Our specimens were similar to those of Hasle and Syvertsen (1996), but their cell diameters were wider than those of our specimens. External processes varied from short tube-shaped in N. robusta and D. phuketensis , claw or screw-shaped in P. calcar-avis , narrow tube-shaped in G. delicatula , slight hook-shaped in G. striata , and oblique tube-shaped in D. fragilissimus . The genus Proboscia was distinguished within the first group because the external processes were longer, and the valve was changed to a probosic structure. As the external process of Rhizosolenia was in the shape of a needle and tube, this genus is separated from other genera (Yun and Lee 2010, Yun et al. 2011). The external process of the family Rhizosoleniaceae is an important taxonomic key character. Areolae occurred in various forms on the external view; circular to sub circular pore-shaped in P. alata, P. indica , and N. robusta and circular to slightly oval pore-shaped in Pseudosolenia calcar-avis . We were unable to count the number of areolae in G. delicatula, G. striata, D. fragilis-simus , and D. phuketensis , but areolae of D. phuketensis are slit-like with a parallel to pervalvar axis (Hernández-Becerril 1995). The number of areolae in the valves varied from 52-90 in 10 μm in P. alata , 30-60 in 10 μm in P. indica , 13-16 in 10 μm in N. robusta , and 16-34 in 10 μm in P. calcar-avis . Hasle and Syvertsen (1996) reported that N. robusta (as Rhizosolenia robusta ) had 19-20 in 10 μm and 28-32 in 10 μm in P. calcar-avis . Sunesen and Sar (2007) reported that P. alata had 54 in 10 μm, 17 in 10 μm in N. robusta , and 23-32 in 10 μm in P. calcar-avis . No differences were observed from previous reports. The number of areolae in the seg-ments varied from 25-62 in 10 μm in P. alata , 17-22 in 10 μm in N. robusta , and 21-38 in 10 μm in P. calcar-avis . N. robusta has 24-26 in 10 μm (Hasle and Syvertsen 1996 as Rhizosolenia robusta ), 22-23 in 10 μm in N. robusta , and 28-32 in 10 μm in P. calcar-avis (Sunesen and Sar 2007). Our specimens had a similar number of areolae in the segments compared with those of previous reports. We could not count number of areolae in P. indica, G. delicat-ula, G. flaccida, G. striata, D. fragilissimus , and D. phuke-tensis , because of delicate cells. Abundant distributions of G. flaccida and P. indica were found in Korean coastal waters. G. flaccida was widely distributed at 6 stations, and D. fragilissimus and N. robusta were sporadically found at 4 stations in Korean coastal waters. P. calcar-avis has been frequently found in the Korean coastal waters, and this species is a warm and temperate water species (Cupp 1943, Hendey 1964, Sundström 1986, Hernández-Becerril 1995, Hasle and Sy-vertsen 1996). Dactyliosolen phuketensis was new to Ko-rean coastal waters.