A variety of Cyclotella atomus were discovered among the epiphytic diatoms on eelgrass (Zostera marina L.) in Korean waters. Cyclotella atomus var. marina has a flat and disc-shaped valve, and is clearly differentiated into a striate marginal zone and a smooth central zone. The valves have a very small diameter ranging from 3.1-3.4 μm. This small diatom does not have any central ornamentation. The marginal striate zone has numerous granules toward the mantle. The valves have five or six marginal fultoportulae with two satellite pores and a single marginal rimoportula. Genus Cyclotella comprises a large complex of centricdiatoms, which have been divided into different groups(Hakansson 2002). This genus was first introduced byKutzing (1833) as a subgenus of Frustulia Ag., which includedone species, Frustulia operculata Ag. Brebisson(1838) adopted the Cyclotella group of Kutzing as a newgenus, which included two species: Cyclotella ovalis andC. operculata. However, no generitype was designated byBrebisson. C. operculata, the monotype of the subgenus,automatically became the generitype of the genus. Cyclotella(Kutz.) Brebisson was predominantly a freshwatergenus in the family Stephanodiscaceae Makarova andhighly diverse with more than 100 species (van Landingham1969). Cyclotella has been widely reported in various habitatsincluding terrestrial fossil species, freshwater species,brackish or marine water species, and even attached specieson phytoplankton and macrophyte (Sullivan 1982,Chang and Steinberg 1988, Hakansson et al. 1993, Hasleand Syvertsen 1997). However, description of Cyclotellaattached to seagrass has been rare Some taxonomical studies on Cyclotella have been carriedout in Korea (Lee and Lee 1988, Lee et al. 1994, Leeet al. 1995b, Cho 1996). Several Cyclotella species, suchas C. badanica, C. comta, C. meneghiniana, C. stelligera,C. striata, C. striata var. ambigua, and C. stylorum, werereported as the benthic diatom from the coastal areas(Choi 1990) and 15 Cyclotella species were reported asthe plankters from the freshwater, brackish and marinewaters of Korea (Lee et al. 1995b). Presently, we describe the newly reported variety Cyclotellaatomus var. marina in Korean waters based onscanning electron microscopy (SEM) observation. Eelgrass (Zostera marina L.) samples were collected for epiphytic diatom research from the region of Yulim-ri (34˚36?N, 127˚47?E) (Fig. 1), Yeosu City, Korea, in July 1998. The site has densely populated eelgrass beds, and is a small, semi-closed bay with narrow openings. Eelgrass sampling was carried out during ebb tide. The eelgrass samples were randomly collected in the upper 1 cm of sediments in a 10 × 10 cm quadrate, and then refrigerated and moved to the laboratory (Heijs 1985, Moncreiff et al. 1992). The epiphytic diatoms on eelgrass were rinsed with distilled water, and removed with a nylon brush and a rubber knife. The collected epiphytic diatoms were cleaned with HNO ₃ and H ₂ SO ₄ , and mounted on cover slips for observing with SEM (TOPCON sm-300; Topcon, Co., Tokyo, Japan) as described previously (Hasle 1983). The diameter of this species was measured on 50 frustules using Optimas V. 6.5 image analysis software (Media Cybernetics, Silver Spring, MD, USA). The valve structure elements were also measured using the software and as described previously (Genkal and Kiss 1993), using the formula: where τ is number of striae in 10 μm; n, the number of striae on circumference of valve face; and D, the diameter of the valve. Terminology is according to Anonymous (1975) and Lee et al. (1995a). Nutrient factors were measured by the method of Strickland and Parsons (1972), and water temperature and salinity was recorded using a YSI MODEL 33 SCT meter (Yellow Springs Instrument Co. Inc., Yellow Springs, OH, USA). Cyclotella atomus var. marina Tanimura , Nagumo et Kato 2004 Description: According to Hakansson (2002), Lowe (1975), and Servant-Vildary (1984), we describe in detail Cyclotella atomus var. marina. The cells are solitary and disc-shaped. The valves are circular, flat, but those with a slight undulation can be found. The diameter of valves range from 3.1 to 3.4 μm. The details of the central area, the marginal area, the mantle and the girdle band of the external view ( Figs 2 - 7 , Figs 18 - 21 ) are as follows. The central zone is smooth without any depressions and surrounded by striae. The striae (점무늬열) consist of 15 or 16 areola in 10 μm and have a simple alveolate structure. Each alveolus (벌집구멍) is opened externally by means of rows. In the marginal area, the fultoportulae (strutted process, 받침돌기) number 5 or 6 for every cell. The external pores of the fultoportulae range from 0.07-0.11 μm in diameter. Towards the mantle the regular rows gradually increase in number to 4-6 before reaching the margin of the shallow mantle. The striate zone has numerous granules toward the mantle. The girdle is composed of several bands, which are open (Figs 6, 7 & 14-17). The internal view with the central area, marginal area, the mantle fultoportulae with their satellite pores, the rimoportula (labiate process, 입술돌기) and the position (Figs 8-17) are as follows. The valve shows the alveoli, with one of the costae bearing the rimopotula and the costa bearing a fultoportula. The central area of the silica has no visible structure internally and extends over to nearly two-thirds the length of the costae and alveoli. The marginal fultoportulae are arranged at every second or third interstria with two satellite pores, and close to the mantle margin. They number 5 or 6 for every cell. The internal pores of the fultoportulae range from 0.14-0.18 μm. There is one marginal rimoportula per cell. This is situated on a recessed costa on the very short mantle in nearly the same or slightly over position as one of the fultoportulae, and is externally visible as an elongated opening. The internal lips are small and vertical to the proximal costa. Habitat: metaphytic which means to be epiphytic or planktonic form, and abundant in marine waters around 26 psu salinity (Table 1) . A system of infrageneric classification of Cyclotella was initially proposed by Lowe (1975) and further extended by Servant-Vildary (1984). Apart from the criteria used by Lowe’s classification, which are 1) presence and spacing of marginal fultoportulae, 2) presence and spacing of fultoportulae on the valve face, 3) presence and number of rimoportulae, 4) presence of spines, 5) other special features such as granulae and structure of striae, Servant-Vildary (1984) has further emphasized the importance of the structure of the alveoli. According to this diagnostic features proposed by Lowe and Servant-Vildary, we compared Cyclotella atomus var. marina with other species of Cyclotella (Table 2) . All the compared species represent the following characteristics; a) a simple alveolus structure, b) presence of marginal rimoportula with simple internal lips, c) marginal fultoportula with simple external and internal pores. C. atomus var. marina in this paper, C. atomus in Trigueros et al. (2000) (Figs 12-15), and C. atomus var. atomus in Genkal and Kiss (1993) (Fig. 13), have also similar fultoportulae, which have short tubes and the two satellite pores range parallel to the alveolae on the internal valve face. However, C. atomus var. marina in this paper differs from the other species in the absence of any central structure including pore, fultoportulae, and rimoportula. In contrast, C. atomus, C. atomus var. atomus and C. atomus var. gracilis have a central pore and fultoportulae. In addition, it is very difficult to discern the morphological features in detail by light microscopy observation because of very small size. However, it can be seen that there is no fultoportula on the valve face and a simple alveolate structure at the margin. The cell structure and size of C. atomus var. marina in this paper are similar to only C. atomus var. marina reported by Tanimura et al. (2004). Both have a similar size, a number of marginal fultoportula and rimoportula, and do not have any central structure. C. atomus var. marina was first recorded by Tanimura et al. (2004) in Tokyo Bay, Japan. This species grows abundantly at a salinity of around 30 psu salinity and is abundant in water with higher nutrients, such as in bays. The presently-described C. atomus var. marina was also found in nutrient-rich marine waters (Table 1) . C. atomus var. marina described in this study was found among epiphytic diatom on eelgrass, but Tanimura et al. (2004) reported this species in the water column of Tokyo Bay. Actually, it seems to be not a true epiphyta because we could not find any attaching material. This species may not be a true epiphyic, but rather a metaphytic diatom that exists in both benthic and/or planktonic forms (Hakansson personal communication). According to this ecological and morphological evidence, we believe that the species in the present study is identical with the C. atomus var. marina described as a new variety by Tanimura et al. (2004). This is the first report of this species in Korean waters.