In 1844 Ehrenberg established the diatom genus Chaetoceros from Antarctic Ocean specimens. Chaetoceros species have a worldwide distribution, and they often dominate marine ecosystems (Evensen and Hasle 1975,Rines and Hargraves 1988, Hernandez-Becerril 1996). Some have caused important blooms in both oceanic and coastal habitats (Shevchenko and Orlova 2010). Researchers have described about 400 species in the genus(Vanlandingham 1968, Rines and Hargraves 1988, Hasle and Syvertsen 1996), but Ferrario et al. (2004) asserted 175 species in the genus.

Chaetoceros is mainly characterized by its chain-forming cells and their setae structures. One of their most conspicuous characteristics is number of setae possessed per cell (Rines and Hargraves 1988, Hasle and Syvertsen 1996, Hernandez-Becerril 1996). The seta structure is extended empty processes that begin on the valve surface corners. These setae generally comprise terminal and intercalary setae, the former at the end of a chain and the latter within the chain.

Sometimes, a few species have unusual intercalary setae,called “special intercalary setae” (Shevchenko et al. 2006). These are thicker and much more strongly silicified than are common intercalary setae (Cupp 1943, Hendey 1964, Shevchenko et al. 2006). Chaetoceros species with these special intercalary setae are C. coarctatus, C. compressus, C. compressus var. hirtisetus, C. contortus, C. diversus, and C. messanensis, and they occur all over the world (Rines and Hargraves 1990, Hernandez-Becerril 1991, Sunesen et al. 2008).

Records note nearly 80 taxa of Chaetoceros species in Korean coastal waters, and, to date, light microscope observations on samples from Korean coastal waters have already documented C. coarctatus, C. diversus, and C. messanensis (Lee 1995). Of course, they possess distinctive morphologies allowing identification under the light microscope. No studies using the scanning electron microscope on the setae ultrastructure of the genus Chaetoceros have been performed in Korea.

This study aimed to examine morphological differences of the ultra setae structures in the special intercalary setae of C. coarctatus, C. compressus var. hirtisetus, C. contortus, C. diversus, and C. messanensis specimens from Korean coastal waters.

From August 2008 to September 2009, we collected Chaetoceros species in the coastal waters of South Korea (Table 1), using a 20 ㎛ mesh plankton net. The samples were immediately fixed with 4% neutralized formalin or brought alive into the laboratory. To examine the setae poroids, we eliminated the organic matter in the Chaetoceros cells’ silicified frustules per the method of Hasle and Fryxell (1970), as follows: we (1) washed samples with distilled water, to remove NaCl, (2) cleaned the rinsed samples by adding equal amounts of KMnO₄ and HCl in a boiling water bath, until the sample becomes clear or only slightly colored, (3) washed samples again with distilled water, and (4) mounted the specimens on glass slides, using Pleurax for light microscopes, and on aluminum stubs, for the scanning electron microscope. To make our morphological observations, we employed 154a light microscope (Axioskop 40; Zeiss, Jena, Germany) with an MRc 5 camera (Zeiss) and a scanning electron microscope (JSM-5600LV; Jeol, Tokyo, Japan). Cells’ sizes were analyzed via image calculation software (Axio Vision AC version 4.5; Zeiss).

For this study’s terminology, we adopted the general proposals made by the majority of authors (Cupp 1943, Brunel 1966, 1972, Anonymous 1975, Ross et al. 1979, Rines and Hargraves 1988, Hernandez-Becerril 1996).

This study adopted Simonsen’s system (1979) based on phylogenetic ideas as follows.

Class Bacillariophyceae Haeckel 1878

Order Centrales Hustedt 1930

Suborder Biddulphiineae Simonsen 1979

Family Chaetoceraceae Smith 1872

Genus Chaetoceros Ehrenberg 1844

Subgenus Phaeoceros Gran 1897

Chaetoceros coarctatus Lauder 1864

Subgenus Hyalochaete Gran 1897

Chaetoceros compressus var. hirtisetus Rines et Hargraves 1990

Chaetoceros contortus Schutt 1895

Chaetoceros diversus Cleve 1873

Chaetoceros messanensis Castracane 1875

We examined 5 Chaetoceros species having special intercalary setae. Table 2 summarizes these species’ morphological characteristics.

Lauder 1864, p. 79, Pl. 8, Fig. 8; Hustedt 1930, p. 655, Fig. 370; Cupp 1943, p. 107, Fig. 62; Hendey 1964, p. 121, Pl. 12, Fig. 1; Hernandez-Becerril 1991; Hasle and Syvertsen 1996, p. 200, Pl. 40.

Synonyms. Chaetoceros borealis var. rudis Cleve 1897, p. 20, Pl. 1, Fig. 5; Chaetoceros rudis Cleve 1901, p. 308.

Cells form a straight, heteropolar, robust chain, usually long, having 12-16 cells. Chloroplasts appear numerous and have small, rounded bodies. Frustules appear cylindrical and elliptical in valve view, with an apical axis of 25-35 ㎛ and a transapical axis of 13-20 ㎛. Each valve Pehas an elliptical shape, flat face, and numerous small rimoportula. The valve mantle is usually high, often but a little higher than the girdle is. Foramina are absent or very small. Posterior terminal setae appear quite thick, large, smoothly curved, and heavily spiny, and they are shorter than the others setae. Common intercalary setae are straight or smoothly curved. The setae cross section diameter is 2.4 to 8.5 ㎛. Spines on the setae have a robust,long shape and appear in straight lines. One edge of a seta has 2-4 spines per 10 ㎛. Setae poroids possess small pores and a straight line arrangement, with one side having 63-104 poroids per 1 ㎛. The special intercalary setae curve strongly near their tips. Spines on these setae are more strongly siliceous than are spines of common intercalary setae. All intercalary setae are about 45° from the apical axis, toward the posterior end. They originate and fuse inside the chain edge, with 4 sides. These setae are coarse and thick arising from the valve margin.

Distribution. Reports place C. coarctatus off lower California, and it is a common oceanic species in tropical or subtropical waters (Cupp 1943, Hendey 1964). Hasle and Syvertsen (1996) stated the species occurs in warm water regions. This species has been recorded at times in all Korean coastal waters, from 1956 to 1995 (Lee 1995). In the present study, we found C. coarctatus in the Yellow Sea on Sep 22, 2008.

Taxonomic remarks. C. coarctatus forms straight chains attached to Vorticella species (Cupp 1943). We found C. coarctatus in this study (Fig. 1A). The species has two terminal setae forms, based on the chains’ position, shape, and direction. The setae on the anterior valve are “anterior terminal setae,” and setae on the posterior valve are “posterior terminal setae” (Hernandez-Becerril 1991).

The anterior terminal setae appear less robust than posterior setae do, and they curve toward the posterior end, with common and special intercalary setae possessing longer and stronger spines than those found in other species.

Rines and Hargraves 1990, p. 114, Fig. 1-29; Shevchenko et al. 2006, p. 242, Fig. 35-39; Sunesen et al. 2008, p. 310, Fig. 5A-F.

Synonym. Chaetoceros sp. “C”, Rines and Hargraves 1988, p. 104, Fig. 212-217.

These cells’ straight chains usually twist to the pervalvar axis. Frustules appear rectangular to quadrangular in girdle view. The apical axis is 7-9 ㎛. Valve face is centrally inflated and more or less flat. Foramina appear relatively wide and slightly constricted centrally. Usually,foramina near the special intercalary setae are wider than are those near the common intercalary setae. Valve mantle is shallow. Terminal setae appear thin and delicate and align almost parallel to the chain axis. Common intercalary setae are thin and delicate, with small spines and siliceous capilli at the proximal part, and are directed nearly perpendicularly to the chain axis. These setae originate and fuse inside the chain edge. Special intercalary setae have relatively long, thick, coarse, tapering ends, smoothly curved with large, spiral twisted spines, but do not possess siliceous capilli.

Distribution. Rines and Hargraves (1990) examined C. compressus var. hirtisetus from Narragansett Bay, Rhode Island, U.S.A. Another study found this species in Pe-

ter the Great Bay, Sea of Japan (Shevchenko et al. 2006). Sunesen et al. (2008) found the species in summer, fall, and spring off the coast of Buenos Aires Province. We found C. compressus var. hirtisetus in Masan Bay on Oct 17, 2009 in Korea coastal waters, making it a newly-recorded species for this area.

Taxonomic remarks. The most distinctive features of C. compressus var. hirtisetus are the capilli on the intercalary setae spines. This species most closely resembles C. compressus and C. radicans. C. compressus var. hirtisetus can be distinguished from C. radicans by the number of chloroplasts, presence of heavy intercalary setae, and number of central processes on the terminal valve. In addition, this species is distinct from C. compressus due to the capilli on the common intercalary setae (Fig. 2C). We have not examined valve shape or rimoportula in C. compressus var. hirtisetus. Rines and Hargraves (1990) reported this species’ valve is elliptically compressed and has multiple (2-7, usually 3) rimoportula located on the terminal valve face.

Schutt 1895, p. 44; Rines 1999, p. 541, Figs 7-43; Sunesen et al. 2008, p. 310, Fig. 5G-M; Shevchenko and Orlova 2010, p. 246-247, Figs 2 & 3.

Cells form long, straight chains that twist slightly about the chain axis. Chloroplasts comprise several small bodies. Frustules appear rectangular, with rounded corners, in girdle view. Apical axis is 7-15 ㎛, and transapical axis is 5-15 ㎛. The valve shape is broadly elliptical to circular, with a flat to centrally-inflated surface. Foramina appear relatively wide and have slight central constriction. Usually, foramina near the special intercalary setae are wider than are those of the common intercalary setae. Rimoportula occur on the terminal valve face. Terminal setae appear thinner and shorter than do common intercalary setae, which are thin and delicate, having small spines almost perpendicular to the chain axis. These setae originate and fuse inside the chain edge. Spines on the setae have a fine or robust shape and align in a helicoid arrangement, with 1 row of 5 to 6 spines per 10 ㎛. Setae walls are thin. Special intercalary setae appear relatively short, thick, coarse, and smoothly curved, with a tapering end having large, spiral-twisted spines in the direction of the chain axis. These setae possess 1 row of 6-11 spines per 10 ㎛. Setae poroids are invisible. The setae’s wall is thick. The common intercalary setae’s diameter is 0.4 to 1.3 ㎛ in cross-section.

Distribution. C. contortus has been reported from the coast of Buenos Aires Province, Argentina (Sunesen et al. 2008) and Peter the Great Bay, Sea of Japan (Shevchenko and Orlova 2010). This study located C. contortus in Korean coastal waters 6 times: Geoje Isl. on Jun 7, 2009, Jeju Isl. on Sep 15, 2009, Daechon Coast on May 3, 2009, Anmyun Isl. on Aug 23, 2008, Aengkang Bay on Jul 19, 2009, and Oido Isl. on Apr 5, 2009. C. contortus is a newly-recorded species for Korean coastal waters.

Taxonomic remarks. Schutt first described and recorded this species in 1895, in the Baltic Sea. Gran (1897) pointed out that Schutt simply gave new names to different-sized specimens of the same species, which Gran and Yendo (1914) synonymized under the name C. compressus, based on its apparent intermediate forms. Since 1914, all material fitting the general description has been assigned the name C. compressus. Rines and Hargraves (1990) concluded that the synonymy by Gran and Yendo (1914) was erroneous and that C. compressus and C. contortus should have separate taxa. The C. contortus valve is broad and nearly round, whereas that of C. compressus is strongly compressed, becoming broadly elliptical only at the lower end of its size range (Rines 1999). We compared the different special intercalary setae of C. contortus and C. compressus var. hirtisetus. C. compressus var. hirtisetus possess long special intercalary setae that curve to resemble an arc and become nearly parallel to the chain axis (Fig. 2A & B). In contrast, special intercalary setae of C. contortus are short and straight (Fig. 3A & B).

Cleve 1873, p. 9, Fig. 12; Hustedt 1930, p. 716, Fig. 4099; Cupp 1943, p. 132, Fig. 87; Hendey 1964, p. 130, Pl. 17, Fig. 4; Moreno Ruiz et al. 1993, p. 422-424, Fig. 2-28; Shevchenko et al. 2006, p. 248. Fig. 71-73.

Synonym. Chaetoceros laevis Leuduger-Fortmorel 1892, p. 38, Pl. 6, Fig. 2.

Cells usually form a short straight chain. Each cell contains 1 chloroplast on the girdle side. Frustules appear rectangular in broad girdle view. Apical axis is 9-11 ㎛, and transapical axis is 4-6 ㎛. The valve has an elliptical shape and a face that is flat or slightly raised in the center.The valve mantle is high, and the girdle band is small. Foramina appear very narrow and lack slits. Rimoportula occur on the terminal valve. Terminal setae are always thin and differ in position from intercalary setae. Common intercalary setae appear thin and hair-like, slightly curved or, often, straight, and usually slightly turned toward the chain ends. Cross section shows the setae to be circular, 0.5 to 1.5 ㎛ in diameter. Setae spines posses a

fine shape and straight-line arrangement, with 1 row of 6-12 spines per 10 ㎛. Poroids of the setae have small slits and a straight alignment, with 1 row of 16-26 poroids per 1 ㎛. Special intercalary setae appear thick, heavy, and, frequently, covered with fine spines. The setae are clearly angular in cross-section, with spirally-arranged little spines. All intercalary setae bent toward the nearest end of the chain. They originate and fuse at the chain edge, arising at the valve corners without basal parts and running almost parallel to the chain axis in the outer part.

Distribution. C. diversus was first found in the Sea of Java (Cleve 1873). This species occurs frequently as a tropical or sub-tropical species of the North Sea (Cupp 1943, Hendey 1964) and the southern Gulf of Mexico (Moreno Ruiz et al. 1993). From 1967 to 1995, C. diversus was occasionally reported in Korean coastal waters, regardless of season (Lee 1995). In this study, we found the species on Jan 20, 2009 in the Korea Strait.

Taxonomic remarks. Leuduger-Fortmorel (1892) placed C. diversus in a close relationship to C. laevis. His drawing of the species was quite poor because of a broken and incomplete chain of two valves, with the characteristically thick setae diverging and lacking spines and with a lack of terminal setae. Since that time, many authors have mentioned that the orientation of the thick setae is a diagnostic characteristic for separating these two species (Hustedt 1930, Allen and Cupp 1935, Cupp 1943, Sournia 1968). Moreno Ruiz et al. (1993) concluded C. laevis was only a morphotype of C. diversus, which has been characterized in the literature as consisting of straight chains of 2 to 6 cells or slightly curved chains of 2 to 8 cells (Moreno Ruiz et al. 1993). This study examined only straight chains comprising 2 to 6 cells (Fig. 3A) and chains of 2 cells with terminal intercalary setae of one type (Fig. 3B).

Castracane 1875, p. 394, Pl. 1, Fig. 1a; Hustedt 1930, p. 718, Fig. 410; Cupp 1943, p. 133, Fig. 89A & B; Hendey 1964, p. 129, Pl. 22, Fig. 3; Hasle and Syvertsen 1996, p. 216, Pl. 45; Shevchenko et al. 2006, p. 250, Fig. 90-92.

Synonym. Chaetoceros furca Cleve 1897, p. 21, Pl. 1, Fig. 10.

Cells form straight chains and have 1 chloroplast, near the girdle. Frustules appear rectangular with conspicuous corners beside adjacent cells in broad girdle view. Apical axis is 7-33 ㎛, and transapical axis is 2-6 ㎛. The valve is narrow and elliptical. Valve faces are flat, and valve mantles are low. Foramina are lanceolate or hexagonal.Rimoportula occur on the terminal valve. Girdle is usually deep. Terminal setae usually are thin and short, turning in different directions. Common intercalary setae are thin and short, arising as a thick pair through the fusion of contiguous cells’ setae. The setae have thicker basal parts, becoming more slender toward the ends. Node-like spines occur in spirals. The setae’s direction is nearly perpendicular to the chain axis. Setae spines are quite fine and helically arranged, with 1 row of 10-12 spines per 10 ㎛. Setae poroids have small pores and align in a helicoid arrangement, with 16-18 per 1 ㎛ in 1 row. Special intercalary setae are thick, long, and strongly silicified. These setae fuse a long distance and then diverge,unlike other setae. All setae originate and fuse at the chain edge.

Distribution. C. messanensis is an oceanic species from subtropical to tropical waters (Cupp 1943), and it has occurred in warm water regions of North America’s west coast (Hasle and Syvertsen 1996) and of Peter the Great Bay, Sea of Japan (Shevchenko et al. 2006). From 1933 to 1995, C. messanensis reportedly appeared in Korea Strait, Yellow Sea, Ulsan Bay, Kori coastal area, Southwestern Sea, Kunsan coast, Chonsu Bay, Jeju Isl., and Samchonpo coastal waters (Lee 1995). In this study, we found the species on Jan 20, 2009, in the Korea Strait.

Taxonomic remarks. Usually, the special intercalary setae of this species occur 1 or 2 to a chain. We observed forked special intercalary setae, which have two forms. The first type has rows of spines in spirals (Fig. 4G & H), while the second type has no spines (Fig. 4E & F). The forked special intercalary setae are this species’ most conspicuous characteristics. We found all of the types in the Korea Strait in this study.