lb. Cell wall coarsely structured . . L. mediterraneus1 (H. Peragallo) Hasle 2 a Two (seldom one) elongate chloroplasts, chains sometimes slightly undulated L. minimus Gran
2b. Numerous small rounded chloroplasts L. danicus Cleve
L. danicus—cosmopolitan—absent or scarce in the subantarctic/Antarctic (recorded from the Drake Passage Rivera, 1983); recorded as dominant species at 81°43'N (Heimdal, 1983); one of the dominant summer diatoms in Norwegian fjords.
L. mediterraneus—cosmopolitan—recorded from the Antarctic to the Arctic, seldom or never as a dominant species (Hasle, 1976a); scarce in the Arctic.
L. minimus—cosmopolitan—apparently absent from the Subantarctic/ Antarctic (Hargraves, 1990). How to identify: All three species may be identified in water mounts, L. mediterraneus especially by the presence of the epiphytic flagellate Rhizomonas setigera (Pavillard) Patterson, Nygaard, Steinberg, & Turley (= Solenicola setigera in Fryxell, 1989, Figs. 1-5). Single valves of L. mediterraneus mounted in a medium of a high refractive index are recognized by its coarse, open structure (Hasle, 1976a, Fig. 34), whereas those of L. danicus may be recognized under the best optical conditions by the small spines in the bordering zone between valve face and mantle and the sometimes more coarsely structured valve center. Remarks: It should be noted that of two adjacent L. danicus valves in a chain one is slightly convex and the other concave, and neither are exactly flattened. Leptocylindrus minimus and L. danicus are distinguished not only by size and chloroplasts but also by the shape of the resting spores. The L. minimus resting spore is globular with a cylindrical neck, whereas that of
7 Basionym: Dactyliosolen mediterraneus (H. Peragallo) H. Peragallo (see Hasle, 1975).
L. danicus consists of two unequal valves (Hargraves, 1990). Leptocylindrus mediterraneus is seldom found with chloroplasts and the cell wall has a double-layered structure (Hasle, 1975). Its taxonomic position is therefore questionable. Leptocylindrus includes two taxa in addition to those dealt with here, viz. L. danicus var. adriaticus (Schröder) Schiller from the Adriatic Sea and L. curvatulus Skvortzov from the Sea of Japan. The main character of the former is the ratio between pervalvar axis and diameter, which is greater than that in L. danicus, whereas the latter is distinguished by undulated chains. Their identity as separate taxa remains to be proven.
Genus Corethron Castracane 1886 (Plate 15, Table 19) Lectotype: Corethron criophilum Castracane (vide Boyer, 1927, p.114). References: Castracane, 1886, p. 85, Plate 21, Figs. 3-6, 12, 14, and 15; Karsten, 1905, p. 100, Plates 12-14; Hendey, 1964, p. 144, Plate 7, Fig. 4; Fryxell 8c Hasle, 1971; Fryxell, 1989, p. 9, Figs. 27-32; Thomas 8c Bonham, 1990.
Corethron was placed in Melosiraceae by Simonsen (1979) and in Chaetoc-erotaceae by Glezer et al. (1988), whereas Round et al. (1990) retained the family Corethraceae Lebour 1930 and introduced Corethrophycidae and Core-thrales as a new subclass and a new order, respectively. The position in Chaetoc-erotaceae probably reflects an emphasis on the presence of long siliceous outgrowths, called spines for lack of a better term, from the border of the valve face. The unique shape of the marginal spines may be a reasonable justification for establishing a separate subclass and order for the genus.
Hendey (1937) concluded that Corethron was a monotypic genus, with C. criophilum as the only species, but appeared in different phases. Thomas 8c Bonham (1990) claimed the existence of two Corethron species in the Antarctic.
Cells cylindrical with more or less dome-shaped valves. Girdle composed of many bands.
Valves with marginal long (barbed) and short hooked (clawed) spines. Chloroplasts numerous rounded or oval bodies.
KEY TO SPECIES (mainly after Thomas 8c Bonham, 1990)
la. Cells solitary, robust, heterovalvate, valves with both hooked and long spines or long spines only, bands open, ligulate
C. criophilum Castracane
1 b. Cells usually in colonies, weakly silicified, only end cells of colonies heterovalvate, intercalary valves with short spines interlocking sibling valves, cingulum composed of half bands C. inerme Karsten
TABLE 19 Morphometric Data of Corethron spp. (Hendey, 1937)
Pervalvar axis Diameter
C. criophilum 20-200 5-20
Distribution: If Hendey's concept of Corethron as a monotypic genus is followed, C. criophilum is a cosmopolitan species recorded as far north as ca. 80°N (Heimdal, 1983) and occurring in its greatest abundances in Antarctic waters (Fryxell & Hasle, 1971). As far as known, C. inerme has not been recorded outside the Antarctic.
How to identify: The species may be identified to genus from specimens in water mounts either of whole cells or of a single valve. Remarks: The structure of the spines was described by Karsten (1905) and verified with EM by Fryxell &c Hasle (1971).
Palmeria was classified under this family by Simonsen (1979) as well as by Round et al. (1990). Ethmodiscus was tentatively placed in Stictodiscoideae Simonsen, subfamily in Biddulphiaceae, by Simonsen (1979), in Ethmodisca-ceae Round in the order Ethmodiscales Round by Round et al. (1990), and in Coscinodiscaceae by Glezer et al. (1988).
The genera dealt with here are characterized by: Solitary cells (exception, C. bouvet). No external tubes of processes.
Marginal labiate processes, sometimes in more than one ring [may be absent in Ethmodiscus (Rivera et al., 1989)].
Labiate processes are sometimes also between valve center and margin, irregularly positioned if present in central part of valve. Labiate processes usually of two types (shape and/or size). Areolae loculate; cribra external; foramina internal (SEM).
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