Figure

with terminology.

Raphe-sternum—the usually unperforate strip of silica, often thickened pervalvarly, which contains the raphe (Mann, 1978, p. 27).

Family Achnanthaceae Kiitzing 1844

Diatoms of this family have heterovalvar cells. One of the valves has a raphe with two longitudinal slits. The other valve has no raphe or only short slits, with the raphe being filled out by silica during the formation of the new valves (Round et al., 1990, p. 33). The cells are more or less genuflexed in the transapical axis. Achnanthes taeniata belongs to Arctic and Baltic Sea plankton, evidently as the only truly planktonic species of this family.

Achnanthes taeniata Grunow in Cleve 8c Grunow (Plate 60) References: Cleve 8c Grunow, 1880, p. 22, Plate 1, Fig. 5; Hustedt, 1959, p. 382, Fig. 828; Hasle 8c Syvertsen, 1990b, p. 289, Figs. 12-22. Girdle view: Cells only slightly genuflexed, raphe valve on the inside of the curvature. Cells in ribbons; vegetative cells attached along entire valve face; apices of resting spores in chains not attached. One (?) H-shaped chloroplast along the girdle.

Valve view: Valve linear with rounded apices. Raphe straight, sternum narrow.

Morphometric data: Apical axis, 10-40 /im; transapical axis, 4-6 ju.m; transapical striae, ca. 25 in 10 ju,m.

Distribution: Northern cold water region and the Baltic Sea. How to identify: Achnanthes taeniata may easily be confused with Fragilariopsis species and with Navicula and Nitzschia species-forming ribbons. In some cases the shape of the chains is distinctive but in most cases all these diatoms must be examined in valve view, preferably in permanent mounts of cleaned material.

Family Phaeodactylaceae J. Lewin 1958

Genus Phaeodactylum Bohlin 1897 Type: Phaeodactylum tricornutum Bohlin. Monospecific genus.

Phaeodactylum tricornutum Bohlin (Plate 60)

Synonym: Nitzschia closterium W. Smith f. minutissima Allen 8c Nelson. References: Bohlin, 1897, pp. 519 and 520, Fig. 9; Allen 8c Nelson, 1910, p. 426; Wilson, 1946; Hendey, 1954; Lewin, 1958; Round et al., 1990, p. 560; Gutenbrunner et al., 1994, p. 129, Figs. 2-5. Solitary. Three types of cells: ovate (naviculoid), fusiform, and, more rarely, triradiate. Ovate cells motile with one siliceous valve per cell. Fusiform cells nonmotile and lack a siliceous valve. One chloroplast. Morphometric data: Ovate cells 8 fim in apical axis, 3 fim in transapical axis; striae not resolved with LM; fusiform cells up to 25-35 |iun long.

Achnanthes taeniata

Nanoneis hasleae

Nanoneis hasleae

Meuniera membranacea

PLATE 60 Achnanthes taeniata: (a) chain with resting spores and chloroplasts; (b) valve with raphe; (c) rapheless valve. Scale bars = 10 /im. Phaeodactylum tricornutum: Three cell types. After Wilson (1946). Scale bar = 10 fim, Nanoneis hasleae: (a) stepped chain in girdle view. Scale bar = 10 fim; (b) valve view. Scale bar = 1 /¿m. Meuniera membranacea: short chain in girdle view with chloroplasts. After Sournia (1968) and Gran (1908). Scale bar = 10 fim.

Distribution: Intertidal rock pools—probably cosmopolitan. How to identify: Siliceous valves, recognized only by the raphe, are found in cleaned material mounted in a medium of a high refractive index. Phase or interference contrast will most likely be needed. The presence of nonsiliceous cells can be verified by "elimination" methods since they will disappear in acid-cleaned material or, sometimes be seen with LM as an unstructured cell wall.

Remarks: The fusiform cell type was, in the past, frequently confused with the pennate diatom N. closterium (= Cylindrotheca closterium), e.g., the Plymouth strain "Nitzschia closterium f. minutissima." This strain was used for decades in diatom physiology studies until a microscopical examination showed that it was the fusiform Phaeodactylum tricornutum.

Incertae sedis (Raphid diatoms)

Genus Nanoneis R. E. Norris 1973 Type: Nanoneis basleae R. E. Norris. Monospecific genus.

Nanoneis hasleae R. E. Norris (Plate 60) Reference: Norris, 1973. Girdle view: Valve surfaces slightly convex to flat or concave, with the concavity present between the apices and the middle part of the valve. Cells occurring in irregular chains with a short overlap of cell ends. Valve view: Broadly elliptical to linear. Valve structure not resolved with LM. Raphe central and extending from one pole to near center; raphe of opposite valve extending from the opposite pole to near center. Morphometric data: Apical axis, 5-12 //.m; transapical axis, 1-1.5 /u.m; transapical interstriae, ca. 40 in 10 fim (TEM). Distribution: Warm water region—open ocean. How to identify: Electron microscopy may be needed.

Family Naviculaceae Kiitzing 1844

The genera treated here under the family name Naviculaceae have been placed in a variety of families in the classification systems by Glezer et al. (1988) and Round et al. (1990). In a manual to be used for species identification, like this chapter, we prefer Simonsen's (1979) broader delineation of Naviculaceae. Naviculaceae differs from Achnanthaceae and Phaeodactylaceae by being isovalvar; both valves of a cell have a "naviculoid" raphe not subtended by the fibulae present in Bacillariaceae.

Navicula is the largest of all diatom genera with 1860 "acceptable" and 2000 "unacceptable species," mainly bottom living forms (Mann, 1986, p. 216). Many of the few marine planktonic Navicula species were transferred to other genera, especially after Cox (1979) typified and emended the description of Navicula sensu stricto.

Tropidoneis is another genus under revision. Patrick &c Reimer (1975) found that the name Tropidoneis had to be rejected in favor of Plagiotropis. Plagiotropis has about 30 species (Paddock, 1990) and is the largest of the genera to which the former Tropidoneis species have been transferred. It comprises brackish water and marine, mostly bottom living species.

Pachyneis with one planktonic tropical and subtropical species was suggested as a possible transition form between Haslea, the former Navícula fusiformes sensu Hustedt, 1961, and some planktonic Tropidoneis species (Simonsen, 1974). Pleurosigma includes marine species, some of which are planktonic.

For practical reasons the genera here referred to as Naviculaceae are divided into four groups.

A. Former and present Navícula species.

1. Former Navícula sp. in ribbons: Meuniera membranacea.

2. Present Navícula spp. in ribbons: N. granii, N. pelagica, N. septentrio-nalis, and N. vanhoeffenii (Table 68).

3. Former solitary Navícula spp.: Haslea spp. (Table 69).

4. Present solitary Navícula spp.: N. directa, N. distans, N. transitans var. derasa, and N. transitans var. derasa f. delicatula (Table 70).

B. Pleurosigma spp.: P. directum, P. normanii, and P. simonsenii (Table 71).

C. The Tropidoneis group.

1. Former Navícula sp., usually solitary: Ephemera planamembrañacea.

2. Former Tropidoneis spp., usually in ribbons.

a. Valves lying in girdle view, vaulted to a high ridge: Banquisia and Membraneis (Table 72).

b. Valves lying in girdle or valve view; valve with only low ridge: Manguinea and Plagiotropis (Table 72).

D. Incertae sedis (Naviculaceat):Pachyneis.

A. Former and present Navícula species. Common characters:

Valves linear, lanceolate, or elliptical. Raphe generally straight. Raphe not raised on a ridge.

Stauros or stauros-like structure present in some species.

Characters showing differences between species: The presence or absence of chains. Number and shape of chloroplasts. Valve striation pattern. Extension of stauros or stauros like structure. The presence or absence of raphe fins.

1. Former Navícula species in ribbons.

Genus Meuniera P. C. Silva nom. nov.

Type: Meuniera membranacea (Cleve) P. C. Silva comb. nov.

Monospecific genus.

Meuniera membranacea (Cleve) P. C. Silva comb. nov. (Plate 60) Basionym: Navícula (Stauroneis) membranacea Cleve. Synonyms: Stauropsis membranacea (Cleve) Meunier; Stauroneis membranacea (Cleve) Hustedt.

References: Cleve, 1897a, p. 24, Plate 2, Figs. 25-28; Meunier, 1910, p. 319, Plate 33, Figs. 37-40; Cupp, 1943, p. 193, Fig. 142; Hustedt, 1959, p. 833, Fig. 1176; Hendey, 1964, p. 221, Plate 21, Fig. 3; Paddock, 1986, p. 89, Figs. 1-8.

Girdle view: Rectangular, valves flat or slightly concave in the center. Cell wall weakly silicified. Stauros narrow and distinct in girdle view. Raphe fins at corners of cells in girdle view (LM). Four ribbon-like and folded chloroplasts per cell, two along each side of the girdle. Valve view: Valves narrow and elliptical with pointed ends. Structure barely visible with LM. Stauros extending from the central nodule to valve margin.

Morphometric data: Pervalvar axis, 30-40 /im; apical axis, 50-90 ¿im. Distribution: Temperate.

How to identify: Meuniera membranacea is readily recognized with LM in girdle view by the distinct stauros and the typical chloroplasts. Remarks: Stauropsis Reichenbach 1860 had been used for an orchid genus; therefore, the diatom genus had to be given a new name (see Taxonomic Appendix).

2. Present Navicula spp. in ribbons (Plate 61, Table 68).

Meunier (1910) regarded the following four marine, mainly planktonic species to belong to the genus he described as Stauropsis. Paddock (1986) disagreed after having reviewed them on the basis of LM and EM data. Characters showing differences between the species are evident from the key and the illustrations.

References: Grunow, 1884, p. 105, Plate 1, Fig. 48; Cleve, 1896a, p. 11, Plate 1, Fig. 9; Gran, 1897a, p. 21, Plate 1, Figs. 1-3; j0rgensen, 1905, p. 107, Plate 7, Fig. 25; Gran, 1908, p. 123, Figs. 167-170; Meunier, 1910, p. 321, Plate 33, Figs. 26, 27, and 33-36; Cleve-Euler, 1952, p. 25, Fig. 1381; Heimdal, 1970, Figs. 1-11; Syvertsen, 1984; Hasle & Syvertsen, 1990b, p. 288, Figs. 1-4.

Navícula

N. vanhoeffenii

TABLE 68 Morphometric Data of Navícula spp. in Ribbons

Species

Pervalvar axis (p.m)

Apical axis (nm)

10 (XITl

N. granii

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