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Distribution:

S. microtrias—southern cold water region—particularly common on and in fast sea ice and in the surrounding plankton. S. stellaris—warm water region to temperate, planktonic. How to identify: The species may be identified in valve view in water mounts although examination of valves cleaned of organic matter and mounted in a medium of a high refractive index may be needed in most cases, especially to identify S. stellaris.

Remarks: Stellarima microtrias forms heavily silicified endogenous resting spores, usually identified as C. symbolophorus, inside weakly silicified cells previously identified as C. furcatus (Syvertsen, 1985). Stellarima stellaris has not been found to form resting spores. The labiate processes are readily observed with LM in the lightly silicified S. stellaris (e.g., Cupp, 1943, Fig. 16) and are less easily seen in the S. microtrias resting spores. Stellarima microtrias was found in long chains in living natural material (Fryxell, 1989) as well as in cultures (G. Hasle, personal observations on a M. Elbrachter isolate). The attachment point was central to eccentric on sibling valves.

Incertae sedis (Stellarimaceae)

Genus Gossleriella Schtitt 1892 Type: Gossleriella tropica Schutt. Monospecific genus. The taxonomic position of this genus is disputed. Hargraves (1976) suggested Rhizosoleniaceae as a possible family; Simonsen (1979)

PLATE 19 Stellarima stellaris: (a) girdle view; (b) valve with central labiate processes. Scale bar = 20 (Lm. Stellarima microtrias: (a) girdle view with endogenous resting spore; (b) valve with central labiate processes. From Hasle & Syvertsen (1990c). Scale bar = 20 /¿m. Gossleriella tropica: valve with girdle spines. From Hustedt (after Schtitt) (1930). Scale bar = 100 fim. Actinocyclus curvatulus: (a) cell in girdle view; (b) valve with curvatulus structure indicated in two sectors with the pseudonodulus located in one. Scale bars = 20 /xm.

retained it in Coscinodiscaceae, and Round et al. (1990) introduced the monotypic family Gossleriellaceae Round. Nikolaev (1983) placed it in the family he described as Symbolophoraceae which was later renamed Stellarimaceae (Sims 8c Hasle 1990). The justification for this classification is the absence of a marginal ring of processes and a single central labiate (SEM) process similar in shape to those of Stellarima. The genus most probably comprises only the generitype.

Gossleriella tropica Schutt (Plate 19)

References: Schutt, 1892, p. 258, Fig. 63; Hustedt, 1930, p. 500, Fig. 280; Hargraves, 1976; Delgado 8c Fortuflo, 1991, Plate 44, Fig. b. Girdle view: Discoid with flat or slightly convex valves and a ring of siliceous spines attached to the cingulum. Chloroplasts numerous small plates, each with a pyrenoid. Valve view: Valve surface poroid. A single process in or near valve center.

Morphometric data: Pervalvar axis ca. 5-7.5 /im; diameter of a valve without spines 96-250 fim, with spines 162-293 fim (Hargraves, 1976). Distribution: Warm water region—lower photic zone (Hargraves, 1976). How to identify: Whole cells are easily identified in water mounts; the valve structure is visible with LM by examination of cleaned material in permanent mounts.

Family Hemidiscaceae Hendey 1937 emend. Simonsen 1975 ex Hasle 1995 (this publication)

Terminology specific to Hemidiscaceae

Pseudonoduius—a marginal to submarginal structure, always only one per valve, with LM evident as an open hole or an area covered by densely packed smaller areolae.

The pseudonoduius is the principle diagnostic feature of the Hemidiscaceae as emended by Simonsen (1975) and encompassing Actinocyclus, Hemidiscus and Roperia. Watkins 8c Fryxell (1986) questioned the taxonomic preeminence of the pseudonoduius, and Fryxell et al., 1986b, p. 33) found that the best place for Azpeitia, which has no pseudonoduius, was within Hemidiscaceae. This placement was "based in part on the fact that the Coscinodiscaceae is ill-defined at the present dynamic stage of diatom systematics."

Glezer et al. (1988) placed Actinocyclus, Hemidiscus, and Roperia in Hemidiscaceae whereas the monotypic family Azpeitiaceae Glezer 8c Makarova was introduced for Azpeitia. Round et al. (1990) followed Fryxell et al. (1986b), placing Azpeitia and the three genera mentioned in Hemidiscaceae.

Characters common to Actinocyclus, Azpeitia, Hemidiscus, and Roperia: Cells low cylindrical to discoid.

Few bands, unperforate (hyaline).

Areolae loculate (SEM).

Cribra external, foramina internal (SEM).

Valve areolation radial, often fasciculate.

Valve face and mantle more or less different in areolation.

One marginal ring of large labiate processes, similar in shape and generally also in size.

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