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References: Castracane, 1886, pp. 89, 90, Plate 9, Fig. 8, Plate 24, Fig. 9; Cleve, 1896a, p. 11, Plate 2, Fig. 21 (no number on the plate); Schutt, 1896, p. 83, Fig. 135; Gran, 1900, p. 113, Plate 9, Figs. 15-20; Pavillard, 1913, p. 126, Fig. la; Hustedt, 1930, pp. 551 and 554, Figs. 314 and 315; Cupp, 1943, p. 76, Fig. 36; Hendey, 1964, p. 142, Plate 5, Fig. 4, Plate 7, Fig. 6, p. 143, Plate 7, Fig. 7; Hasle, 1973a, pp. 15-27, Figs. 44-86; Syvertsen, 1979, p. 55, Figs. 63-69; Round et al., 1990, pp. 142-143; Takano, 1990, pp. 168-169.

Generic characters: Tight chains. Cylindrical cells.

Weakly silicified vegetative valves.

Valve surface with radial ribs and few well-developed areolae.

One central strutted process.

One marginal ring of strutted processes.

One marginal labiate process.

Characters showing differences between species: Size of pervalvar axis and diameter.

Shape of external tubes of marginal strutted processes (SEM). Size of labiate process.

Presence or absence of resting spore formation.

Hasle (1973a) distinguished between three Detonula species. Detonula confervacea (Cleve) Gran (basionym: Lauderia confervacea) and D. pumila are common in marine plankton, while there are few records in the literature of the much larger D. moseleyana (Castracane) Gran (basionym: Lauderia moseleyana Castracane). The single connecting thread from a strutted process in a central valve depression is usually conspicuous in D. pumila and less so in D. confervacea. Detonula pumila is also distinguished from D. confervacea by the longer external tubes of the marginal strutted processes linked in a distinct zig zag pattern. The external tubes of D. confervacea are laterally expanded into a T shape, whereas those of D. pumila and D. moseleyana have the shape of half tubes. The marginal labiate process of D. pumila is smaller than that in the two other species, and D. moseleyana differs from D. pumila mainly by its greater diameter.

Distribution:

D. confervacea—northern cold water region to northern temperate. D. pumila —probably cosmopolitan with a preference for warmer waters. D. moseleyana—Indian Ocean. How to identify: The species may be distinguished in girdle view in water mounts especially by their size (Table 2).

Remarks: Based on Cleve's (1900a, p. 22, Plate 8, Figs. 12 and 13) original description of Thalassiosira condensata as well as on the description and illustrations of the species in Lebour (1930, p. 63, Fig. 35) and Hendey (1937, p. 238, Plate 11, Fig. 11) we suggest that this species also should be put into synonomy with D. pumila. Resting spores (exogenous) are common in D. confervacea and not reported for the two other species. The resting spore valves are coarsely areolated and smoothly curved, similar to the valves of Thalassiosira spp. with one central process. The external tubes of the marginal strutted processes lack the lateral expansions linking vegetative cells together. Chains of D. confervacea of maximum size and Bacterosira bathyomphala are similar, both having many chloroplasts and a short or no distance between adjacent cells. The distinction is seen by paying attention to the external parts of the marginal strutted processes of end valves of D. confervacea chains, visible with LM in this species but not in B. bathyomphala.

Genus Lauderia Cleve 1873 Type: Lauderia annulata Cleve. Monospecific genus.

Lauderia annulata Cleve (Plate 1) Synonym: Lauderia borealis Gran.

References: Cleve, 1873a, p. 8, Plate 1, Fig. 7; Gran, 1900, p. 109, Plate 9, Figs. 1-8; Hustedt, 1930, p. 549, Fig. 313; Cupp, 1943, p. 74, Fig. 35; Hasle, 1973a, p. 3, Figs. 1-3; Syvertsen & Hasle, 1982; Takano, 1990, pp. 170-171.

Girdle view: Cells in chains fairly close (separated by occluded processes on marginal part of valve). Pervalvar axis slightly longer than diameter. Valve view: Valve surface with faint radial ribs. Prominent central annulus, sometimes with a few processes. A large marginal labiate process. Numerous strutted processes on valve face and margin. Long occluded processes in marginal zone (types of processes not differentiated with LM).

Morphometric data: Cell diameter 24-75 ¡jlm, pervalvar axis 26-96 /nm, more than 30 radial ribs in 10 jj.m on valve face. Distribution: Warm water region to temperate.

How to identify: Whole cells of B. bathyomphala, D. putnila and Lauderia annulata are distinguished in water mounts by the way the cells are linked together in chains. At a certain focus, adjacent cells of B. bathyomphala seem to be separated by a central lenticular opening. The external structures of the marginal strutted processes of D. pumila are linked midway between adjacent valves, and the tubular occluded processes of L. annulata run from one adjacent valve to the next. In critical cases cleaned valves mounted in a medium of a high refractive index may be examined to show the differences in process patterns.

Genus Minidiscus Hasle 1973 (Plate 2, Table 3)

Type: Minidiscus trioculatus (F. J. R. Taylor) Hasle.

Basionym: Coscinodiscus trioculatus F. J. R. Taylor.

References: Taylor, 1967, p. 437, Plate 5, Fig. 43; Hasle, 1973a, p. 29, Figs.

101-108; Takano, 1981; Rivera & Koch, 1984; Takano, 1990, pp.

Generic characters:

Usually observed as single cells.

Valves with a more or less prominent hyaline margin.

Mantle usually high.

Processes more or less concentrated in valve center. KEY TO SPECIES

1 a. Hyaline valve margin prominent 2

lb. Hyaline valve margin missing or extremely narrow

M. comicus Takano

2a. Processes close together in a nonareolated, undulated central part

M. chtlensis Rivera

2b. Processes separated by one to several areolae

Distribution:

M. comicus—described from Japanese waters (Takano, 1981) and recorded from Argentine waters (Lange, 1985), the English Channel, the Adriatic, and the Gulf of Mexico (G. Hasle and E. Syvertsen, unpublished observations).

Planktoniella blanda

Planktoniella muriformis

Pervalvar axis Diameter Species (|tm) (|xm)

M. comicus

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