M. chilettsis—described from Chile (Rivera & Koch, 1984) and recorded from localities in the Pacific as well as the Atlantic Oceans between ca. 60°N and 50°S (Sancetta, 1990, and Hasle and Syvertsen, unpublished observations) and off the Argentine coast (Ferrario, 1988). M. trioculatus—described from the Indian Ocean and regarded as cosmopolitan in distribution (Hasle, 1973a). How to identify: Minidiscus spp. belong to the smallest known planktonic, centric diatoms. Due to the small size they can hardly be identified to species with LM. With electron microscopy (EM) they are distinguished by differences in areolation and location of the processes. Remarks: Since these species are numerically important in marine nanoplankton but easily overlooked, their worldwide distribution is still uncertain.

Genus Planktoniella Schutt 1892 (Plate 2, Table 4) Type: Planktoniella sol (Wallich) Schutt. Basionym: Coscinodiscus sol Wallich.

References: Wallich, 1860, p. 38, Plate 2, Fig. 1; Schmidt, 1878, Plate 59, Figs. 35-37; Schtitt, 1892, p. 258, Fig. 64; Hustedt, 1930, p. 464, Fig. 259; Cupp, 1943, p. 63, Fig. 27; Loeblich et al., 1968; Fryxell & Hasle, 1972, Figs. 34-36; Round, 1972; Desikachary, 1989, p. 9, Plates 742-744; Hallegraeff, 1992, Figs. 1-12; Hasle & Syvertsen, 1993, p. 303, Figs. 17-31.

PLATE 2 Minidiscus trioculatus, M. comicus, and M. chUensis: Single valves. Structure seen with EM. Scale bar = 1 fim. Planktoniella blanda: cells in valve view having varying number of lobes; cell to the left with areolation and process pattern. Scale bar = 10 /xm. Planktoniella sol: cell in valve view with wing, central strutted process, and two marginal labiate processes. Scale bar = 10 fjLm. Planktoniella muriformis: colony of many cells in valve view connected by an organic matrix. Scale bar =10 /um.


Diameter (|im)

Areolae in 10 |jim

Marginal proc. in 10 jim

No. of labiate proc.

P. blanda

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