Ehrenb. (Ehrenberg, 1837: 61, pl. 18, figs 5–7)
van Valkenberg (1980: 335), Moestrup & Thomsen (1990: pl. 1–16), Henriksen et al. (1993: 29, figs 3–11)
Distephanus speculum (Ehrenb.) Haeckel (1887)Cannopilus calyptra Haeckel (1887)
Cells uniflagellate. Silica skeleton to 70 µm diameter (distance between the tips of the longest spines), comprising 2 hexagonal rings of different sizes connected by 6 bars and with 6 conspicuous protruding spines; cytoplasm contained between the hexagonal rings. Bars attached to corners of a small ±circular ring. Large hexagonal ring c. 17–23 µm diameter; small hexagonal ring c. 7–11 µm diameter. Spines radiating from the corners of the large ring c. 15 µm long; smaller spines (usually 1–3) projecting from the small ring. Flagellum located near one radial spine, c. 25 µm long, covered in flagellar hairs. Protoplast with a large central nucleus surrounded by numerous discoid golden brown chloroplasts and other organelles.
Drygalski Glacier, Ross Sea and coastal waters off Davis Station, East Antarctica; common in the Southern Ocean (Hada, 1970); Weddell Sea (Balech & El-Sayed, 1965; Fryxell, 1989); Weddell-Scotia Confluence (Garrison et al., 1987).
This silicoflagellate is one of the most spectacular and readily identified organisms in the Southern Ocean.
There is evidence of plasticity of skeletal configuration in this genus (van Valkenberg 1980, Hendricksen 1993).
A small percentage of D. speculum skeletons can have morphological anomalies, with a 5-fold or a 7-fold rather than 6-fold symmetry (sometimes referred to as var. pentagonus Lemmerm. and var. septanarius (Ehrenb.) Jörg., respectively). Twin cells with opposed and partly interlocked skeletons represent dividing cells.