(Gourret) Cleve (1900b: 15)/(Ehrenb.) Dujard. (1841: 378) complex
Dodge (1982: 231), Steidinger & Tangen (1997: 472)
Cells large, fusiform, 250 µm to more than 1 mm long, 15–25 µm diameter; right antapical horn small or absent; left hypothecal horn straight. Cingulum c. 10 µm wide, only slightly excavated, ornamented with pores; cingular lists very narrow. Cell surface generally smooth, with scattered pores; small spines are sometimes present.
Southern Ocean, 43–62°S, south of Australia; south of 70°S (Balech, 1968); cosmopolitan in cold-temperate to tropical waters (Steidinger & Tangen, 1997).
A distinctive, armoured genus with long, apical and antapical horns, a prominent cingulum, clear tabulation and longitudinal ribs (Jörgensen, 1911). Tabulation of Ceratium is typically 4', 7'', 6c, 12s, 6''', 1p, 1''''. The central body is sometimes dorsally compressed and chloroplasts are present, although some species have food vacuoles due to phagocytic activity. Only a few taxa have been collected from Antarctic waters, the most prominent being the C. pentagonum/C. lineatum complex and the C. extensum/C. fusus complex.
There is considerable morphological overlap between Ceratium extensum and C. fusus. Both are large fusiform dinoflagellates with fully developed apical and left antapical horns but reduced right antapical horns. The difference between the two is based on the relative length of the hypotheca; in C. extensum it is distinctly longer than the epitheca. However, we found this character to be unreliable.Okamoto & Takahashi (1984) did not find this complex south of 50°S, and they reported it to be rare at water temperatures below 7°C.