(Kofoid & Skogsb.) Balech (1973: 8, pl. 1, figs 6–17)
Phalacroma contractum Kofoid & Skogsb. (Kofoid & Skogsberg, 1928: 83, fig. 3, 1)
Phalacroma ruudii Braarud (1935: 112, fig. 32)Prodinophysis contracta (Kofoid & Skogsb.) Balech (1944: 429)Prodinophysis ruudii (Braarud) A.R.Loebl. (Loeblich, 1965: 17)
Balech (1976: 35, fig. 23), Dodge (1982: 48, fig. 4H; pl. 2b)
[see Dodge (1982) for a detailed description] Cells small, rounded to ellipsoidal, 20–30 µm long, 17–25 µm diameter. Epitheca large and conical, occupying up to one-third the length of the cell. Cingulum 5 µm wide, deeply incised, smooth, with rows of marginal pores; cingular lists greatly reduced. Sulcal lists narrow, inconspicuous by LM. Cell surface densely covered with pores; apical and antapical excrescences absent. Chloroplasts present.
Southern Ocean, south of Australia; Southern Ocean, near South Georgia (Dodge & Priddle, 1987); Falkland Is. and the south-eastern Atlantic (Balech, 1988).
Dinophysis is a distinctive genus with up to ten representatives in the Southern Ocean. While diversity is lower than in many other marine environments, occasionally species of Dinophysis have been observed to dominate Southern Ocean dinoflagellate floras (Dodge & Priddle, 1987).The genus is characterised by a small rounded to flat epitheca and a much larger, rounded to elliptical hypotheca. The cingulum is well defined and bordered by prominent lists. Lists are also associated with the sulcus which, because of the lenticular shape of the cell, are usually located on the side.Many authors consider Phalacroma to be synonymous with Dinophysis (e.g. Tai & Skogsberg, 1934; Abé, 1967). The genera overlap in many aspects of their morphology, but can be separated the distinctive, funnel-shaped, anterior cingular list of Dinophysis. Other features, such as the development and direction of cingular lists in combination with the height and shape of the epitheca, also help to distinguish the genera. Members of the genus Dinophysis are specialised predators, and Hansen (1991) observed D. rotundata Clap. & J.Lachm. (= Phalacroma rotundatum ) and D. hastata Stein ingesting the prostomatid ciliate Tiarina fusus (Clap. & J.Lachm.) Bergh.
The large, conical epitheca distinguishes this from most other Dinophysis species. Dinophysis tenuivelata also has a prominent epitheca, but it lacks a deeply incised cingulum. This is the most common Dinophysis species in coastal waters of eastern Antarctica; however, it never comprises more than 10 percent of a dinoflagellate community.