Gymnodinium pulchellum

G. pulchellum is a planktonic species first described from southeastern Australia. This species is a bloom-former associated with extensive fish and invertebrate kills in southeast Florida. During one red tide event waters turned an orange-red color with cell levels recorded as high as 19.7 X 106 cells/L (Steidinger et al. 1998).

The name 'pulchellum' originates from the Latin word pulchellus, 'beautiful little' (Larsen 1994).

This species is found in temperate to tropical neritic waters. It has been reported from Hobsons Bay (Melbourne area), Australia, where it is often common during the austral summer and early autumn (Larsen 1994). It has also been recorded from Tasmanian waters (Hallegraeff 1991), Japanese waters (Fukuyo et al. 1990; Onoue et al. 1985; Takayama 1985) and from the Mediterranean (Carrada et al. 1991). More recently it has been identified in the western Atlantic off the east coast of Florida (Steidinger et al. 1998). Due to its minute size, G. pulchellum may have been greatly overlooked in phytoplankton assessments.

G. pulchellum is a photosynthetic species with several yellowish-brown chloroplasts. Pyrenoids are also present (Figs. 3, 4). The large nucleus is ellipsoidal and located in the left central part of the cell (Figs. 5, 6) (Fukuyo et al. 1990; Larsen 1994; Steidinger & Tangen 1996; Steidinger et al. 1998).

Holotype: Gymnodinium pulchellum Larsen, 1994: 32, fig. 58
Type Locality: Tasman Sea: Hobsons Bay, Melbourne, Australia

G. pulchellum reproduces asexually by binary fission.

Sharing the same habitat and locale, and the same general shape, G. pulchellum can be confused with G. mikimotoi. G. pulchellum, however, is smaller in size and has a distinctive sigmoid apical groove; the apical groove of G. mikimotoi is straight (Larsen 1994).

Gymnodinium pulchellum is an unarmoured, marine, planktonic dinoflagellate species. This species produces red tide blooms and has been associated with fish and invertebrate kills in Japan and Florida.

Gymnodinium type '84-K Onoue et al., 1985

Gymnodinium pulchellum is an athecate species; i.e. without thecal plates. Cells are small and broadly oval with slight dorso-ventral compression (Figs. 1-5). The ventral surface is flattened; the dorsal surface is rounded. A conspicuous and well-defined sigmoid apical groove is present on the epitheca (Figs. 1, 2); the groove is a characteristic reversed S-shape (Fig. 2). Cells range in size from 16-25 µm in length to 11-16 µm in width (Fukuyo et al. 1990; Larsen 1994; Taylor et al. 1995; Steidinger & Tangen 1996; Steidinger et al. 1998).
The epitheca is slightly smaller than the hypotheca. The wide and deeply excavated cingulum is premedian, and is displaced in a descending fashion 1-1.5 times its width (Figs. 1, 3, 6). The sulcus slightly invades the epitheca as a finger-like projection (Fig. 2). The sulcus widens and deepens towards the posterior of the cell creating a bilobed hypotheca (Figs. 1, 3, 4)(Larsen 1994; Taylor et al. 1995; Steidinger & Tangen 1996; Steidinger et al. 1998).

G. pulchellum is a toxic species associated with fish and invertebrate kills from southeast Florida. The presence of this species at two separate fish kills in the Indian River, FL, suggests it is ichthyotoxic (Steidinger et al. 1998). Onoue et al. (1985) demonstrated that G. pulchellum (as Gymnodinium type '84-K) is ichthyotoxic. Three toxic fractions have been isolated from this species: neurotoxic, hemolytic and hemaglutinative (Onoue & Nozawa 1989). G. pulchellum is most likely responsible for fish kills in the Melbourne, Australia, region (Larsen 1994).