Dinophysis fortii

D. fortii is a planktonic oceanic and neritic species (Abè 1967; Taylor et al. 1995; Steidinger & Tangen 1996). It is a bloom-forming species; noxious blooms have been reported from Australia (Hallegraeff 1987) and Japan (Yasumoto et al. 1980; Osaka & Takabayashi 1985; Igarashi 1986). In northern Japan warm currents in spring and early summer carry populations of D. fortii landward where cells filter into coastal areas of intensive shellfish aquaculture (Taylor et al. 1995). Populations seem to be most abundant in early summer (Yasumoto et al. 1980; Osaka & Takabayashi 1985; Igarashi 1986).
Observations of Miyazono and Minoda (1990) suggest that this species prefers high salinity and low temperatures; however, they can tolerate lower salinities. Early studies of Ishimaru et al. (1988) suggest the capablity of D. fortii to prey upon cryptomonads.

D. fortii is widely distributed in cold temperate waters world-wide, but is also found in subtropical to tropical areas (Abè 1967; Taylor et al. 1995; Steidinger & Tangen 1996).

Dinophysis fortii is a photosynthetic species with large central chloroplasts and a terminal pyrenoid (Hallegraeff & Lucas 1988; Larsen & Moestrup 1992).

Holotype: Dinophysis fortii Pavillard, 1923: 881
Type Locality: unknown

D. fortii is best identified by its wide rounded posterior and the presence of reticulations on the sulcal list (Larsen & Moestrup 1992; Steidinger & Tangen 1996). Variations in cell shape are mostly seen in the placement and size of the hypothecal dorsal bulge (Abè 1967).
Many authors consider Phalacroma to be synonymous with Dinophysis (Steidinger & Tangen 1996).

D. fortii reproduces asexually by binary fission.

Dinophysis fortii is an armoured, marine, planktonic dinoflagellate species. This species is a bloom forming toxic species associated with DSP events. It has world-wide distribution in cold temperate waters, but is also found in subtropical to tropical waters.

Species in this genus are laterally compressed with a small, cap-like epitheca and a much larger hypotheca (dorso-ventral depth of epitheca is 1/2 to 2/3 of hypotheca). The shape of the cell in lateral view is the most important criterion used for identification (Taylor et al. 1995).
Cells of Dinophysis fortii are large, long and subovate, ending in a broadly rounded posterior (a dorsal bulge)(Figs. 1-4). The posterior end is the widest. The left sulcal list (LSL) is well developed and very long; it can extend up to 4/5 of the cell length (Figs. 1-3) (Abè 1967; Taylor et al. 1995; Steidinger & Tangen 1996).
The thick thecal plates of the hypotheca are deeply areolated (Figs. 1, 2), each areolae with a pore (Fig. 4). Cell size ranges: 56-83 µm in length and 43-58 µm in dorso-ventral width (at the base of the third rib of the LSL) (Abè 1967; Taylor et al. 1995; Larsen & Moestrup 1992; Steidinger & Tangen 1996).

The small epitheca is made up of four plates. Well developed cingular lists, both anteriorly inclined, obscure the epitheca (Figs. 1-4). The anterior cingular list (ACL), which is wider than the posterior list (PCL), forms a wide and shallow cup with the epitheca as its bottom (Figs. 3, 4) (Fukuyo et al. 1990; Taylor et al. 1995). The sulcus is comprised of several irregularly shaped plates. The flagellar pore is housed in the sulcal area. The LSL is very long, reticulated (Figs. 1, 4) and supported by three ribs (Fig. 3) (Larsen & Moestrup 1992; Taylor et al. 1995). A well-developed triangular right sulcal list (RSL) is also present; it is approximately half the length of the LSL (Figs. 1, 4) (Steidinger & Tangen 1996).
The hypotheca, with four large plates, comprises the majority of the cell. The dorsal margin and posterior end are smoothly convex with a slight concavity near the cingulum (Figs. 2, 3). The ventral margins are fairly straight, slanting at an angle of 110-120 degrees to the cingulum (Figs. 2, 3) (Abè 1967; Larsen & Moestrup 1992; Taylor et al. 1995; Steidinger & Tangen 1996).

Dinophysis fortii is a known toxin-producing species (Lee et al. 1989; Yasumoto 1990; Larsen & Moestrup 1992; Taylor et al. 1995; Steidinger & Tangen 1996). It is the most noxious cause of diarrhetic shellfish poisoning (DSP) in Japanese waters. It produces Dinophysistoxin-1 (DTX1), Dinophysistoxin-2 (DTX2), and okadaic acid (OA) (Lee et al. 1989; Yasumoto 1990), although clones in warmer waters show very low

Dinophysis fortii was the first species found to be associated with DSP; concentrations as low as 200 cells/L can cause human intoxication (Yasumoto et al. 1980).