Identifying Harmful Marine Dinoflagellates

Dinophysis acuta
Ehrenberg, 1839
Plate 12, Figs. 1-4

Species Overview: Dinophysis acuta is an armoured, marine, planktonic dinoflagellate species. It is a toxic species associated with DSP events and is commonly found in cold and temperate neritic waters.

Taxonomic Description: 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 acuta are large and robust, and are among the largest species in the genus Dinophysis (Fig. 1). Cells are oblong with a slightly pointed or rounded posterior end (Figs. 1-4). The left sulcal list (LSL) extends beyond the midpoint of the cell (about 2/3 of cell length) ending at or above the widest portion of the cell (Fig. 3) (Balech 1976; Dodge 1982; Larsen & Moestrup 1992; Taylor et al. 1995; Steidinger & Tangen 1996).
The thick thecal plates of the hypotheca are coarsely areolated, each areola with a central pore (Figs. 1, 2, 4). The areolation becomes very faint or disappears near the edge of the plates. Cell size ranges: 54-94 Ám in length and 43-60 Ám in dorso-ventral width (widest below the middle) (Fig. 3)(Balech 1976; Dodge 1982; Larsen & Moestrup 1992; Taylor et al. 1995; Steidinger & Tangen 1996).

Nomenclatural Types:
Holotype: Dinophysis acuta Ehrenberg, 1839: 124, 151, plate 4 (fide Schiller, 1933)
Type Locality: Mediterranean Sea: Gulf of Lion, France

Thecal Plate Description: The small epitheca is made up of four plates. It is low, flat or weakly convex, and is not visible in lateral view (Balech 1976; Larsen & Moestrup 1992; Taylor et al. 1995).
The cingulum is made up of four unequal plates, all with pores. Well developed cingular lists are present: an anterior cingular list (ACL), and a posterior cingular list (PCL). They are generally smooth and rarely ornamented (Fig. 3). The high ACL obscures the low epitheca (Balech 1976; Dodge 1982; Larsen & Moestrup 1992).
The sulcus is comprised of several irregularly shaped plates. The flagellar pore is housed in the sulcal area. The left sulcal list (LSL), supported by three ribs that radiate outward, is rather broad with a convex ventral margin. It is wider posteriorly and slightly areolated. The second sulcal rib is closer to the first than to the third. The third rib is the longest (Figs. 1-4) (Balech 1976; Dodge 1982; Taylor et al. 1995).
The hypotheca, with four large plates, comprises the majority of the cell. The anterior 2/3 of the hypotheca has convex margins, while the posterior third of the hypotheca forms a broad asymmetrical triangle with a straight dorsal edge and occasionally a slightly concave ventral edge (Figs. 1-4). The tapered and roughly pointed antapex is directed slightly ventrally (Figs. 1-4) (Balech 1976; Dodge 1982; Taylor et al. 1995). Balech (1976: figs. 2H, 2I) depicts two specimens with two to three small knob-like spines on the posterior end.

Morphology and Structure: Dinophysis acuta is a photosynthetic species with yellow chloroplasts (Dodge 1982; Larsen & Moestrup 1992).
Dimorphic cells, one half resembling D. acuta and the other half resembling D. dens (the proposed gamete form), have occasionally been observed in this species (Reguera et al. 1990; Hansen 1993; Moita & Sampayo 1993). It is highly probable that these cell forms represent a stage in gametogenesis (Hansen 1993).

Reproduction: D. acuta reproduces asexually by binary fission. Hansen (1993) speculates that sexual reproduction, with sexual dimorphism, is part of the life cycle for this species.

Ecology: D. acuta is a planktonic oceanic and neritic species (Dodge 1982; Taylor et al. 1995; Steidinger & Tangen 1996). This is a bloom-forming species; blooms are often associated with shellfish toxicity (Taylor et al. 1995).

Toxicity: D. acuta is a toxic species that produces okadaic acid (OA), as well as Dinophysistoxin-1 (DTX1) (Lee et al. 1989; Yasumoto 1990). D. acuta has been associated with DSP outbreaks in Chile (Larsen & Moestrup 1992), Portugal (Alvito et al. 1990; Sampayo et al. 1990), Scandinavia (Dahl & Yndestad 1985; Krogh et al. 1985; Underdahl et al. 1985; Edler & Hageltorn 1990), and the USA (Freudenthal & Jijina 1985).

Species Comparison: D. acuta is very similar to D. norvegica in their general shape, and thus can easily be misidentified. D. acuta can be differentiated by its larger size and different shape: D. norvegica is widest in the middle region of the cell, whereas D. acuta is widest below the mid-section. Moreover, D. acuta has a longer left sulcal list relative to its cell length (Balech 1976; Dodge 1982; Larsen & Moestrup 1992; Taylor et al. 1995; Steidinger & Tangen 1996). D. acuta also strongly resembles a warm-water species, D. schroederi Pavillard, 1909 (Schiller 1933; Balech 1976; Burns & Mitchell 1982).

Habitat and Locality: Dinophysis acuta is widely distributed in cold and temperate waters world-wide (Larsen & Moestrup 1992; Steidinger & Tangen 1996).

Remarks: Many authors consider Phalacroma to be synonymous with Dinophysis (Steidinger & Tangen 1996).

Figure 1: Morphology of a Dinoflagellate