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Ostreopsis siamensis Schmidt, 1902

Species Overview:

Ostreopsis siamensis is an armoured, marine, benthic dinoflagellate species. It was first identified from plankton samples from the Gulf of Siam (Thailand).

Taxonomic Description:

Species in this genus are anterio-posteriorly compressed and are observed in apical or antapical view. The epitheca and hypotheca are not noticeably different in size. Unique features of this genus are on the cingulum. In ventral view the cingulum reveals two prominent structures: a ventral plate (Vp) with a ventral pore (Vo), and an adjacent curved ridged plate (Rp). The distinguishing feature at the species level is the shape of the first apical plate (1') on the epitheca (Fig. 1) (Faust et al., 1996).

Cells of O. siamensis are tear-shaped in apical view and attenuated laterally (Figs. 1,2). The thecal surface is smooth with evenly scattered round pores (Figs. 1,2). Large (0.5 µm diameter) and small (0.1 µm diameter) pores are present (Fig. 3). Cells have a dorsoventral diameter of 108-123 µm and a transdiameter of 76-86 µm (Faust et al., 1996).

Thecal Plate Description:

The plate formula for Ostreopsis siamensis is: Po, 3', 7'', 6c, 6s?, Vp, Rp, 5''', 1p, 2''''. On the epitheca, a narrow curved apical pore plate (Po) is closely associated with the narrow apical plate 2' (Fig. 4). The apical pore appears as a curved slit 2 µm long (Figs. 1,4). The 1' plate is narrow and pentagonal (Fig. 1). The hypotheca is composed of eight plates (Fig. 2). The posterior intercalary plate (1p) is large, elongated (26 X 55 µm), and pentagonal (Fig. 2). Plate 1'''' contacts the sulcal region (Figs. 2,5) (Faust et al., 1996).

The narrow cingulum is deep with a smooth edge (Figs. 1,2) and is composed of six plates. In the cingulum the Vo is situated on the Vp next to a protuberant Rp (Figs. 5,6). The Vo may be open or closed. The sulcus is small, recessed and hidden below plates 1'''' and 2'''' (Fig. 5) (Faust et al., 1996).

Morphology and Structure:

Cells of O. siamensis are photosynthetic and contain numerous golden-brown chloroplasts (Fig. 8). A large nucleus is posterior (Faust et al., 1996).


O. siamensis reproduces asexually by binary fission.

Species Comparisons:

O. siamensis differs from other species of the genus by a number of features: 1. tear-drop shape; 2. large cell size; and 3. small round evenly distributed thecal pores (Faust et al., 1996).


O. siamensis are benthic, epiphytic and can be tycoplanktonic (Steidinger and Tangen, 1996). They have been observed in plankton samples, but it is most frequently associated with sand and as epiphytes on macroalgae. Engulfed cells were often observed in this species collected from Belizean waters indicating mixotrophic feeding. The ventral pore (Vo) is the proposed feeding apparatus (Faust et al., 1996). These cells swim very slowly and spin around the dorso-ventral axis (Fukuyo, 1981).


This species is a known toxin producer; it produces an analog of palytoxin (Nakajima et al., 1981, Usami et al., 1995).

Habitat and Locality:

Ostreopsis siamensis has been observed in various tropical regions of the world. Populations were originally discovered in plankton samples collected from the Gulf of Siam (Thailand) (Schmidt, 1902: figs. 5-7) and then seldom observed again for over 70 years. Cells were later found as epiphytes on macroalgae in the Pacific Ocean (Taylor, 1979a, Yasumoto et al., 1980a, Fukuyo, 1981, Nakajima et al., 1981, Holmes et al., 1988), the SW Indian Ocean (Quod, 1994), the Florida Keys (Bomber, 1985), and the Caribbean region (Carlson, 1984, Tindall et al., 1984, Ballantine et al., 1988, Faust, 1995, Faust and Morton, 1995). They have also been associated with sand in the Caribbean (Faust et al., 1996).

Ostreopsis siamensis