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Ostreopsis mascarenensis Quod, 1994

Species Overview:

Ostreopsis mascarenensis is an armoured, marine, benthic dinoflagellate species. It was discovered in shallow barrier reef environments and coral reefs in the Mascareignes Archipelago, SW Indian Ocean.

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. mascarenensis are very large and broadly oval (Figs. 1,2). This is the largest species in the genus. The thecal surface is smooth with evenly distributed pores that often contain ejected trichocysts (Figs. 1,3,4). The pores are round containing two small openings with smooth edges (Fig. 4) and a diameter of 0.6 µm. Cells have a dorsoventral diameter of 155-178 µm and a transdiameter of 118-134 µm (Quod, 1994, Faust et al., 1996).

Thecal Plate Description:

O. mascarenensis is a large cell with very large plates (Figs. 1,2). The plate formula for this species is: Po, 3', 7", 6c, 6s?, Vp, Rp, 5''', 1p, 2''''. On the epitheca, the apical pore plate (Po) bears a long curved slit-like apical pore (26 µm) with an array of minute openings (Figs. 1,5,6). The 1' plate is large, long and hexagonal, 102 µm long and 40 µm wide (Figs. 1,6). Plate 3' has a trapezoid shape (Fig. 1). In the hypotheca, the posterior intercalary plate (1p) is long and wide (Fig. 2). Plate 1''' is large compared to other species in the genus (Fig. 2) (Quod, 1994, Faust et al., 1996).

The lipped cingulum is narrow with a smooth edge. It houses the Vo situated on the Vp, and the Rp (Figs. 7,8). The sulcus is recessed and hidden (Fig. 3) (Quod, 1994, Faust et al., 1996).

Morphology and Structure:

Cells of Ostreopsis mascarenensis are photosynthetic with light golden-colored chloroplasts. This species has two pusules in the sulcus and one dorsal red pyrenoid (Quod, 1994).


O. mascarenensis reproduces asexually by binary fission.

Species Comparisons:

O. mascarenensis differs from other species of the genus by its large size, thecal morphology, geotropic swimming behaviour (Quod, 1994) and dissimilar plates, in particular, plates 1', 2', 3', 1''' and 1p (Quod, 1994).


Cells of O. mascarenensis are commonly associated with dead corals and sediments and as epiphytes on macroalgae (Quod, 1994, Faust et al., 1996). Cells exhibit geotropic swimming. Cells may form blooms, reaching a density of >10,000 cells.g fresh weight of algal tissue (Quod, 1994). There is evidence of mixotrophy in this species: prey organisms are engulfed via the ventral pore (Vo), the proposed feeding apparatus (Faust et al. 1996).


This species produces an unnamed toxin which may cause ciguatera (Quod, 1994). This toxin has never been detected in fish (S.L. Morton, pers. com.).

Habitat and Locality:

Populations of O. mascarenensis can be commonly found in shallow (2-5m) barrier reef environments and coral reefs in the SW Indian Ocean. This species has been observed as an epiphyte on Turbinaria sp., Galaxaura sp., dead corals and sediments at Mayotte, Reunion and Rodriguez Islands (Quod, 1994). Cells were also discovered from the lagoonal island, Tobacco Cay, Belize, in the Caribbean Sea (Faust et al., 1996).

Ostreopsis mascarenensis