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Ostreopsis heptagona Norris et al., 1985

Species Overview:

Ostreopsis heptagona is an armoured, marine, benthic dinoflagellate species. It was discovered in the Florida Keys.

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 Ostreopsis heptagona are large, broadly oval, oblong and pointed (Figs. 1-2). Thecal surface is smooth with scattered small round pores (0.3um in diameter) that can only be observed at the SEM level (Fig. 3). Cells have a dorsoventral diameter of 80-108 µm, and a transdiameter of 46-59 µm (Faust et al., 1996).

Thecal Plate Description:

The plate formula of Ostreopsis heptagona is: Po, 3', 7'', 6c, 6s?, Vp, Rp, 5''', 1p, 2''''. The epitheca contains 11 plates. The apical pore plate (Po) is 15 µm long, narrow and curved (Figs. 1,4), situated between apical plates 1', 2' and 3', with a long, slit-like apical pore. The 1' plate is large and irregularly heptagonal (seven-sided), and the distinguishing plate for this species (Fig. 1). The hypotheca has eight plates. The posterior intercalary plate (1p) is one of the most characteristic plates of O. heptagona; it is long and narrows dorsally, extending along the dorso-ventral axis (Figs. 2,5) (Faust et al., 1996, Norris et al., 1985).

The cingulum is equatorial and narrow (Figs. 2,4). Within the cingulum the Vo is situated on the Vp, adjacent to the Rp (Figs. 6,7) (Faust et al., 1996). Norris et al., 1985 identified 5 sulcal plates and a transitional plate (t) in this species (Fig. 6,10).

Morphology and Structure:

Ostreopsis heptagona is a photosynthetic species.

Reproduction:

Cells of O. heptagona reproduce asexually by binary fission.

Species Comparisons:

Ostreopsis heptagona is distinguished by two major features: a) an irregulary-shaped asymmetric heptagonal (seven-sided) 1' plate that occupies the left center of the epitheca (Faust et al., 1996, Steidinger and Tangen, 1996); and b) the pentagonal and dorso-ventrally elongate 1p plate in the hypotheca (Faust et al., 1996).

In O. heptagona plate 5'' is pentagonal as it contacts plates 1', 3' and 6'', and plate 6'' is quadrangular and does not touch 3'. In both O. siamensis and O. ovata plate 5'' is quadrangular and does not touch 1', while 6'' is pentagonal and contacts two apical plates, 1' and 3'. Plate 1p in O. heptagona is rather narrow, and is always curved, concaved to the left and gradually narrows dorsally (Faust et al., 1996). Plate 1p in O. siamensis is also narrow, but maintains nearly the same width throughout its length. This plate is different in O. ovata: 1p is comparatively wider and shorter, and widens dorsally (Norris et al., 1985).

Ecology:

Cells of O. heptagona are frequently found as epiphytes on macroalgae in the Caribbean (Morton and Faust, 1997). Live cells exhibit an unusual jerky swimming motion and a strong positive geotropic tendency. Cells almost immediately attach to the nearest substrate (another dinoflagellate, diatom, detritus or petri dish). Cells attach tenaciously by a network of mucilage strands (Fig. 4) which are expelled by thecal pores (Norris et al., 1985).

Mixotrophy has been documented in other species of this genus with the ventral pore (Vo) as the proposed feeding apparatus (Faust et al., 1996).

Toxicity:

This species was determined to be toxic (J. Babinchak, according to Norris et al., 1985).

Habitat and Locality:

Populations of O. heptagona have been reported as epiphytic on macroalgae in the Caribbean Sea (Morton and Faust, 1997), and found in the plankton in the Florida Keys (Steidinger and Tangen, 1996). Maximum densities were reported for O. heptagona associated with Dictyota dichotoma (Bomber, 1985) and Acanthophora spicifera (Morton and Faust, 1997).

Ostreopsis heptagona