Ostreopsis labens Faust and Morton, 1995
Ostreopsis labens is an armoured, marine, benthic dinoflagellate species. This species was discovered in sand and lagoonal water from the barrier reef of Belize, Central America, and associated with macroalgae from coral reef habitats of Oshigaki and Iriomote Islands, Japan.
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 labens are large and broadly ovate (Figs. 1,2). Thecal surface is smooth covered with numerous large pores arranged in a regular pattern, which can be recognized under the light microscope (Figs. 1-3). Thecal pores are round with smooth edges (average diameter 0.3 µm). Trichocyst pores appear in round elevated structures (Fig. 4). Thecal valve is relatively thick. Cells have a dorsoventral diameter of 86-98 µm, and a transdiameter of 70-80 µm wide (Faust and Morton, 1995).
Thecal Plate Description:
The plate formula of Ostreopsis labens is: Po, 3', 7'', 6c, 6s?, Vp, Rp, 5''', 1p, 2''''. The epitheca in apical view (Fig. 1) is convex and composed of 11 asymmetrical plates. The apical pore plate (Po) is about 18 µm long with a raised edge and a narrow, curved slit-like apical pore (Fig. 1). The large Po, long and narrow, comes in contact with plates 1' and 3'' on the right-ventral side (Fig. 5). It is associated with apical plate 2' and positioned off-center (Fig. 1). The 1' plate is large, pentagonal, and is situated in the left center of the epitheca (Fig. 1). Plates are delineated by a network of intercalary bands (Figs. 1,4) (Faust and Morton, 1995).
The deep lipped cingulum is equatorial and very narrow (Figs. 1,3,6,7) . It is about 3-4 µm wide, composed of six plates and enclosed by smooth edged lists. The inner surface is lined with parallel rows of equally spaced, round pores (Fig. 7). Within the cingulum, in ventral view, the Vo is present on the Vp adjacent to the Rp (Figs. 8,9). The Vo is elongate and protuberant, situated between plates 1'' and 1'''' (Fig. 8). The shape and size of the Vo can vary (Faust and Morton, 1995).
The sulcus is small, narrow, recessed, and hidden under antapical plate 1'''' (Figs. 10,11). A very distinctive plate is present: a protuberant curved ridged plate, Rp, that appears as an extension of the sulcus (Figs. 9-11). Antapical plate 1'''' extends to a slightly curved sulcal list that partially covers the sulcus and plate 2'''' (Figs. 10,11). Two flagella emerge posteriorly (Fig. 11) (Faust and Morton, 1995).
Morphology and Structure:
Ostreopsis labens is a photosynthetic species that contains chloroplasts, starch granules, and a spherical nucleus situated posteriorly (Fig. 15) (Faust and Morton, 1995).
Ostreopsis labens reproduces asexually by binary fission.
Ostreopsis labens is close in size to O. heptagona, but they differ in general shape and epithecal plate pattern. The most distinguishing feature between the two species is the shape of the first apical 1' plate: it is pentagonal in O. labens, and heptagonal in O. heptagona (Faust and Morton, 1995). Moreover, the hypothecal plate designation and size of plates of O. labens and O. heptagona are the same. However, the shape of plate 1p in O. heptagona is broader ventrally (Norris et al., 1985: fig. 3), whereas in O. labens it is elongate extending approximately to the middle of the hypotheca (Faust and Morton, 1995).
The shape of plate 2' is similar in O. labens, O. ovata and O. mascarenensis, but different from that in O. heptagona (Norris et al., 1985), O. lenticularis and O. siamensis (Fukuyo, 1981).
Cells of O. labens are frequently associated with plankton, sand, coral reefs and macroalgae. Cells swim in a distinct gliding motion (Faust and Morton, 1995).
This species displays mixotrophic behavior: cells with large ingested particles within the cytoplasm have been observed (Fig. 16). A number of food vacuolate cells have been reported containing engulfed prey with an intense red color (Faust and Morton, 1995). Engulfment of prey most likely occurs through the ventral pore (Vo) which is the proposed feeding apparatus (Faust et al., 1996).
The toxicity of this species is not known.
Habitat and Locality:
Populations of Ostreopsis labens are found in warm tropical waters in the East China and Caribbean Seas. This species has been observed in several different habitats: plankton, colored sand, coral reefs and macroalgal associations. Cell populations in colored sand can reach cell densities of 2500 cells/g sand and 1200 cells/L in lagoonal waters. Cells epiphytic on macroalgae can be as high as < 50 cells per approximately 10 cm^2 of plant surface (Sargassum, Pedina, Turbinaria and Galaxaura) (Faust and Morton, 1995).