Alexandrium ostenfeldii (Paulsen) Balech and Tangen, 1985
Species Overview:
Alexandrium ostenfeldii is an armoured, marine, planktonic dinoflagellate. Generally, it is a cold-water coastal species found in low numbers mainly along the west coast of Europe.
Taxonomical Description:
A distinctive species, cells of A. ostenfeldii are medium-sized and nearly spherical (Fig. 1). Cells are single, but are often found in two-celled colonies. Epitheca and hypotheca equal in height (Figs. 1). This species has thin thecal plates and a characteristic large ventral pore on the first apical plate (1' ) (Figs. 2,3). Faint surface pores are numerous and unevenly distributed. Cells range in size between 40-56 µm in length and 40-50 µm in transdiameter width (Balech, 1995, Balech and Tangen, 1985, Konovalova, 1993, Larsen and Moestrup, 1989, Taylor et al., 1995, Steidinger and Tangen, 1996).
Thecal Plate Description:
The plate formula for A. ostenfeldii is: Po, 4', 6'', 6c, 10s, 5''', 2''''. The apical pore plate (Po) is relatively large with a large comma-shaped foramen (Figs. 3,7). It can be either in direct contact with the first apical plate (1') or indirectly connected via a thin suture (thread-like process) (Fig. 7). The most distinctive plate of this species is the 1' plate: a) it bears a large characteristic ventral pore; and b) a 90 degree angle is formed at the point where the ventral pore and the 4' plate come in contact (Figs. 3,4). The distinctive sixth precingular plate (6'') is wider than high (Fig. 6) (Balech, 1995, Balech and Tangen, 1985, Larsen and Moestrup, 1989, Taylor et al., 1995).
The broad epitheca is convex-conical, while the hypotheca is hemispherical with an obliquely flattened antapex (Fig. 1). The slightly excavated cingulum is equatorial and displaced in a descending fashion less than one time its width; it has narrow lists (Fig. 1,6). The sulcus is slightly depressed and inconspicuous (Balech, 1995, Balech and Tangen, 1985, Konovalova, 1993, Larsen and Moestrup, 1989, Taylor et al., 1995).
Morphology and Structure:
A. ostenfeldii is a photosynthetic species with radiating chloroplasts. The nucleus is U-shaped and equatorial (Balech and Tangen, 1985).
Reproduction:
A. ostenfeldii reproduces asexually by binary fission. This species also has a sexual cycle with isogamous mating types; a planozygote is formed (Jensen and Moestrup, 1997).
Species Comparisons:
A. ostenfeldii is easily misidentified as other Alexandrium species; detailed thecal plate observation is often necessary for proper identification (Balech, 1995, Larsen and Moestrup, 1989).
A. ostenfeldii and A. tamarense are often confused for each other since they overlap in size and often co-occur; however, A. ostenfeldii is slightly larger and is more widely distributed (has a wider salinity range) than the latter species (Moestrup and Hansen, 1988). Other differences between these two species include: A. ostenfeldii has a much larger ventral pore on the first apical plate 1'; and the 6'' plate is wider than high, whereas the width and height of the 6'' plate in A. tamarense are equal (Balech, 1995, Hansen et al., 1992).
This species also closely resembles another Alexandrium species, A. peruvianum. Both species are large cells with distinctive large ventral pores on the 1' plate; however, morphological differences are evident in the 1' plate and the APC. Moreover, A. ostenfeldii is a larger cell and produces PSP toxins (Balech, 1995, Steidinger and Tangen, 1996, Taylor et al., 1995).
REMARKS:
Belonging to the Alexandrium complex, this A. ostenfeldii has a long and complex taxonomic history.