Alexandrium minutum Halim, 1960
Species Overview:
Alexandrium minutum is an armoured, marine, planktonic dinoflagellate. It is a widely distributed species associated with toxic PSP blooms in coastal regions.
Taxonomical Description:
Cells of A. minutum are small, nearly spherical to ellipsoidal, somewhat dorsoventrally flattened and occassionally longer than wide (Figs. 1-4). Cells are single with a characteristic ventral pore on the first apical plate, 1' (Figs. 2-4,8,9). Thecal plates thin. Thecal surface ornamenation can vary from light to heavy reticulation (mostly confined to the hypotheca) with small scattered pores. Intercalary bands are present (Figs. 1-3). Large range in size in this species: between 15-30 µm in length and 13-24 µm in transdiameter width (Balech, 1989, Balech, 1995, Hallegraeff, 1991, Taylor et al., 1995, Steidinger and Tangen, 1996, Hwang et al., 1999).
Thecal Plate Description:
The plate formula for A. minutum is: Po, 4', 6'', 6c, 10s, 5''', 2''''. The epitheca is larger than the hypotheca. The apical pore complex (APC) is oval to broadly triangular and pointed posteriorly (Fig. 3). The apical pore plate (Po) is large, narrow and oval with a wide foramen (Figs. 3,9). The Po can be either in direct contact with the first apical plate (1') (Figs. 3,9a) or indirectly connected via a thin suture (thread-like process) (Fig. 9b). This connection can be obscured by plate growth overlap by plates 2' and 4'. A characteristic ventral pore is located on the slender and rhomboidal 1' plate (Figs. 2-4, 8,9). The distinctive sixth precingular plate (6'') is long and narrow (Fig. 1) (Balech, 1989, Balech, 1995, Hallegraeff, 1991, Taylor et al., 1995, Steidinger and Tangen, 1996, Hwang et al., 1999).
The epitheca is hemielliptical to conical with convex sides (Figs. 1,2,4). The apex is broadly rounded. The short hypotheca is hemielliptical with a convex to flat antapex (Figs. 1,2,4). The deeply excavated cingulum is displaced in a descending fashion one time its width with thickened margins (Figs. 1,2,4). The sulcus is shallow with narrow lists (Figs. 1,2) (Balech, 1989, Balech, 1995, Hallegraeff, 1991, Taylor et al., 1995, Steidinger and Tangen, 1996, Hwang et al., 1999).
Morphology and Structure:
A. minutum is a photosynthetic species with an elliptical nucleus (Balech, 1989, Balech, 1995).
Reproduction:
A. minutum reproduces asexually by binary fission. This species also has a sexual cycle that produces a characteristic resting cyst (Figs. 6,7) (Bolch et al., 1991).