Alexandrium monilatum (Howell) Balech, 1995
Species Overview:
Alexandrium monilatum is an armoured, marine, planktonic dinoflagellate. It is a coastal warm water species associated with toxic red tides and massive fish and shellfish kills.
Taxonomical Description:
A very distinctive chain-forming species, A. monilatum typically occurs in long chains of 16 or more cells. Single cells are medium to large, wider than long, and flattened anterio-posteriorly (Figs. 1,2). Epithecal shoulders are occasionally observed. Thecal plates are thin with many delicate pores. Cells range in size between 28-52 µm in length and 33-60 µm in transdiameter width (Balech, 1995, Taylor et al., 1995, Steidinger and Tangen, 1996).
Thecal Plate Description:
The plate formula for A. monilatum is: Po, 4', 6'', 6c, 10s, 5''', 2''''. The large apical pore complex (APC) is broadly triangular and slightly curving posteriorly (Fig. 3). The large apical pore plate (Po) is ovate with a small comma-shaped foramen. The anterior attachment pore (aap) is large and round (Fig. 3). Small pores are present along the margin of the Po. The characteristic first apical plate (1') is not connected to the Po; it is short and broadly pentagonal (Figs. 2,3). The 1' plate is typically without a ventral pore, however, specimens from Florida reveal a pore at the juncture where the 1', 2' and 4' plates meet (Fig. 3) (Balech, 1995, Taylor et al., 1995, Steidinger and Tangen, 1996).
The epitheca and hypotheca are nearly equal. The antapex is slightly concave. The median cingulum is deeply excavated, devoid of lists, and is displaced in a descending fashion one time its width (Figs. 1,2). The sulcus of this species bears a diagnostic feature: a large and rhomboid-shaped posterior sulcal plate (s.p.) (Fig. 4). The s.p. is concave and recessed with radial markings, and contains a large central posterior attachment pore (pap) (Fig. 4) (Balech, 1995, Taylor et al., 1995, Steidinger and Tangen, 1996).
Morphology and Structure:
A. monilatum is a photosynthetic species with central radiating brownish chloroplasts. The quarter-moon shaped nucleus is equatorial (Balech, 1995).
Reproduction:
A. monilatum reproduces asexually by binary fission; plane of fission is oblique. This species also has a sexual cycle with armoured isogamous gametes that fuse with cingula at oblique angles (Fig. 5). Gametes range in size from 36 X 36 µm to 47 X 56 µm. After fusion, a planozygote forms which then encysts into a characteristic resting cyst (Figs. 6,7) (Walker and Steidinger, 1979).