Alexandrium acatenella (Whedon and Kofoid) Balech, 1985b

Species Overview:

Alexandrium acatenella is an armoured, marine, planktonic dinoflagellate. It is associated with toxic PSP blooms in Pacific coastal regions.

Taxonomical Description:

A non-chain forming species, cells of A. acatenella are small to medium sized, longer than wide, and angular to round in ventral outline (Figs. 1,2). A characteristic ventral pore is present (Fig. 3). Two short antapical spines are present; no apical horn is present (Fig. 3) . The thecal surface is sculptured with large and small pores. Cells range in size between 35-51 µm in length and 26-35 µm in transdiameter width (Whedon and Kofoid, 1936, Balech, 1995, Taylor et al., 1995, Steidinger and Tangen, 1996).

Thecal Plate Description:

The plate formula for A. acatenella is: Po, 4', 6'', 6c, 9s, 5''', 2''''. The epitheca in this species is longer than the hypotheca: often it is equal to the length of the hypotheca plus the cingulum. The cone-shaped epitheca is low with convex sides (Figs. 1-3). The apical pore complex (APC) is roughly rectangular. The apical pore plate (Po) is broadly oval and narrows ventrally; it bears a relatively large foramen (Fig. 4). The first apical plate (1') comes in direct contact with the Po, and also bears the characteristic ventral pore (vp) (Fig. 4) (Whedon and Kofoid, 1936, Balech, 1995, Taylor et al., 1995, Steidinger and Tangen, 1996).

The post-median cingulum is deeply excavated, and displaced in a descending fashion about 1 time its width without overhanging. Narrow lists are present on the cingulum (Figs. 1-3). The deeply excavated sulcus widens posteriorly flaring to the right, slightly invading the hypotheca. The short hypotheca is broadly rounded with two posterior antapical spines (Figs. 1-3). The antapex region between the spines is slightly concave (Whedon and Kofoid, 1936, Balech, 1995, Taylor et al., 1995, Steidinger and Tangen, 1996).

Morphology and Structure:

A. acatenella is a photosynthetic species with elongated chloroplasts. Cells can be highly pigmented and reddish-brown in color. The elliptical nucleus is C-shaped and equatorial (Whedon and Kofoid, 1936, Prakash and Taylor, 1966, Balech, 1995).

Reproduction:

A. acatenella reproduces asexually by binary fission (Whedon and Kofoid, 1936).

Species Comparisons:

A. acatenella is very similar morphologically (size, shape and thecal plate formula) to the toxic Atlantic species, A. tamarense. Differences lie in the general shape of the cell, thecal sculpture, length of epitheca in relation to the hypotheca, and size and shape of the apical plates. The former species is roundish, while the latter is wider (shoulders) and roughly pentagonal. Thecal plates in A. acatenella are clearly porolated, while in A. tamarense they are relatively smooth. The epitheca in A. acatenella is distinctly longer than the hypotheca; they are nearly equal in A. tamarense. The size and shape of the apical plates differ in these two species (Balech, 1995).

A. acatenella also shares some common characteristics of A. catenella. However, the former species is a non-chain former without a posterior attachment pore, bears a ventral pore on 1', and is usually found in warmer waters (Prakash and Taylor, 1966, Balech, 1995).