Bysmatrum subsalsum (Ostenfeld) Faust and Steidinger, 1998
Species Overview:
Bysmatrum subsalsum is an armoured, marine, benthic dinoflagellate species. Cells are commonly found in temperate and tropical marine environments.
Taxonomic Description:
Unique to the Bysmatrum genus is the recessed chamber-like apical pore complex (APC) containing a large apical pore plate (Po) with a central raised dome, and an elongated canal plate (X plate). The most striking feature of this genus is the separation of intercalary plates 2a and 3a by apical plate 3' (Horiguchi and Pienaar, 1988b, Faust and Steidinger, 1998).
Cells of Bysmatrum subsalsum are round to convex (Figs. 1,2,3,7) with an oblique axis (Figs. 6,10), and are compressed dorso-ventrally (Fig. 6). Epitheca and hypotheca are nearly equal in size. The epitheca is conical with broadly convex sides and apex (Figs. 2,13). The hypotheca is trapezoidal with a broadly indented center situated between the two posterior lobes (Figs. 1,2). Cell length varies from 32 to 41 µm and width from 28 to 37 µm (Steidinger and Balech, 1977, Faust, 1996a, Faust and Steidinger, 1998).
The thecal surface is distinctly vermiculate to reticulate (Figs. 1-3,5) appearing as striated longitudinal thin lines under LM. Intercalary bands are transversely striated (Figs. 2,3) (Faust, 1996a).
Thecal Plate Description:
The plate formula of B. subsalsum is: Po, X, 4', 3a, 7'', 6C, 4S, 5''', 2''''. The APC is approximately 8 µm long, the longest in the Bysmatrum genus. It is directed toward the ventral center of the cell (Figs. 1,3) and is easily recognized under LM. The Po has a smooth and raised three-segmented collar (Fig. 4). Apical plate 1' is pentagonal and reaches the cingulum (Fig. 1). Characteristic of this genus, the intercalary plates 2a and 3a are separated by apical plate 3' (Figs. 3,11) (Steidinger and Balech, 1977, Faust, 1996a, Faust and Steidinger, 1998).
The cingulum, consisting of six plates, is deep, recessed and finely striated. It is displaced 1 time its width, and aligned with a row of fine pores (Figs. 1,2,5). The sulcus is narrow and deep, and widens posteriorly. It appears to extend to antapical plates 1'''' and 2'''' (Figs. 1,5). The sulcus does not reach the antapex (Steidinger and Balech, 1977, Faust, 1996a).
Antapical plates 1'''' and 2'''' indent the hypotheca ventrally forming a wide space and two posterior lobes (Figs. 1,5). An antapical spine is absent (Faust, 1996a).
Morphology and Structure:
B. subsalsum is a photosynthetic species with an equatorial nucleus and numerous yellow-brown chloroplasts, oval to rod-shaped, and radially arranged. An accumulation body is present as an orange globule, which is variably located. Large median pusule is usually located on the right side. An eyespot is evident in the midsulcal area (Steidinger and Balech, 1977, Faust, 1996a).
Reproduction:
Bysmatrum subsalsum reproduces asexually by binary fission.
Species Comparison:
Bysmatrum subsalsum is similar to three species: Bysmatrum arenicola, B. caponii and Peridinium sociale. These species share several thecal plate arrangement characteristics, most notably the lack of contact between intercalary plates 2a and 3a, as well as a pentagonal and asymmetric 1' apical plate and a chamber-like APC (Steidinger and Balech, 1977, Horiguchi and Pienaar, 1988a, Faust and Steidinger, 1998).
B. subsalsum differs from B. arenicola, a tide pool species, by a number of features: a. smaller average size; b. reticulated thecal plates; c. an eyespot; and d. different habitat (Horiguchi and Pienaar, 1988b).
Bysmatrum caponii, another tide pool species, shares a number of features with Bysmatrum subsalsum: a. round to convex shape with an oblique axis; b. presence of a pusule and stigma; c. radiating chloroplasts; d. elongated nucleus; and e. reticulate thecal pattern that gives the appearance of longitudinal striae (Steidinger and Balech, 1977, Horiguchi and Chihara, 1983).
B. subsalsum, however, differs from B. caponii by several features: a. larger average cell size; b. position of nucleus; c. lack of antapical spines; and d. different habitats (Horiguchi and Pienaar, 1988a).
Peridinium sociale differs from Bysmatrum subsalsum by a number of characteristics: a. larger average size; b. Po very angular; c. lacks oblique axis; d. possesses antapical spines; and e. different habitat (Biecheler, 1952, Steidinger and Balech, 1977).