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Bysmatrum arenicola Horiguchi and Pienaar, 2000

Species Overview:

Bysmatrum arenicola is an armoured, marine, sand-dwelling, benthic dinoflagellate species. It was discovered in tidal pools and sandy substrates along the east coast of South Africa.

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 arenicola are almost ellipsoidal (Fig. 1), dorso-ventrally flattened with a slightly oblique longitudinal axis, inclining ventrally (Fig. 2). The epitheca is conical (Figs. 2,7,8), whereas the hypotheca is hemispheroidal, occasionally observed with an obliquely flattened antapex (Figs. 1,7,8). Cells range in size from 40 to 58 µm long, 34 to 46 µm wide, and 21 to 28 µm in dorso-ventral depth (Horiguchi and Pienaar, 1988b).

Thecal plate surface is reticulate: they are covered with minute, wart-like projections which are linearly arranged. Trichocyst pores are mostly found in the peripheral region of each plate. Smooth intercalary bands are only occasionally observed (Fig. 6) (Horiguchi and Pienaar, 1988b).

Thecal Plate Description:

The plate formula of B. arenicola is: Po, X, 4', 3a, 7'', 6C, 4S, 5''', 2'''' (Fig. 11). The APC is trapezoidal with rounded corners; an elongated canal plate (X plate) is present (Fig. 11). Raised margins around the APC are formed by the overlapping borders of apical plates 1'-4'. The Po is about 0.8 µm in diameter with a raised margin. Because of the asymmetrical arrangement of the epitheca, the APC is directed to the cell's right (Fig. 11). Apical plate 1' is pentagonal and reaches the cingulum (Fig. 11). Characteristic of this genus, the intercalary plates 2a and 3a are separated by apical plate 3' (Figs. 5,11) (Horiguchi and Pienaar, 1988b, Faust and Steidinger, 1998).

The cingulum, made up of six plates, is well excavated and is displaced almost 2 times its width (Figs. 1,3). The deep sulcus, with smooth narrow lists, indents the hypotheca and widens posteriorly (Fig. 1) (Horiguchi and Pienaar, 1988b).

The length of the longitudinal flagellum is about 1.5 times that of the cell (Fig. 3). The transverse flagellum emerges from the middle of the sulcus (Fig. 3) (Horiguchi and Pienaar, 1988b).

Morphology and Structure:

Cells of Bysmatrum arenicola are photosynthetic, with many brown to reddish-brown, rod-shaped and radially arranged chloroplasts. The nucleus is oval and situated on the dorsal side of the hypotheca or almost in the center of the cell (Fig. 8). Pyrenoids are present in both the epitheca and hypotheca. One large vacuole is usually situated in the right side of the hypotheca (Fig. 7). A pusule is also present (Horiguchi and Pienaar, 1988b).

A peduncle is present in the sulcus (Fig. 4), and emerges from almost the same point as the longitudinal flagellum. It is short and thin with a narrow, thread-like tip (Figs. 3,4) (Horiguchi and Pienaar, 1988b).

This species is capable of forming an apical stalk, a stub-like structure found on the epitheca used to anchor the cell to a substrate. It is formed by extraction of gelatinous material from the apical pore region (Fig. 9) (Horiguchi and Pienaar, 1988b).


Bysmatrum arenicola reproduces asexually by binary fission; plane of fission is oblique (Fig. 10) (Horiguchi and Pienaar, 1988b).

Species Comparisons:

Bysmatrum arenicola is similar to three species: Bysmatrum caponii, B. subsalsum 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. caponii, also a tide pool species, differs from B. arenicola by several features: a. conical shape and almost pentagonal outline; b. smaller average cell size; c. lack of prominent dorso-ventral compression; d. heavily striated thecal plates; e. possession of antapical spines; and f. presence of an eyespot (Horiguchi and Pienar, 1988b).

B. subsalsum differs from B. arenicola also by a number of features: a. smaller average size; b. reticulated thecal plates; c. an eyespot; and d. different habitat (Horiguchi and Pienaar, 1988b).

P. sociale differs from B. arenicola by a number of features: a. almost pentagonal shape; b. lack of dorso-ventral compression; c. reticulated thecal plates; d. presence of antapical spines; e. presence of an eyespot; and f. different habitat (Horiguchi and Pienaar, 1988b).

Stalk forming dinoflagellates are few in number. All are photosynthetic and have specialized habitats: tide pools (Bysmatrum arenicola and Scrippsiella hexapraecingula); interstitial water (Stylodinium littorale); and freshwater (Stylodinium tarnum, S. polymorphum) (Horiguchi and Pienaar, 1988b).


Cells of the benthic Bysmatrum arenicola form dense patches on the surface of the sand within tide pools. They exhibit verticle migration within the interstitial space: cells migrate from the sand surface to the bottom of the sand layer just before the incoming tide. During the day, motile cells swim just above the surface of the substratum and remain non-motile or slowly rotate (Horiguchi and Pienaar, 1988b).

The apical stalk is used to anchor the cell firmly to the substratum via mucous strands (Fig. 9). This species is capable of ecdysis (Horiguchi and Pienaar, 1988b).


This is not a known toxin producer.

Habitat and Locality:

Populations of B. arenicola have been collected from dense brown-colored patches of sand in tide pools from the east coast of the Republic of South Africa: Rocky Bay, Widenham, Mauwalume, Palm Beach, and Umtentweni (Horiguchi and Pienaar, 2000).

Bysmatrum arenicola