Guide to Cymatosira

Species in the genus Cymatosira occur primarily in sand and salt marsh sediments in temperate and warm brackish and marine waters. Filaments of Cymatosira are often suspended in the tychoplankton of coastal waters. Around 17 species have been described, most of which are fossil. Our understanding is that there are only two extant species – the generitype C. lorenziana and C. belgica Grunow in Van Heurck. The description presented here is based on these two species.

Cells are joined in filamentous colonies by prominent linking spines. Frustules are heterovalvar, in that at least one valve is convex; the other valve is either convex, concave, or straight. Typically, a single chloroplast is present in each cell. The chloroplast is centrally located, but may extend toward both apices, filling the cell.

The valve outline varies from broadly to narrowly lanceolate, with rounded, often drawn-out apices. The valve mantle is either steep, broad, and non-areolate, or gently curves into an areolate mantle. A distinct sternum is absent, and the areolae tend to be distributed irregularly across the valve. The poroid areolae are occluded by a cribrum, which is suspended by 4-6 short pegs and usually possesses several minute spinules. Both apices of the valve terminate in an ocellulus; these are slightly angled in opposite directions to the apical axis. The rim of the ocellulus is prominent and forms a funnel-shaped opening enclosing numerous porelli.

There are 2 types of valves in a filament – intercalary and end (separation). The former hold the chain of cells together and the latter occur only at the two ends of a filament. Complex, T-shaped (sometimes Y- shaped) spines from one intercalary valve protrude from just inside the valve margin and interdigitate with those of an adjacent intercalary valve to provide a secure connection between the cells of a filament. The linking spines are usually only located at the valve middle, but may extend for most of the valve length. In uncleaned material the linking spines can be seen to possess fine filigree on their surfaces below the T-shaped upper parts. The morphology of the shorter, non-linking, intercalary valve spines is variable in that they may fuse to form a bar, a fence-like structure, or occur as isolated structures. In some cases, they fuse to form a centrally-located spine-like structure which terminates at the ocelluli. Typically, the morphology of the spines on the two valves of the same intercalary cell differs slightly. For the details of the complex spine morphology in this genus see Fryxell and Miller (1978) and Hasle et al. (1983). The rimoportulae of the intercalary valves are located within the row of T-shaped linking spines and externally have the same shape as the spines, or they are tubular externally and offset slightly toward the valve center between the marginal areolae. Usually, only one valve of an intercalary cell possesses a rimoportula but in some cases rimoportulae occur on both valves - or are lacking on both. Internally, the rimoportulae are sessile.

Separation valves differ morphologically from intercalary valves in at least three ways: 1) they have larger ocelluli, 2) their rimoportulae are tubular (externally) and situated slightly off center (subcentral) on the valve away from the marginal spines, and 3) their marginal spines are reduced and often spear-shaped and are either fused or unfused. Both separation valves of a filament may possess rimoportulae or only one of the separation valves has a rimoportula.

The cingulum consists of 4-7 open, ligulate bands. The valvocopula and the intercalary bands possess a single row of pores at the junction of the pars interior and pars exterior. The most abvalvar band, which is the connecting band, lacks pores and has a fimbriate margin.

Cymatosira is distinguished from all other genera in the Cymatosiraceae, except Campylosira Grunow ex Van Heurck, by complex linking spines. It differs from the latter genus in the shape of the valve (broadly to narrowly lanceolate versus sub-lunate) and the position of the rimoportulae (within or immediately adjacent to the T-shaped spines in intercalary valves and subcentral in separation valves versus the midpoint of the convex margin in all intercalary and separation valves of Campylosira).

Sabbe et al. (2010, p. 246, Figs 17-20, 28-31) describe a third extant species, C. minutissima Sabbe and Muylaert, but this taxon may not be aligned with the genus. The authors state that its assignment to Cymatosira was based on negative morphological features (i.e. the lack of certain characters that prevents its assignment to one of many genera of the family Cymatosiraceae as defined by Hasle et al. 1983). However, this taxonomic approach is problematic, as C. minutissima lacks the diagnostic characters of the genus Cymatosira such as subcentral rimoportulae and complex T-shaped linking spines. No filaments of this taxon could be found in their samples and, correlated with this, was the absence of valves that could be classified as separation valves (those possessing larger ocelluli, subcentral rimoportula, and simple reduced spines). This is significant as chains of C. lorenziana contain four different types of valves whereas six different types exist in the chains of C. belgica. In contrast, Sabbe et al. (2010) could only identify one morphological valve type in their material. Thus, Cymatosira is a small genus in need of further study.