Diatomella colonialis, Vijver & Cohu, 2017

Vijver, Bart Van De & Cohu, René Le, 2017, Diatomella colonialis, a new diatom species (Bacillariophyta) from the sub-Antarctic Region, Phytotaxa 306 (4), pp. 281-286 : 282-283

Diatomella colonialis sp. nov. ( Figs 1–38 )

LM ( Figs 1–31 ):—Frustules rectangular in girdle view, forming short chains of up to 10 cells ( Figs 30, 31 ). Girdle very broad, composed of several copulae ( Figs 30, 31 ). Valvocopula with one large, usually quadrangular opening in the middle and two smaller rounded openings near the apices. Valves linear-lanceolate (larger specimens) to almost elliptical (smaller specimens) with almost parallel to weakly convex margins and broadly rounded, never protracted apices. Valve dimensions (n=50): length 6.0–30.0 μm, width 3.0–6.5 μm. Axial area very broad, reaching almost 90% of the total valve width. Central area absent. Raphe branches short, straight with indistinct, simple proximal raphe endings. Distal raphe endings not discernible in LM. Striae very short, reduced to the valve margins, 18–20 in 10 μm.

SEM ( Figs 32–38 ):—Frustules connected to each other by their valve faces with small, bifurcated marginal spines ( Fig. 32 ). Spines positioned between striae on the valve face/mantle junction ( Fig. 33 ). Occasionally, valves lacking spines present ( Fig. 35 ). Valve face and girdle bands often covered by small silica granules and plaques ( Figs 33, 35 ). Externally, striae very short, biseriate, composed of only a few areolae on the valve face. Areolae paired, very small, rounded to squarish ( Fig. 34 ). Transition between valve face and mantle curved with biseriate striae continuing onto the valve mantle abutting a wide, non-porous valve margin ( Figs 33, 34 ). Towards the apices, striae longer with up to 4 paired areolae ( Fig. 35 ). External raphe branches very short, confined to both apices, slightly undulating with very simple, straight proximal endings ( Figs 34, 35 ). Terminal raphe fissures lacking; external distal endings simple ( Fig. 35 ). Internally, a large axial plate, expanding laterally from the axial area occludes more than 75% of the valve face ( Figs 36, 37 ). Striae alveolate, wider than their intervening virgae ( Fig. 37 ). Paired areolae separated by low silica vimines ( Fig. 37 ). Internal raphe branches simple, short ( Figs 36, 37 ). Proximal raphe endings not co-axial, unexpanded, asymmetrically deflected to the same side ( Fig. 36 ). Distal raphe endings straight. Helictoglossae almost absent ( Figs 36, 37 ). Cingulum composed of four open, non-perforated copulae ( Fig. 33 ). Valvocopula (VC) almost twice as wide as the other three (C1–C3). Copula C1 possessing a large ligula ( Fig. 33 ). Irregularly shaped siliceous plaques present on the abvalvar side of the valvocopula and scattered on the other copulae, forming a granular ornamentation ( Fig. 33 , see arrows). Valvocopula with two large, interdigitating projections ( Fig. 38 ), defining one large, usually quadrangular opening in the middle, and two smaller rounded openings near the apices ( Fig. 38 ).

Type:— SUB-ANTARCTICA. Iles Kerguelen , Ile Longue, sample Port Bizet, coll. date 1975 (holotype, slide no. BR! 4470; isotype, slide PLP! 318, PC! 0145188) .

Etymology:—The specific epithet ‘ colonialis ’ refers to the feature of this species to form long colonies.

Ecology & Distribution:— Diatomella colonialis is typically planktonic, collected in a pond lacking any pollution. So far the species has only been observed in this part of the Kerguelen Archipelago and is only known from the type locality. On the nearby sub-Antarctic islands of Crozet or Heard Island, the species has never been observed. Given the age of the sample, apart from climatic data and the conductivity (250 μS/cm), no physico-chemical measurements were made. The sample was dominated by D. colonialis , together with Sellaphora subantarctica Van de Vijver & Beyens (in Van de Vijver et al. 2002: 109), Frankophila maillardii (Le Cohu in Lange-Bertalot & Le Cohu 1985: 213–214) Lange-Bertalot (1997: 71), Achnanthes muelleri Carlson (1913: 23) , Planothidium subantarcticum Van de Vijver & C.E.Wetzel (in Van de Vijver et al. 2013: 111), P. cyclophorum (Heiden in Heiden & Kolbe 1928: 580) Van de Vijver (in Van de Vijver et al. 2002: 99) and an unknown Nitzschia species.

Figures

FIGURES 1–31. Diatomella colonialis sp. nov. Light microscopy views. Holotype population from Iles Kerguelen (sample Port Bizet). 1–3. Initial valves. 4–25. Valve views showing the broad range of valve dimensions. 26–29. Observations on scalariform valvocopulae. 30-31. Two colonies with frustules connected by linking spines. Scale bar represents 10 μm.

FIGURES 32–38. Diatomella colonialis sp. nov. SEM views taken from the holotype population from Iles Kerguelen (sample Port Bizet). 32. SEM of a long chain with several frustules connected by their linking spines. 33. SEM of an entire frustule in girdle view showing the structure and composition of the girdle (VC = valvocopula, C1–C3 = copula 1 to 3). The arrows indicate plaques and the granular surface structure. 34. SEM external valve face view. Note the presence of marginal linking spines and the reduced raphe structure. 35. SEM external valve face view of an entire valve lacking marginal spines. 36. SEM internal valve face view of an entire valve showing the large silica plate covering almost the entire valve interior. 37. SEM internal detail of half a valve showing the reduced internal raphe branch and the alveolate structure of the striae. 38. SEM view of a scalariform valvocopula. Scale bar represents 10 μm.