LM (Figs 1–22):—Valves disc-shaped, circular, almost flat with a narrow mantle. Valve diameter (n=50): 7–14 μm. Central area distinct, almost smooth, clearly separated from the marginal, chambered striae, diameter 3.5–5.5 μm. Usually two central fultoportula present in the central area. Larger specimens occasionally having three central fultoportulae (Fig. 3), smaller specimens often possessing only one (Figs 10, 11 & 14). Marginal fultoportulae present on the costae, visible in LM in different focus (Figs 9 & 12, arrows). Rimoportula marginal hardly distinguishable from the marginal fultoportulae. Radial striae separated by thickened costae, 8–10 costae in 10 μm. Striae very fine, not discernible in LM.

SEM (Figs 23–29):—Central area smooth, flat with very shallow, irregular narrow grooves (Figs 23–26). Spines, granules or other distinct structures entirely lacking in the central area (Figs 23–26). Central fultoportula openings circular, small, slightly rimmed (Fig. 23), usually in pairs although often irregularly scattered over the entire central area (Fig. 27). Marginal striae located on raised, wedge-shaped ribs, separated by hyaline, non-perforated furrows (Figs 15, 17). Striae uniseriate,> 70 in 10 μm, composed of very small, slightly rimmed areolate. Distinct small, rounded depressions present on almost each furrow near the valve margin (Fig. 28, arrow), occasionally separated in several smaller depressions (Fig. 23, arrow). Marginal fultoportula openings rounded, weakly protruded, located under 1–3 short, blunt spines (Fig. 28, double-headed arrows). Irregular pattern of small spines and granules present at the valve margin (Fig. 28). Spines often broken although when not, clearly showing their rather short length (Fig. 27, arrow). External rimoportula opening not different from the marginal fultoportulae. Internally, marginal fultoportulae located on each costa (Fig. 24), surrounded by three satellite pores (Fig. 29). Rimoportula with a shortly stalked labium having a radially oriented long slit (Figs 24, 29). Striae very finely areolae (Fig. 29). Central fultoportulae surrounded by 3 satellite pores (Fig. 24).

Type:— SUB-ANTARCTICA.Iles Kerguelen, Port-aux-Français, sample BW424, coll. date 07/02/1998 (holotype, slide no. BR! 4506; isotype, slide PLP! 337).

Etymology:—The species is named in honor of Mr. Tom De Ceuster (DCM) to thank him for his support of and interest in the Botanic Garden Meise (Belgium).

Ecology & Distribution:—The new species is quite rare in the sub-Antarctic Region and so far only found on two archipelagos, Iles Kerguelen and Iles Crozet. Cyclotella deceusteriana is typically found in highly eutrophic pools located in large (marine) animal colonies such as a large Macaroni penguin (Eudyptes chrysolophus) rookery on Iles Crozet and elephant seal (Mirounga leonina) wallows on Iles Kerguelen. These pools have a circumneutral to slightly alkaline pH (6.7–7.5) and a very high conductivity (740–3900 μS/cm). Other taxa in the samples include Pinnularia australoshetlandica Carlson (1913: 16), various Luticola taxa and Planothidium delicatulum (Kützing 1844: 75) Round & Bukhtiyarova (1996: 353).