Eunotia globicephala J.C.Taylor, Cocquyt & G.Walsh, 2024

Taylor, Jonathan C., Cocquyt, Christine & Walsh, Gina, 2024, Tropical African diatoms from the Eunotia asterionelloides (Bacillariophyta) species complex, with descriptions of new species, Plant Ecology and Evolution 157 (1), pp. 88-99 : 88

Eunotia globicephala J.C.Taylor, Cocquyt & G.Walsh sp. nov.

Figs 5 , 6

Type.

REPUBLIC OF THE CONGO • Confluence of outflow from Lake Nanga and the Kouilou River; 4°19 ’37.9” S, 11°52 ’01.0” E; sample 13-335; holotype: slide SANDC-13-335 (SANDC), the valve representing the type is illustrated in Fig. 5F ; isotype: slide BR 4756 (BR) .

LM description.

Valves linear, arcuate, morphologically variable over cell cycle. Ventral margins convex, dorsal margins concave. Apices distinctly capitate in large cells, subcapitate in smaller cells. Apices rostrate to weakly cordiform, strongly asymmetrical in larger cells, expanded ventrally, slightly indented in larger cells forming a shallow cleft. Cells planar to highly flexed round the apical axis, flexing may reach 90° (Fig. 5G ). Terminal nodule barely visible. Cells weakly heteropolar in girdle view, most often exhibiting flexing around the apical axis. Cells constricted mid-valve. Valve length 25-40 µm, width 1.0-2.5 µm in the middle, and 3.0-4.5 µm at the poles. Striae visible under LM, 15-22 in 10 µm.

SEM description.

Striae uniseriate externally and internally, composed of round areolae, occluded externally by hymenes. Striae continue onto valve mantle near the apices, becoming discontinuous close to the apices (Fig. 6F ). Striae interrupted externally at the margin by a slightly thickened silica ridge. Striae evenly spaced, parallel at the centre of the valve becoming radial at the apices (Fig. 6A, B ). Raphe slits clearly visible externally and internally, extending onto the valve face terminating very close to the valve apices (Fig. 6D ). Valve mantle shallow, at 90° to the valve face (Fig. 6E ). Conical spines present at valve face/mantle junction in larger cells (Fig. 6C, E ), absent in smaller cells (Fig. 6A ). Internally, terminal raphe branches terminating onto small but well-defined helictoglossae (Fig. 6D ). Girdle composed of several open, perforated copulae, bearing one row of occluded areolae (Fig. 6C ). One rimoportula present, always located close to the terminal raphe ending at the ventral part of one of the apices (Fig. 6F ).

Distribution.

At present only known from the type locality.

Ecology.

This species was collected at the confluence of Lake Nanga and the Kouilou River. The site is situated in a seasonally flooded, forest-grassland-wetland mosaic in the Kouilou plain. Water at the time of sampling was slightly turbid with circumneutral pH (6.94), low conductivity (74.8 µS.cm-¹), and moderate oxygen levels (53.8% saturation).

Etymology.

The epithet Eunotia globicephala refers to the genus of the long-finned pilot whale [ Globicephala melas (Traill, 1809)] as the apices of Eunotia globicephala bear a strong resemblance to the head and rostrum of this whale when seen in profile.

Registration.

http://phycobank.org/104057

Notes.

In the Eunotia globicephala population observed from the Republic of the Congo cells can be planar to highly flexed around the apical axis, flexing may reach 90° (Figs 5G, N , 6B ). Flexing was only observed in larger cells. This twisting may have an advantage for colony formation in that twisted cells will allow for the formation of rather more complex three-dimensional net-like structures rather than flattened colonies as are found in Asterionella formosa Hassall. As cell attachment usually is in the form of a mucus pad forming on the cell apex near to the ventral margin a cell twisted through 90° would provide an attachment site that would be perpendicular to that cell. A three-dimensional net-like structure could provide greater resistance to sinking and help maintain the position of the colony within the euphotic zone. Interestingly, the raphe never seems to be reduced in this taxon as is the case in E. zasuminensis and E. magnaparva .

Figures

Figure 5. Eunotia globicephala sp. nov., LM from type material, sample 13 - 335. Type represented by Fig. 5 F. A - M. Size diminution series, valve view. N - O. Girdle view. Eunotia tukanorum, LM from sample CCA 1847. P - S. Diminution series, valve view. T - U. Girdle view. Scale bar = 10 µm.

Figure 6. Eunotia globicephala sp. nov., SEM from type material, sample 13 - 335. A. External view of valve of smaller cell, no marginal spines present. B. External view of valve of large cell, flexed 90 ° around the apical axis. C. External view of dorsal margin and apices, note the marginal spines and open copulae. D. External view of the ventral mantle showing the position of the raphe (arrowhead). E. External view of mid-valve showing the conical marginal spines and areolae occluded externally. F. Internal view of the valve apex showing helictoglossa and rimoportula (arrowhead) located very close to the ventral margin. Scale bars = 2 µm.