Williamsella iraqiensis Al-Handal et Kociolek, 2016

Al-Handal, Adil Y., Kociolek, J. P. & Abdullah, D. S., 2016, Williamsella iraqiensis sp. nov., a new diatom (Bacillariophyta, Fragilariophyceae) from Sawa Lake, South Iraq, Phytotaxa 244 (3), pp. 289-297 : 291-293

Williamsella iraqiensis Al-Handal et Kociolek , sp. nov. (LM Figs 2–7 , SEM Figs 8–21 )

Cells solitary with four plate-like plastids distributed in the middle third of the valve ( Fig. 7 ). Valves linear, narrow, with slightly flattened rounded apices ( Figs 8, 9, 12 , 22 ), 40–60 μm long, 2–3 μm wide at the center and 1.0–1.5 μm at the apices (n = 50). External valve face flat. Sternum distinct in the middle but becoming very narrow towards the apices ( Figs 8, 12 , 18 ). Sternum is wider on the internal side of the valve and not narrowing near the apices ( Figs 9, 10, 11 ). Striae uniseriate, opposite, parallel, 24–30 in 10 μm, composed of 4–6 areolae in the middle and 2–3 areolae near the apices ( Fig. 8 ). Striae do not reach valve margins externally. Areolae round and poroid. Areolae are covered with a velum ( Fig. 13 ). An apical pore field is present on each apex, lying in a very shallow ocellimbus and composed of three rows (rarely four) of porelli, 5–8 in each row ( Figs 13, 14 , 17 ). Rimoportula single, located transapically at the central axis of the valve at a distance of two straie from valve apex, externally as a rectangular opening (very rarely oblique) and as a slit surrounded by convex, dome-like labiate processes internally ( Figs 10, 12 , 16 ). Cingulum composed of one band with fine pores, 16–20 in 10 μm ( Figs 19, 20, 21 ).

Type:— IRAQ. Sawa Lake, 31º 18’ N, 45º 00’ E, 2 m a.s.l., Al-Handal, 13 May 2013 (holotype: BM!, individual on slide 101/809, deposited in the Natural History Museum, London, UK, here illustrated as Fig. 2 . Isotype: slide no. SL52020, Department of Marine Biology, Marine Science Center, Basra, Iraq).

Etymology:—The epithet iraqiensis refers to the geographic location where this second Williamsella species was found.

Differential diagnosis: — Williamsella iraqiensis is similar to W. angusta but can be readily distinguished from it under LM by the relatively wide sternum which results from shorter striae along valve margin. In the later species the axial area is very narrow and linear. Transapical striae are denser in W. iraqiensis as compared to W. angusta (24–30 in 10 μm versus 17–20 in 10 μm, respectively). Areola number in each stria is much lower in W. iraqiensis than in W. angusta . The apical pore field in both species appears similar in the shape of the pores but their number is different. It is wider in W. iraqiensis and is not located deep inside the ocellimbus as in W. angusta . Shape and structure of the copulae look also similar.

Under light microscope, W. iraqiensis may look similar to some taxa belonging to Fragilaria , particularly in valve outline and striae organization. Examination of live material of this species showed absence of any colonial mode of life, a criterion that is currently accepted for recognizing the species of Fragilaria ( Round et al. 1990) . SEM images of many specimens did not show any spines or other structures to support chains or other colonial forms.

Williamsella , in general, shares morphological similarities under LM such as valve outline and striation pattern with several other genera, such as Tabularia , Ulnaria or Synedropsis . However, large differences in ultrastructure between Williamsella and these genera can be seen under SEM; including the fine structure and arrangement of striae, the apical pore field and the structure and position of the ocellimbus, the number and shape of the rimoportula, and the number and structure of the cingulum bands. In their description of Williamsella, Graeff et al. (2013) detailed some ultrastructural features which set this genus apart from other allied general. Most important in distinguishing this genus from those listed here is the external occlusions on the areolae, a feature absent in these other taxa. Table 1 summarizes the main characteristics of these genera in comparison to Williamsella .

Ecology: — Williamsella iraqiensis was common on the sediment of Sawa Lake, Iraq. Salinity of the lake water during sampling was 34 psu indicating highly saline conditions of an inland water impoundment. In addition to high salinity, the lake water is characterized by high concentrations of gypsum (CaSO 4) which almost reached saturation levels (6952 ppm, Al-Handal et al. 2014). Some healthy frustules of W. iraqiensis were also found as epiphytes on Chara sp. , a submerged macrophyte widely distributed in the lake.

Figures

FIGURES 2–7. LM images of W. iraqiensis showing variation in valve outline and arrangement of striae. 7. Shape and distribution of chromatophores.

FIGURES 8–14. SEM micrographs of W. iraqiensis. 8. External view of whole valve. 9. Internal view of the valve. 10. Internal view of the rimoportula showing its structure and position. 11. Internal view of the valve showing valve curvature and arrangement of areolae. 12. External side of the upper third of the valve showing a rimoportula, arrangement of areolae and the flat nature of the valve. 13. Valve apex showing the structure of the apical pore field, and areolae covered with vela. 14. Internal view of valve apex showing a rimoportula and the apical pore field.

FIGURES 15–22. SEM micrographs of W. iraqiensis. 15.Valve apex without rimoportula.16.Valve apex with rimoportula and the shallow ocellimbus. 17. Valve apex with four rows of areolae in the apical pore field. 18. Middle of the valve showing arrangement of areoale. 19. Cingular band with a single row of areolae. 20, 21. Shape of the cingular band. 22. Whole view of the valve in low magnification.