Craspedostauros laevissimus (West & G. S. West, 1911) Sabbe

Zidarova, Ralitsa, Haan, Myriam De, Ivanov, Plamen, Hineva, Elitsa & Vijver, Bart Van De, 2022, The genus Craspedostauros E. J. Cox (Bacillariophyta) on the coasts of Livingston Island, Maritime Antarctica, Phytotaxa 572 (1), pp. 1-24 : 4-7

Craspedostauros laevissimus (West & G.S.West 9: 8) Sabbe (00: 5) (Figs – 8, 7, 9– 7, 80–85)

LM observations (Figs – 8, 7, 80–85):— Frustules rectangular, lanceolate in girdle view with weakly convex margins ( Figs 7, 8 ), constricted in the middle (i.e. biarcuate), with numerous copulae ( Fig. 18 ). Longest valves linear ( Figs 80, 83, 85 ), linear-lanceolate, smaller valves becoming elliptic-lanceolate ( Figs 4 , 13, 15, 27). Apices broadly rounded ( Figs 4 , 15, 27), to almost cuneate (Figs 2, 3, 13). Valve margins in the middle ranging from occasionally almost straight ( Fig. 83 ) to, usually, weakly (Figs 3, 5) to moderately ( Figs 4 , 9) convex. Valve dimensions (n = 26): valve length 20–42 µm, valve width 4.5–7.0 µm. Occasionally, weakly silicified valves, probably in state of formation, and easily “squashed” on the slides, appear to have a larger valve width (see for instance Figs 81, 82, 84 ). Axial area very narrow, linear. Central area variable in size, from very narrow ( Figs 9 , 11 ), almost rectangular (Figs 3, 16) to bow-tie-shaped fascia, widening towards the margin (Figs 2, 4, 5, 27). Raphe filiform, slightly curved to straight, with enlarged central raphe endings, and elongated, unilaterally bent, terminal raphe fissures. Striae fine, parallel, occasionally very weakly radiate in the middle, 28–30(32) in 10 µm. Areolae not or occasionally slightly individually discernible in LM.

SEM observations ( Figs 9– 7 , 86–87 ):— Externally, valve face curving into a deep mantle ( Figs 29, 38 ). Raphe almost straight ( Figs 29, 32, 36, 39 ) or sometimes weakly undulating ( Fig. 35 ). Central raphe endings weakly curved and enlarged ( Figs 29–30, 32–33, 35, 37 ), occasionally straight and only weakly expanded ( Figs 39–40 ). At the apices, axial area triangularly widening, forming a silica flap ( Figs 38, 41 ) on one side covering the terminal raphe fissures. Fissures continuing shortly onto the mantle ( Figs 31, 34, 35, 38, 39, 41 ), clearly unilaterally bent ( Figs 35, 36 ) to weakly hooked ( Fig. 38 ). Striae uniseriate, composed of cribrate areolae of almost equal size ( Figs 30, 35, 39, 40, 45 ) to slightly larger near the axial area ( Figs 37, 40, 41 ). Larger areolae, apparently formed by merging two adjacent areolae occasionally present near the axial area ( Figs 31 , 87 , arrows). Areolae continuing around the apices ( Figs 31, 34, 38 ). Cribra composed of usually four ( Figs 34, 37, 40, 42 ), rarely five ( Figs 33, 34, 40 ) peripheral pores, although very rarely, up to 6–7 very small peripheral pores were observed ( Fig. 41 ), or very occasionally only 1–3 ( Fig. 45 ). In general, areolae near the axial area more complex, with more peripheral pores, whereas areolae on the rest of the valve face simplified having a lower number of peripheral pores ( Figs 40, 45 ). Central cribrum pores usually single or absent ( Figs 30, 37, 40 ), occasionally 2–3 ( Figs 31, 33, 34, 41 ). Areolae uniformly distributed over the valve face, ca. 40 in 10 µm. Internally, areolar openings square to rectangular ( Figs 46, 47 ), to almost rounded ( Figs 43, 45 ). Internal raphe branches straight, located on a distinctly raised sternum. Central raphe endings terminating onto double helictoglossae ( Figs 43, 45, 46 ). Stauros narrow, located on a wider hyaline fascia ( Figs 43–46 ). Terminal raphe endings finish onto broad helictoglossae ( Figs 46, 47 ).

Ecology and associated diatom flora:— The species was often found in the epilithon of tidal pools at Hannah Point (samples 11 and 13, and LT10), having variable salinity levels between 11.5 and 33.7 (Table 2), but it was most abundant in sample 13. The diatom flora of this sample included Melosira spp. and several unidentified Navicula taxa in larger numbers.

Figures

FIGURES 9–. SEM images of Craspedostauros laevissimus (sample 11). 29. External valve view.30.Close up of the central area of the same valve showing the expanded central raphe endings and cribrate areolae.31. Close up of the apex of the same valve showing the bent and expanded distal raphe endings and a cribrate areola with two central pores (white arrow). Note the presence of areolae and pores around the raphe endings on the apex (black arrows).32. External valve view.33. Same valve, close up of the central area with the expanded and slightly curved central raphe endings.Arrow shows a cribrate areola with three central pores. 34. Same valve, close up of the apex with white arrows showing the presence of both areolae with and without central pores. Note also the presence of areolae and pores on the apex (black arrows). Scale bars = 5 μm (Figs 29, 32); 1 μm (Figs 30, 31, 33–34).

FIGURES 5 –56. SEM images of Craspedostauros confusus sp. nov. from the type population (sample 14). 52. Internal valve view. 53. Close up of the central area internally, showing the narrow stauros on a wider hyaline area, the central raphe endings with double helictoglossa and the rounded to elliptic internal areolar openings. Arrow shows an areola with seven central pores, visible internally. 54. Close up of the apex with the pores of cribrate areolae internally (arrows). 55. Close up of the central area of another valve of the same population, showing once again the narrow stauros, the internal central raphe endings with double helictoglossa and rounded internal areolar openings. 56. Close up of the apex with rounded to elliptic and even somewhat irregular in shape internal areolar openings. Scale bars = 5 μm (Fig. 52); 1 μm (Figs 53–56).

FIGURES 60–85. LM images of Craspedostauros confusus sp. nov. and C. laevissimus from several populations from Livingston Island. 60–67. Craspedostauros confusus sp. nov. (sample DNA5). 68–77. Craspedostauros confusus sp. nov. (sample LT6). 78–79. Craspedostauros confusus sp.nov. (sample MO’), Fig.78 showing afrustule withnumerous copulaein girdleview.80–84.Craspedostauros laevissimus (sample LT10). 85. Craspedostauros laevissimus (sample 13). Scale bar = 10 µm.

FIGURES – 8.LM images of Craspedostauros laevissimusand C.confusus sp.nov. 2–8.Craspedostauros laevissimus (sample 11).9–18. Craspedostauros laevissimus (sample 13). 19–26. Craspedostauros confusus sp. nov. (type population, sample 14). 27. Craspedostauros laevissimus (enlarged view of Fig. 4, sample 11). 28. Craspedostauros confusus sp. nov. (enlarged view of Fig. 22, type specimen). Scale bars = 10 µm.

FIGURES 86–90. SEM images of Craspedostauros confusus sp. nov. and C. laevissimus from several populations from Livingston Island. 86. External valve view of Craspedostauros laevissimus (sample LT10). 87. Close up of the striae of the same valve, showing the occasionally present enlarged areolae near the raphe, without central pores (arrow). Note the density of areolae in comparison to Fig. 88 below. 88–90. Close ups of the central area (Fig. 88) and apices (Figs 88–90) of Craspedostauros confusus sp. nov. (sample MO’), showing the typically enlarged cribrate areolae near the raphe, usually with several central pores. Scale bars = 5 μm (Fig. 86); 1 μm (Figs 87–90).