Sellaphora styxii, Novis & Braidwood & Kilroy, 2012

Novis, Phil M., Braidwood, Jasmine & Kilroy, Cathy, 2012, Small diatoms (Bacillariophyta) in cultures from the Styx River, New Zealand, including descriptions of three new species, Phytotaxa 64 (1), pp. 11-45 : 34-36

Sellaphora styxii sp. nov. P.M. Novis, J. Braidwood & C. Kilroy ( Figs 130–147 , 165 )

Frustules elliptical in valve view, 5.0–8.0 µm long, 2.5–2.9 µm wide, with rounded poles, biraphid. Live cells usually contain a single H-shaped plastid, but this sometimes fragments to give two parietal plastids appressed to each valve margin ( Fig. 130 ). Valve face planar and curved at the margins. Striae radiate, uniseriate or (more commonly towards the poles) biseriate, 22–25 in 10 µm. Central area “bow tie”-shaped, less obviously in smaller cells ( Figs 143–146 ). Areolae round, variable in size and spacing, not aligned longitudinally. Transapical ribs not deflected; groove parallel to the raphe absent. Raphe central, with sinuous external fissures; central endings deflected towards primary side; apical endings hooked towards the secondary side. The apical raphe endings are curved in opposite directions on the hypovalve and epivalve ( Fig. 147 ). Transapical bars (as in S. pupula (Kütz.) Mereschk. ) absent.

Type: — NEW ZEALAND: Canterbury: Styx River (lat. 43°27.800, long. 172°36.213), periphyton, P.M. Novis and J. Braidwood, 4 November 2009 ( CHR618419 !; cleaned frustules made from culture LCR-S: 19:1:1) .

Distribution: —The single known site so far for this new species is site 4 in the Styx River, New Zealand.

Etymology: —named for the Styx River, the site of its isolation.

Molecular data: —The closest match to the Styx strain was Sellaphora cf. seminulum clone TM37, isolated from Threipmuir Reservoir, Scotland (p-distance = 0.034). Phylogenetic analyses placed the two strains in a robust clade ( Fig. 165 ). The length of the rbc L fragment of the Styx strain was 955 bp, in an overall dataset of 1473 bp with 270 variable sites (190 parsimony informative, 111 occurring within the Styx fragment). The model chosen and implemented in the Bayesian analysis was T92+G+I.

Observations: — Sellaphora seminulum (Grunow) Mann 1989 is very similar in morphology to the Styx strain; as illustrated by Morales & Vis (2007) the only differences are a more irregular arrangement and size of the areolae and a slightly smaller frustule size in the former. These specimens agree well with Mann’s (1989) account of Sellaphora in many respects, but there are some key differences. For instance, the striae from the Styx strain are not always uniseriate, particularly in larger individuals, where biseriate striae usually occur, especially towards the poles, where they are still radiate, rather than parallel or convergent. Although the areolae are round, they are not always small or closely spaced, being somewhat irregular in both size and arrangement. These features, plus the divergent DNA data, justify the new species assignment. Evans et al. (2008) recovered Sellaphora as a monophyletic group, including S. cf. seminulum clone T37, in their comprehensive analyses of rbc L sequences. Using Fallacia Round et al. 1990 as an outgroup, and with the additional taxa of the Styx and Mayamaea atomus strains ( Bruder & Medlin 2008), our analyses failed to do this. Repeating the analysis without the Styx strains (which had somewhat shorter sequences) gave the same result (not shown).

Data on sexual reproduction of the Styx strain are presently lacking. Sellaphora cf. seminulum clone T37 is known to be capable of vigorous selfing, as in Geitler’s (1932) original strain of S. seminulum ; by contrast, other species of Sellaphora tested by Mann et al. (1999) possess mechanisms that prevent or hinder selfing. This, plus the lack of completely uniseriate striae (regarded by Mann 1989 as diagnostic of Sellaphora ) and the results of the molecular analyses, may ultimately justify excluding S. cf. seminulum and S. styxii from the genus Sellaphora .

Figures

FIGURE 165. Bayesian phylogenies of Navicula, Sellaphora, and Eolimna strains from the Styx River, for 18S rDNA (left) and rbcL (right). Support numbers above branches are Bayesian posterior probabilities / Maximum Parsimony bootstrap values. Scale bar = 0.1 subsitutions/site. See text for substitution models and description of data sets.

FIGURE 130–147. Sellaphora styxii, strain LCR-S:19:1:1, CHR618419. Scales: Fig. 130, 10 µm; Fig. 142, 5 µm (use for Figs 131–142); Fig. 144, 1 µm (use in Figs 143–146); Fig. 147, 1 µm. Fig. 130. Live cells viewed by LM, showing parietal plastids. Figs 131–142. Cleaned frustules viewed by LM, showing size variation (girdle view of frustules of different size in Fig. 142). Figs 143–147. SEM of cleaned frustules. Figs 143–146, valves of different size showing striae pattern: longer valves have more uniseriate striae; Fig. 147, epivalve and hypovalve showing opposing apical raphe termini.