Navithidium gen. nov., a new monoraphid diatom (Bacillariophyceae) genus based on Achnanthes delicatissima Simonsen

Figures & data

Figs 1–10. LM images of the SV (Figs 1, 2) and RV (Figs 3–10) of Navithidium delicatissima. Figs 1–5, LM images of material collected from Västervik, Swedish Baltic Sea coast. Figs 6–10. LM images of type material, Slide No. Di 208, Hustedt Collection.

Figs 11–16. SEM images of SV of Navithidium delicatissima. Figs 11, 12, 14. SV internal view of entire valve showing marginal striae and vestigial virgae. Fig. 13. External valve view showing areolae arranged on the mantle. Fig. 15. SV valvocopula. Fig. 16. Internal view of marginal striae composed of a single triangular areola with domed hymenes open only at the sides as very narrow slits which are only open on the peripheral ring.

Figs 17–22. SEM images of SRV of Navithidium delicatissima. Figs 17, 18. External view showing striae interrupted by wide lateral areas on both side of the axial area, straight filiform raphe and linear lanceolate axial area. Note polar raphe ends curved to the same side of valve. Figs 19–21. Internal view of valve showing slightly elevated axial area and central nodule. Striae are separated by well-developed virgae. Fig. 22. Enlarged part of internal valve margin showing stria and areola structure.

Table 1. Comparison of Navithidium gen. nov. with related genera

	Navithidium Al-Handal & Romero	Achnanthidium Kützing	Achnanthes s.l. Bory de St.-Vincent	Psammothidium Bukhtiyarova & Round	Rossithidium Bukhtiyarova & Round	Madinithidium Witkowski, Desrosiers & Riaux-Gobin	Scalariella Riaux-Gobin
Frustule	flat	V-shaped in girdle view, bent about the median transapical plane	V-shaped in girdle view, bent about the median transapical plane	V-shaped in girdle view, bent about the median transapical plane	V-shaped in girdle view, bent about the median transapical plane	flat to slightly concave
Valve outline	elliptical or lanceolate-elliptical	linear, lanceolate or elliptical	linear to lanceolate	elliptical, lanceolate-elliptical or linear-elliptical	linear, elongate or lanceolate	elliptic to linear elleptic	elliptic to linear-elliptic
Apices	always rounded	rounded or capitate apices	capitate to rounded	always rounded	rounded	obtusely rounded to capitate	obtusely rounded to subrostrate
Length:breadth ratio	usually less than twice as long as wide	usually 4–5 times longer than wide	usually twice, rarely 3 times, longer than wide	usually twice, rarely 3–4 times, longer than wide	usually 4–5 times longer than wide	2–3 times longer than wide	3–4 times longer than wide
Mantle	differentiated	well differentiated	more or less sharply differentiated from the concave valve face the SV; merging gradually with the concave face in the RSV	well differentiated	differentiated	shallow	shallow
Sternum valve
Sternum	broadly lanceolate	linear to narrowly lanceolate	often displaced towards one margin, always narrower than the raphe-sternum	narrowly linear to narrowly triangular	linear-lanceolate	narrow, linear	narrow, linear
Central area	absent	fascia	sternum gradually widening toward the valve centre; fascia	variable	sternum gradually widening toward the valve centre	absent	absent
Areolae	dash-shaped, short	poroids	poroids	poroids	poroids	elongated, finely perforated	elongated, finely perforated
Striae	radiate, uniseriate	uni- to multiseriate	uni- to triseriate	radiate	uni- or biseriate, parallel	parallel	parallel to slightly radiate
Raphe-sternum valve
Central area	not expanded	variably shaped, fascia	usually fascia or stauros	stauros or stauros-like	varies in both size and shape	reduced	absent
Raphe slits	straight	straight	straight	straight	straight	filiform, slightly elevated	filiform
Central pores	externally expanded, coaxial; internally coaxial	externally simple or slightly expanded	externally straight and expanded; internal endings coaxial and simple or slightly hooked to the same side	variably developed	externally expanded	close to each other	close to each other
Terminal fissures	externally extending onto the mantle and curving around the final valve face striae	externally extending onto the mantle and curving around the final valve face striae, extending onto the mantle	curved	variably developed	absent	doubly hooked, bent to same side	doubly hooked, bent to same side
Helictoglossa	raised	slightly raised	raised	slightly raised		not observed	weakly developed
Striae	uniseriate	uni-to multiseriate	bi- to multiseriate	uniseriate, radiate	uni- or biseriate, parallel	uniseriate, parallel	uniseriate, parallel to slightly radiate
Cells	solitary	solitary or in short chains	solitary or forming short chains attached to substratum by a mucilage pad			solitary	solitary
Habitat	Brackish water, attached to substrata	freshwater, haptobenthic. Attached to a substrata by a mucilage stalk secreted from one end of the R-bvalve	usually haptobenthic, predominantly marine but a few in freshwaters where they are mainly subaerial	adnate attaching to the substrata (commonly sand grains) by the RV face	freshwater	marine, tube dwelling	marine, benthic
Main references	this study	Round et al. (1990)	Lange-Bertalot & Krammer (1989), Round et al. (1990)	Bukhtiyarova & Round (1996)	Bukhtiyarova & Round (1996)	Desrosiers et al., (2014)	Riaux-Gobin et al. (2012)

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