Observations on the diatom Navicula hedinii Hustedt (Bacillariophyceae) and its transfer to a new genus Envekadea Van de Vijver et al. gen. nov.

Figures & data

Figs 1–16. Light micrographs of Envekadea (Navicula) hedinii. Figs 1–11. Valve views of specimens from Lake Vrana (Croatia). Fig. 12. Girdle view of specimen from Lake Vrana (Cr.). Figs 13–15. Valve views of the type population from Mapiek Köll (eastern Turkestan). Fig. 16. Valve view of specimen (Croatian population) showing the chloroplast structure. Scale bars: 10 µm.

Figs 17–19. Light micrographs of valve views of Envekadea (Navicula) pseudocrassirostris from Eleusis (Athens, Greece). Scale bar: 10 µm.

Figs 20–22. Transmission electron micrographs of Envekadea (Navicula) hedinii from Lake Vrana (Croatia). Fig. 20. Entire valve showing the square to polygonal shape of the areolae when the external hymenes have been removed. Fig. 21. Detail of the stria structure with the hymenes covering the areolae. Fig. 22. Detail of the hymenes showing their porous structure. Scale bars: 10 µm (Fig. 20), 1 µm (Fig. 21) and 0.5 µm (Fig. 22).

Figs 23–27. Scanning electron micrographs of external views of Envekadea (Navicula) hedinii from Lake Vrana (Croatia). Fig. 23. Presence of the hymenes covering the external areola openings. Fig. 24. An eroded valve showing the raphe path with the fissures deflected into opposite directions and the typical areola structure. Fig. 25. Detail of the terminal raphe fissure. Note the almost straight course of the raphe branch and the irregular areola structure. Fig. 26. Detail of the delta-shaped central raphe pores. Fig. 27. A tilted valve showing the stria structure at the valve face/mantle junction. Scale bars: 10 µm (Figs 23, 24, 27) and 1 µm (Figs 25, 26).

Figs 28–33. Scanning electron micrographs of valve and girdle views of Envekadea (Navicula) hedinii from Lake Vrana (Croatia). Fig. 28. Detail of the external valve apex with the wedge-shaped hyaline area and the clearly deflected terminal raphe fissure. Fig. 29. External detail of the irregularly shaped areolae near the valve apex. Fig. 30. Internal view of an entire valve with a partly detached valvocopula lying in the valve. Fig. 31. Detail of the external valve apex showing the girdle. Fig. 32. Detail of the valvocopula in Fig. 30, showing the lack of perforations and the undulated pars interior. Fig. 33. End of a frustule showing girdle bands and valve apices. Scale bars: 10 µm (Fig. 30) and 1 µm (Figs 28, 29, 31–33).

Figs 34–38. Scanning electron micrographs of internal views of Envekadea (Navicula) hedinii from Lake Vrana (Croatia). Fig. 34. Entire valve. Fig. 35. Detail of the central part of the valve with the raised internal nodule. Note the raised rims between the striae and the round shape of the areolae. Fig. 36. Detail of the central raphe endings. Note the slightly deflected ends. Fig. 37. Detail of the valve apex with the typical, very simple helictoglossa. Fig. 38. Detail of the valve apex with the typical, tunnel-like helictoglossa. Scale bars: 10 µm (Fig. 34) and 1 µm (Figs 35–38).

Figs 39–44. Scanning electron micrographs of Envekadea (Navicula) hedinii from the type population of Mapiek Köll. Fig. 39. External valve view of an entire valve. Presence of the hymenes covering the areola openings. Fig. 40. Detail of the central part of the valve showing the central raphe endings. Fig. 41. Detail of an external valve apex with the typical, deflected terminal raphe fissure. Fig. 42. Internal view of an entire valve. Although broken along the axial area, it is still clear that the central raphe endings are weakly deflected. Fig. 43. Detail of the internal structure of a valve apex with the simple helictoglossa. The presence of the raised rims between the striae and the rounded shape of the areolae is well visible. Fig. 44. Detail of a broken valve showing the presence of external hymenes covering the external areola openings. Inside the valve, it is clear that internal hymenes are completely absent. Scale bars: 10 µm (Figs 39–42, 44) and 5 µm (Fig. 43).

Figs. 45–50. Scanning electron micrographs of Envekadea (Navicula) pseudocrassirostris from Eleusis (Athens, Greece). Fig. 45. External view of an entire valve in a slightly tilted position to show the valve face/mantle margin. Note the sigmoid course of the raphe. Fig. 46. Detail of the external central area with the delta-shaped central raphe pores and the reduced central area. Fig. 47. Detail of an external valve apex with the golfclub-like terminal fissures. The internal raphe branch does not seem to follow the terminal fissure path. Fig. 48. Internal view of an entire valve. Fig. 49. Detail of the girdle. Three unperforated bands can be seen. Fig. 50. Detail of the internal structure of the apex with the simple helictoglossa and the wedge-shaped hyaline area around the terminal ending. Scale bars represent 10 µm (Figs 45, 48, 49) and 1 µm (Figs 46, 47, 50).

Table 1. Comparison between the new genus Envekadea and some freshwater genera presenting a similar set of features.

	Envekadea	Adlafia	Kobayasiella	Veigaludwigia	Sellaphora	Cosmioneis	Cavinula
Valve size (µm)	25–52	<25 (40)	Relatively small	Variable	Variable	Variable	Relatively small
Valve outline	Linear–lanceolate with (sub)- capitate apices	Linear to linear–lanceolate, protracted, subcapitate or rostrate apices	Linear to elliptic–lanceolate, protracted, subcapitate or rostrate apices	Linear with undulate margins	Linear, lanceolate or elliptical, bluntly rounded to capitate ends	Lanceolate to elliptical, rostrate or strongly capitate ends	Linear–lanceolate, rhombic–lanceolate or elliptical, slightly rostrate poles
Raphe	Straight, filiform	Filiform, weakly curved	Filiform, umbilicus present	Filiform, weakly curved	Straight, filiform	Straight, filiform	Straight, filiform
Central raphe endings	Greatly expanded, forming conical depressions, straight	Indistinct	Indistinct	Indistinct	Expanded, slightly deflected	Greatly expanded, forming conical depressions, straight	Expanded, pore-like
Central internal endings	Very slightly deflected to one side	Clearly deflected, indistinct	T-shaped, straight	Strongly hooked	Deflected to the primary side	Anchor-shaped	Straight, unexpanded
Terminal raphe endings	Very short terminating in expanded, golfclub-like pores bent in opposite directions, giving the raphe a sigmoid path	Deflected to the same side	Strongly deflected or hooked to the same side	Deflected to the same side	Deflected or hooked to the primary side	Curved towards the secondary side	Very short or bent to one side, fissures turned into opposite directions, giving the raphe a sigmoid look
Striation pattern	Very dense, radiate in the centre, then abruptly convergent close to the apices	Very dense, clearly radiate, at the apices convergent	Radiate in the centre, then abruptly convergent close to the apices	Clearly radiate in the middle, distinctly convergent towards the apices	Parallel to radiate, convergent near the poles	Radiate	Radiate
Striae	Uniseriate, continuing over the face/mantle junction	Uniseriate, continuing without interruption from face to mantle	Presence of a hyaline ridge at the face/mantle junction	Uniseriate, continuing over the face/mantle junction	Uniseriate (to biseriate)	Uniseriate, continuing over the face/mantle junction	Uniseriate
Areolae	Large, rectangular to polygonal pores, usually occluded externally by porous hymenes	Simple, small rounded poroids, occluded externally by hymenes	Transapically elongated, finely hymenate	Rounded, depressed in outer surface, externally closed by hymenes	Small, simple rounded poroids, internally occluded by hymenes	Small, round poroids internally occluded by hymenes	Small, round poroids, sometimes transapically elongated, internally occluded by hymenes
Girdle structure	Two to three non-porous bands	Two bands each with double row of poroids	Two bands with one to two rows of poroids	Unknown	A few open, non-porous bands	Seven open bands with one row of poroids	Narrow, open bands with two rows of poroids
Plastids	One, H-shaped in valve view	Unknown	One, H-shaped, asymmetrical	Unknown	One, H-shaped	Two, H-shaped in girdle view	One to two, H-shaped in girdle view
Habitat	Freshwater-marine	Freshwater	Freshwater	Freshwater	Freshwater	Freshwater	Freshwater

Table 2. Comparison between the new genus Envekadea and some marine genera presenting a similar set of features.

	Envekadea	Stenoneis	Climaconeis	Berkeleya
Valve size (µm)	25–52	Variable	Variable	Variable
Valve outline	Linear–lanceolate with (sub)- capitate apices	Linear, bluntly rounded, sometimes protracted apices	Narrowly linear, linear–lanceolate or lunar with slight capitate apices	Linear to linear–lanceolate with slightly capitate apices
Raphe	Straight, filiform	Straight, filiform	Straight, filiform	Short, straight, filiform
Central raphe endings	Greatly expanded, forming conical depressions, straight	Expanded, straight	Simple to slightly expanded, sometimes deflected to the ventral margin	Straight, weakly expanded
Central internal endings	Very slightly deflected to one side	Turned to one side	Simple	Straight
Terminal raphe endings	Very short terminating in expanded, golfclub-like pores bent in opposite directions, giving the raphe a sigmoid path	More-or-less absent, terminating in expanded pores, deflected slightly to the same side	Terminal fissures absent	Straight or slightly deflected to the secondary side
Striation pattern	Very dense, radiate in the centre, then abruptly convergent close to the apices	Equidistant, parallel to convergent, usually faint in LM, large hyaline zone in the central area	Equidistant, rather parallel to convergent, a stauros is present in some species	Fairly dense, parallel to convergent near the poles
Striae	Uniseriate, continuing over the face/mantle junction	Uniseriate, rather disorderly	Uniseriate, continuing over the face/mantle junction	Uniseriate
Areolae	Large, rectangular to polygonal pores, usually occluded externally by porous hymenes	Small, round poroids, usually occluded externally by a velum	Small round to oval poroids, closed internally by hymenes	Simple round poroids closed internally by hymenes
Girdle structure	Two to three non-porous bands	Not studied	Numerous bands with two rows of poroids	Five open bands with two rows of round/oval poroids
Plastids	One, H-shaped in valve view	Unknown	Four to twenty H-shaped plastids	One, H-shaped in valve view, long arms appressed to the girdles
Habitat	Freshwater-marine	Marine	Marine	Brackish–marine