Pleurosigma guarreranum, sp. nov. (Pleurosigmataceae, Bacillariophyta), from the Gulf of San Matías, Argentina, and comparison with the allied species Pleurosigma exemptum and P. obesum

Figures & data

Fig. 1. Map of the northern region of the Gulf of San Matías showing the nine sampling stations and locations in the study area of Argentina.

Fig. 2–10. Pleurosigma guarreranum, holotype: LPC 13671, Piedras Coloradas (Figs 3, 7–10); paratype: LPC 13672, Las Grutas (Figs 2, 4–6). LM. 2, 3. Valve showing the central area with bilateral dilatation. 4–7. Details of central areas. Figure 4 shows the central nodule, containing the central raphe endings, surrounded by two symmetrical, almost parallel bars (cf. Fig. 24); Fig. 7 is focused on the external raphe fissures (cf. Figs 12, 14). 8–10. Details of the valve apex, including two different focuses of the same apex (Figs 8, 9). Note that the terminal area is unilaterally dilated and funnel-shaped in apical position (Fig. 9); Fig. 10 shows the helictoglossa (cf. Figs 25–27). Scale bars = 50 µm (Figs 2, 3) and 10 µm (Figs 4–10).

Fig. 11–19. Pleurosigma guarreranum from the Gulf of San Matías: isotype material, Piedras Coloradas. External views, SEM. 11. Whole valve, angled (vaulted) at the raphe. 12, 13. Valve surface, which is verrucose. Note the undulate and overlapping central raphe fissures (Fig. 12). 14, 15. Valve face showing apically elongated slit-like foramina. Note the central raphe fissures (Fig. 14) and the raphe bordered by a hyaline area limited by a row of small areolae (Fig. 15). 16–18. Valve apex showing hook-shaped terminal raphe fissures. 19. Broken valve showing loculate areolae with internal occluded pores with recessed bars (arrows) and a pair of bigger pores lacking the central bar (arrowheads). Scale bars = 50 µm (Fig. 11), 10 µm (Figs 16–18), 5 µm (Figs 12–15) and 2 µm (Fig. 19).

Fig. 20–27. Pleurosigma guarreranum from the Gulf of San Matías: isotype material, Piedras Coloradas (Figs 20, 23–25, 27); paratype material, Las Grutas (Figs 21, 22, 26). Internal views, SEM. 20. Valve showing saddle-shaped central area. 21. Valve broken and tilted showing raphe sternum and central area. 22. Detail of Fig. 21, showing raphe sternum bordered by a row of small areolae. 23. Circular pores (probably occluded by hymenes) crossed by a recessed bar (arrows). Note the pairs of pores lacking the central bar (arrowheads). 24. Detail of the saddle-shaped central area showing central bars surrounding the central nodule. Arrowheads show lateral excavations of the stauros-like structure. 25–27. Valve apex showing polar endings finishing in helictoglossae bordered by an unperforate area; note also the discontinuous row of apical slits (Fig. 27). Scale bars = 50 µm (Fig. 20), 10 µm (Fig. 21); 5 µm (Figs 22, 24, 25, 27) and 2 µm (Figs 23, 26).

Fig. 28–34. Pleurosigma exemptum, sensu emend., LM. Pleurosigma exemptum, isotype slides US D3092 (Fig. 28) and US D3289 (Figs 29, 31, 32); and P. obesum isotype slide US D3309 (Figs 30, 33, 34). 28–30. Valves showing the central area with bilateral dilatation. 31. Detail of central area of valve in Fig. 29. 32. Detail of the apex of valve in Fig. 29. 33. Central part of valve. Note the coaxial central raphe endings. 34. Apex, showing terminal area funnel-shaped. Scale bars = 50 µm (Figs 28–30) and 10 µm (Figs 31–34).

Table 1. Morphometric data obtained from specimens of Pleurosigma guarreranum and from type material, protologues and literature of P.exemptum, P. obesum and P. decorum. Abbreviations: nd, no data given by the original author; *, measured from pictures in the literature cited; n gives the number of valves measured.

Taxon	Reference	Length (µm)	Width (µm)	Length : width ratio	Transverse striae in 10 µm	Oblique striae in 10 µm	Stria angle	Characteristics of raphe sternum	Morphology of the apices	Valve outline
P. guarreranum	This study (n =12)	600–681	76–90	6.2–7.8	14.0–16.0	11.0–12.0	80–86°	sigmoid, approaching opposite sides towards the apices raphe angle +8° to +12°	sub-obtuse	sigmoid-lanceolate
P. exemptum	Mann (1925)	504–1160	90–116	nd *7.3	nd	10.3–11.0	in excess of 90°	sigmoid, approaching opposite sides towards the apices	somewhat blunt ends	valve broadly lanceolate, moderately sigmoid
	Stidolph (2002: type material) (n = 1)	758.5	108.4	7.1	15.0	10.0	100° *95–97°	sigmoid, eccentric towards the apices raphe angle +8°	sub-obtuse	sigmoid-lanceolate
	This study (type material) (n = 2)	500–644	92–100	5.4–6.4	14.0	10.0	95–102°	sigmoid, approaching opposite sides towards the apices raphe angle +9° to +10°	sub-obtuse	sigmoid-lanceolate
P. obesum	Mann (1925)	312–351	63–67	nd *5.0	nd	11.2	90°	sigmoid, strongly oblique to the apical axis at mid-valve, approaching opposite sides towards the ends	blunt rounded ends	valve broad, sigmoid
	Stidolph (2002: type material) (n = 1)	347.2	66.5	5.2	15.5	11.0	95°	sigmoid and eccentric towards ends raphe angle +12°	*sub-obtuse	*sigmoid-lanceolate
	This study (type material)(n = 1)	315	63	5.0	14.0	11.0	94–95°	sigmoid, approaching opposite sides towards the ends raphe angle +11°	sub-obtuse	sigmoid-lanceolate
P. decorum	Smith (1853)	248–317 *279	nd *31	nd *9.0	nd	14	nd	not central *raphe angle +15	acute	lanceolate, twisted
	Sterrenburg et al. (2014, in press)	254–260	27–33	7.8–9.4	18–20	13–14	81–83°	sigmoid and strongly eccentric towards the ends of the valves raphe angle +15°	sub-acute	sigmoid-lanceolate

Mann, A. (1925). Marine diatoms of the Philippine Islands. United States National Museum, Bulletin 100 (6: 1): 1–182.

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Smith, W. (1853). A synopsis of the British Diatomaceae; with remarks on their structure, functions and distribution; and instructions for collecting and preserving specimens. Vol. 1. John Van Voorst, London.

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