Thalassiosira visurgis Hustedt

Thalassiosira visurgis Hustedt (1957): 207, pl. 1: figs 1-4 — Simonsen (1987): 439, pl. 657, figs. 1–9; Hasle (1978 b): 263, 264, figs. 1–4; Mahood et al. (1986): 138, figs. 56–61, 95, 96; Aké-Castillo et al. (1999): 497, figs. 45, 46; Li et al. (2013): 104, 105, figs. 126–128.

Description

Valve face flat, concave or convex; diameter 14.0 (12.5–18.0) μm. Areolae arranged in linear or eccentric lines, 10 (12–15) in 10 μm on valve face, 28–32 (17–20) in 10 μm on mantle. One fultoportula present in centre close to large areola. Valve margin characterised by ring of disorganised fultoportulae, 4–5 (4–10) in 10 µm and distinctly short and dentate external tubes. Two rimoportulae with external tubes having variable length and smooth openings, separated 120 ° – 180 ° apart, located on opposite sides and slightly inside ring of marginal fultoportulae, with each rimoportula taking the place of a fultoportula. Valve mantle ornamented with ribbed rim. Fine silicious granules on valve extend to margin.

Diagnosis

Some variability in morphological characters exists within specimens identified as T. visurgis in literature. The number and position of MFSs in the Galiano Island specimen are similar to those described by Hasle (1978 b) and Li et al. (2013), but dissimilar to those observed in Mahood et al. (1986). The MHMPP specimen has both a pronounced ribbed margin and two slightly larger external tubes (indicated by arrows in Fig. 42) that are presumably rimoportulae with smooth openings, as they are in the correct position, but that are shorter than described in literature, with the exception of fig. 3 a in Hasle (1978 b). The major difference between the MHMPP specimen and literature describing T. visurgis is the organisation of the marginal processes. If we compare T. visurgis, as described in Hasle (1978 b), Mahood et al. (1986) and Li et al. (2013), with T. elsayedii (Fryxell 1975), the latter species has the following characteristics: 1) areolae appear smaller, but are similar in density, at 9–11 in 10 µm; 2) external processes have spines; 3) the number of striations (ribbed margin) are more numerous and 4) Fryxell (1975) makes no mention of granulation on the valve surface, nor do granules appear in her images. Thus, T. elsayedii is similar to T. visurgis in having two labiate processes on opposite sides of the valve, but is dissimilar in four other characteristics. The MHMPP specimen is unlikely to be T. baltica, which has 2–9 central strutted processes. However, the marginal process patterning in T. baltica is clearly in two rows, which is somewhat similar to the MHMPP specimen. Until we study more material, we will consider the MHMPP specimen to be T. cf. visurgis, as it fits most closely with descriptions in literature.

Notes

iNaturalist ID: 260003450 (Fig. 42).