Planothidium frequentissimum

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 : 20-21

Planothidium frequentissimum (Lange-Bert. in Krammer & Lange-Bert.) Lange-Bertalot 1999 ( Figs 13–25 , 161 )

Frustules elliptic-lanceolate in valve view, monoraphid, 12.2–13.6 µm long, 4.1–4.7 µm wide. Striae transverse to slightly radiate at central area to more strongly radiate at poles, 17(–19) in 10 µm, multiseriate. Araphid valve asymmetric at central area where striae are interrupted by a “hoofmark” structure (sensu Round & Bukhtiyarova 1996) appressed to inner valve surface. Internally, the hoofmark takes the form of a thickened silica hood that interrupts the striae ( Fig. 23 ). Araphid valve slightly convex, linear to slightly curved towards hoofmark, raphid valve slightly concave, linear; frustules thus curved in girdle view ( Figs 19, 20 ), 3.1–5.3 µm wide. Raphe straight, with expanded proximal area and terminal fissures curved towards the secondary valve side; internally, central ends are weakly curved. Sternum slightly depressed, and sometimes kinked near one pole.

Reference: — Krammer & Lange-Bertalot 1991, p. 78, fig. 44: 1–38.

Specimens examined: —CHR618407! (cleaned frustules made from culture LCR-S:2:1:1).

Distribution: — P. frequentissimum is recorded from elsewhere in New Zealand (Cassie Cooper in press) and regarded as cosmopolitan. Within the Styx catchment, the species appeared in culture from site 1.

Molecular data: —The only sequence for Planothidium in Genbank is that for 18S of P. lanceolatum (Bréb. ex Kütz.) Lange-Bertalot (1999: 287) strain L1249 (isolated from the USA; Medlin & Kaczmarska 2004). Unfortunately, we failed to obtain an 18S sequence for our strain. The closest match for our strain using rbc L was the closely related P. victori ( Fig. 161 , and see below), with a p-distance of 0.017; the two strains also formed a robust clade in Bayesian and MPB analyses. More sequences of strains in this complex are required before conclusions about relationships can be drawn using molecular data. The full dataset was 1473 bp long, including a fragment of 572 bp for our strain. There were 512 variable sites (379 parsimonyinformative, 162 occurring within the fragment from the Styx strain). The model selected by BIC and implemented in the Bayesian analysis was GTR+G+I.

Observations: —As currently circumscribed, species of the genus Planothidium that contain a hoofmark structure can be distinguished by the nature of the silica thickening underlying the hoofmark, and by the ends of the frustules. P. frequentissimum is unique in possessing a hood-like structure coupled with rounded to slightly drawn-out apices. P. rostratum (Østrup) Lange-Bertalot (1999: 285) , which also possesses a hood, is excluded by its rostrate to subrostrate valve ends. LM observations show that the hoofmark varies in its openness; thus it seems unlikely that this feature can be used to discriminate taxa. Overall there is significant variation within this strain, with some valves having a curved sternum and some linear (especially in the araphid valve), many with alternately arranged striae and others in which the striae are strictly matched across the sternum.

Figures

FIGURE 161. Bayesian phylogenies of Planothidium 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 13–41. Planothidium spp. from the Styx River. Scales: Fig. 17, 5 µm (use for 13–20, 26–39); others 1 µm. Figs 13–25. Planothidium frequentissimum strain LCR-S:2:1:1, CHR618407. Figs 13–20, Cleaned frustules viewed by LM; Figs 13–15, araphid valve showing horseshoe structure; Figs 16–18, raphid valve (vertical pairs are through-focus images of the same frustules). Figs 19, 20, variation seen in girdle view, with convex araphid valve and concave raphid valve. Figs 21–25, SEM of cleaned frustules. Fig. 21, raphid valve; Fig. 22, araphid valve with horseshoe visible; Fig. 23, internal view of araphid valve showing structure of horseshoe; Fig. 24, internal view of half of raphid valve; Fig. 25, araphid valve with depressed pseudoraphe. Figs 26–41. Planothidium victori strain LCR-S:18:1:1, CHR618408. Figs 26–39, cleaned frustules viewed by LM; Figs 26–31, araphid valve showing horseshoe structure and curved valve margin especially in smaller valves; Figs 32–37, raphid valve (vertical pairs are through-focus images of the same frustules); Fig. 38, small araphid valve with horseshoe and hooked apex showing that the valve hook can occur at either end with respect to the horseshoe (compare with 30, 31); Fig. 39, girdle view showing convex araphid valve and concave raphid valve. Figs 40, 41, SEM of cleaned frustules. Fig. 40, araphid valve with horseshoe attachment visible; Fig. 41, oblique view of raphid valve.