Minidiscus vodyanitskiyi A. M. Lyakh & Ye. D. Bedoshvili, 2018

Lyakh, Anton M. & Bedoshvili, Yekaterina D., 2018, Tiny diatoms Minidiscus vodyanitskiyi sp. nov. (Bacillariophyta) from the Sea of Azov and consideration of polygonal areolae pattern, Phytotaxa 375 (2), pp. 171-181 : 174-176

Minidiscus vodyanitskiyi A. M. Lyakh & Ye. D. Bedoshvili , sp. nov. ( Figs 2–13 , 15–17, 19)

Dimensions, μm.

Total frustule height: 3.25–3.58 (n=2).

Valve margin height: 0.63–0.65 (n=2).

Frustule diameter: 2.85–(4.00 ± 0.95)–8.20 (n=38).

Valve diameter: 2.70–(3.75 ± 0.87)–7.80 (n=45).

Frustule has the shape of a round cylinder with oblique ends. Due to the presence of a hyaline conical flange before valves, the valve diameters are smaller than the diameters of frustules ( Figs. 3–4 ).

Cingulum comprises two copulae perforated by rows of equally distributed pores. One row of larger, elongated pores is located on the frustule edges close to each valve ( Figs. 3–4 ).

Valve face is flat. Valve mantle is convex and narrow; it resembles a conical frustum ( Fig. 3 ).

Areolae pattern is deformed polygonal. Areolae pattern polygonality is 6–7. The pattern consists of concentric hexa- or heptagons with deformed sides ( Fig. 2 ). It does not contain radial rows except for the 6–7 primary ones, and there are 6–7 areolae surrounding the valve centre. Areolae density is 2.1–(3.2±0.6)–4.1 areolae in 1 μm (n=29). Areolae shape ( Figs. 5, 7 ) varies from the larger, rounded-elliptic openings in the valve centre to the smaller, elongated polygonal ones near the valve margin. On the valve margin, the areolae are elongated and do not have outer tangential ribs ( Fig. 5 ). Areolae loculate with external foramina and internal cribra. Cribra are perforated by equally located cribral pores ( Fig. 7 ).

Fultoportulae. The valve has one central fultoportula (CFP) and some marginal fultoportulae (MFP). They have no external or internal tubes. Internally and externally they appear similar to round pores surrounded by a low siliceous rim ( Figs. 5, 6 ). CFP is located near the geometric centre of the valve, 6–7 areolae surround it ( Fig. 2 ). Internally CFP is surrounded by two satellite pores (SLP) located opposite each other ( Fig. 9 ). Externally, CFP opens by a round pore. MFPs are equally spaced on a border between the valve surface and valve mantle. Internally, each MFP is surrounded by three SLPs. The angle between SLPs is 90°, and the middle pore is directed towards the valve edge ( Fig. 8 ). Externally, MFPs are placed on small hyaline elevations. The number of MFPs was varied from 4 to 6, except for one exemplar with ten MFPs ( Fig. 5 ).

Rimoportula. The valve has one rimoportula (RMP). It is located at a distance of one areolae from the valve centre ( Fig. 5 ). Externally it opens by a round pore or elongated polygonal pore with ragged edges. Internally, it is a slit surrounded by a plan elliptic rim oriented radially or slightly tangentially ( Figs. 6, 9 ).

Siliceous granulation. The areolae ribs of some of the found specimens consisted of siliceous granulations ( Fig. 10 ). We considered that they were formed due to deficiency of silicon.

Filiform siliceous growth. There were two contrasting groups of diatoms and intermediate forms between them ( Figs. 10–13 ). Both groups have identical taxonomic characteristics except for the presence of tiny filiform siliceous growth on the areolae ribs. The first group has smooth ribs between the areolae ( Figs. 10, 11 ), and the second contains filiform growth on the areolae ribs ( Figs. 12, 13 ). The density and length of the filiform growth was varied. We observed specimens without filiform growth, with rare small filiform growth ( Fig. 12 ), with dense intermediate filiform growth, and with dense long filiform growth that was also formed on a cribra inside areolae chambers ( Fig. 13 ).

There were no significant differences in the diameters of valves with and without filiform growth on the areolae ribs (Fig. 14), except for one exemplar. Its valve diameter was 7.8 μm, and it did not contain filiform growth on the areolae ribs (Fig. 15, right). Morphologically, it was very similar to other diatoms in the population.

Teratological forms. We found teratological valves where only one or two SLPs surround some MFPs, while the other MFP had three SLPs ( Figs. 16, 17 ). Another teratology is the orthogonal direction of RMP slit ( Fig. 17 ), since on normal valves such a slit is directed radially ( Fig. 9 ).

Type: — RUSSIA. Open waters of the Sea of Azov (45°50.23′ N; 36°00.08′ E), water column. Collected by A. Lyakh in June, 2016, and November, 2017. Slide DM.1.aa. Holotype: DM.1.aa.h!, illustrated on Fig. 5 . Paratype: DM.1.aa.p!, illustrated on Fig. 6 .

Slide is deposited to the Collection of the Hydrobionts of the World Ocean, Institute of Marine Biological Research RAS, Sevastopol.

Etymology: —Species is named in honour of a Russian hydrobiologist Prof. Vladimir Alekseevich Vodyanitskiy. He had proved that the Black Sea has high biological productivity, and that there is a mix of waters between the surface and deep layers of the Black Sea. He was a cofounder and a first chief of the Institute of Biology of the Southern Seas, where the slide with species holotype is deposited. Also, the species name is a reminder that described diatoms were found during the expedition on the research vessel ‘Prof. Vodyanitskiy’.

Differential diagnosis: —The diameter of the valve, the variability of areolae density, the position and the morphology of RP, CFP, and MFP, the number of MFPs of M. vodyanitskiyi are identical to those characteristics of T. proschkinae Makarova , M. spinulatus (Takano) J.S. Park & J.H. Lee and M. proschkinae (Makarova) J.S. Park & J.H. Lee.

M. vodyanitskiyi differs from those species in combination of characteristics: three SLPs around marginal fultoportulae, where the angle between adjacent SLPs is 90°, and the middle pore is directed towards the valve edge; two SLPs around the central fultoportulae located opposite each other; the loculate areolae surrounded by siliceous ribs.

Distribution:— Type material from the Sea of Azov. Specimens matching type material are sporadically found in the upper layer (0–5 cm) of the New Azov sediments of the Sea of Azov, and mentioned as Thalassiosira cf. proschkinae ( Gogorev & Kovaleva 2017) .

Figures

FIGURES 1–2. Types of polygonal areolae patterns. 1. Geometric models of polygonal areolae patterns redrawn from (Longuet-Higgins 2001). 2. Deformed hexagonal and heptagonal areolae patterns of M. vodyanitskiyi. SEM.

FIGURES 3–4. Construction of cingulum of M. vodyanitskiyi. SEM.

FIGURES 5–9. Construction of valve and areolae, and different views on fulto- and rimoportulae of M. vodyanitskiyi. 5–6. Outside and inside view of the valve of M. vodyanitskiyi; Fig. 5 shows holotype. 7. Outside and inside view on locular areolae and cribrum, which is perforated by cribral pores. 8. Different views on marginal fultoportulae—they all are surrounded by three satellite pores. 9. Different views on central fultoportula that is surrounded by two satellite pores, and on rimoportula that can be oriented radially, or slightly tangentially. SEM.

FIGURES 10–13. Diversity of areolae constructions in M. vodyanitskiyi. SEM. 14. Dependence between diameters of valves and the presence or absence of filiform growth on areolae ribs in M. vodyanitskiyi; black vertical lines show median values. 15. Variability of valve dimensions: from small to largest; the largest valve has no filiform growth on areolae ribs, but in other characteristics it is identical to the small one. SEM. Scale bar is 10 μm.

FIGURES 16–17. Teratological valves of M. vodyanitskiyi. 16. Two marginal fultoportulae are surrounded by 1 or 2 satellite pores instead of 3. 17. Rimoportula slit is orthogonal to a radial direction, and the number of satellite pores around two marginal fultoportulae is less than 3. SEM.