Amphitetras antediluviana Ehrenberg (1840: 62)

Amphitetras antediluviana Ehrenberg (1840: 62)

Homotypic synonyms: Triceratium antediluvianum (Ehrenberg) Grunow (1867: 24) ; Biddulphia antediluviana (Ehrenberg) Van Heurck (1885: 207) ; Odontella antediluviana (Ehrenberg) Peragallo (1903: 686) .

Morphological description: Living cells of A. antediluviana form chain-shaped, tangled colonies of dark brown color ( Fig. 2 View FIGURES 2–3 ). Each cell with numerous small chloroplasts of discoid shape ( Fig. 3 View FIGURES 2–3 ). The valve is quadrangular with broadly rounded apices and slightly concave margins ( Figs 4–7 View FIGURES 4–10 ). The Black Sea specimens differs from type material by a more smoother frustules shape; the outgrowths with ocelli protrudes less clearly. Frustules are heavily silicified and have significant wall thickness (2.3–2.5 µm on average). Numerous girdle bands of valve have small elevations on corners and slightly depressions on the sides of mantle. In girdle view, frustule length significantly (by 2–3 times) exceeds its width ( Figs 8–10 View FIGURES 4–10 ). Valve diameter in the Black Sea population on the Southern coast of Crimea (Laspi Bay and Cape Fiolent) varies from 23.4–43.6 μm, height 70.0–82.1 μm in girdle view. Parameters of specimens from the Eastern coast of Crimea (Feodosiya Bay) vary from 20.3 to 35,1 μm in valve diameter, and from 62 to 65 μm in valve height. Off the coast of the Azov Sea, cell dimensions were 26,2–28,5 μm in valve diameter, 68–70 μm in valve height ( Figs 7, 10 View FIGURES 4–10 ).

Each valve is equipped with four ocelli located at the corners ( Figs 11–12 View FIGURES 11–16 , 17 View FIGURES 17–22 , 23, 26 View FIGURES 23–28 , 29 View FIGURES 29–34 , black arrows), with a diameter of 8–9.5 μm. Ocelli are riddled with small, densely arranged (50–55 in 10 μm) poroids ( Figs 21 View FIGURES 17–22 , 26 View FIGURES 23–28 ). The diameter of poroids is around 0.17 μm. In SEM, it is visible that poroids are arranged in more or less distinguishable rows, divided by thin hyaline strips ( Fig. 32 View FIGURES 29–34 , black arrowheads). The polar region of each valve is moderately depressed, surrounded by a circular elevation, which is resolvable in angled valve view ( Fig. 29 View FIGURES 29–34 , white arrowhead).

The central part of valve is constructed by thick vimines separating the areolae of two types. Areolae of the first type ( Figs 15 View FIGURES 11–16 , 20 View FIGURES 17–22 , 33–34 View FIGURES 29–34 , white arrows) are more common, round to angular, 1.2–2.9 μm in diameter, arranged by 4–5 in 10 μm. They are covered by cribrate occlusions. The latter are formed by simple poroids (0.08–0.15 μm in diameter) separated by delicate crossbars. Externally, openings are round to hooked. In close view, areolae of the first type possess large spathulate formations with C-shaped slits at the perimeter ( Fig. 27 View FIGURES 23–28 , white arrowheads). In addition, there are numerous particles of silica situated at the crossbars separating the poroids ( Fig. 27 View FIGURES 23–28 , black arrowheads). Internally, cribrate occlusions are supported by several (5–12) relatively wide props ( Figs 16 View FIGURES 11–16 , 22 View FIGURES 17–22 , 34 View FIGURES 29–34 , white arrows). Second-type areolae are smaller (less than 1 μm in diameter) and less common, equipped with volate occlusions ( Figs 15 View FIGURES 11–16 , 20 View FIGURES 17–22 , 33–34 View FIGURES 29–34 , black arrows). In this case, outgrowths of occlusions are spathulate, moderately branched, resulting in slits of irregular shape ( Fig. 28 View FIGURES 23–28 , black arrows). Notably, vimines throughout the valve are equipped with angular, mostly tetrahedral particles of biosilica ( Figs 15 View FIGURES 11–16 , 20 View FIGURES 17–22 , 26 View FIGURES 23–28 , 32, 33 View FIGURES 29–34 , white arrowheads) with a diameter of 0.5–0.8 μm.

The girdle is connected to the epitheca and hypotheca by one valvocopula on each side ( Fig. 25 View FIGURES 23–28 , white arrowheads). There are large, round-squarish areolae at the girdle, arranged in rows by 4–6 in 10 μm ( Figs 13 View FIGURES 11–16 , 19 View FIGURES 17–22 , 25 View FIGURES 23–28 , 29 View FIGURES 29–34 , white arrows).

In this study, the reconstruction of phylogenetic relationships was carried out using the ML and BI methods. Consistently, with previous study on Amphitetras phylogeny ( Ashworth et al. 2013), the analysis identified united clade for Odontella rostrata (Hustedt) Simonsen (1987: 250) , Amphipentas pentacrinus Ehrenberg (1841: 205) , and Amphitetras antediluviana ( Fig. 1 View FIGURE 1 ). The ML and BI phylogenetic analysis of V4 region of the 18S rRNA gene and the plastid rbc L gene associates novel Amphitetras antediluviana strains (BL-5_ 12092019 and AZ-12_ 24122019) with strain Amphitetras antediluviana ETC 3627 with maximum statistical support (BS 100, PP 1.0; Fig. 1 View FIGURE 1 ). It is worth mentioning that during the construction of the phylogram, we analyzed two strains from GenBank library with the same index as the strain from the Sea of Azov investigated herein: “Azo-12 Amphi-A11 OP297425 OP304753” and “Azo-12 Amphi-B4 OP297426 OP304754”. Strain “Azo-12 Amphi-A11…” was originally identified as Amphipentas pentacrinus , while strain “Azo-12 Amphi-B4…” belongs to Amphitetras antediluviana according to the GenBank. This discrepancy can indicate that the mentioned strains were misidentified. However, taking into account the detached position of “Azo-12 Amphi-B4…” at the presented tree, A. antediluviana may be understood as a cryptic taxon from now on.

Type locality of the studied species is Isle Tjörn , situated at the Southwestern coast of Sweden ( Jahn & Kusber 2006). Around the World Ocean, this species has been noted by different authors: as Amphitetras antediluviana from Europe: Greece ( Jahn & Kusber 2006), Albania ( Miho & Witkowski 2005); North America , Mexico ( López-Fuerte & Siqueiros-Beltrones 2016); South America , Argentina ( Garibotti et al. 2011) and Uruguay ( Metzeltin & García-Rodríguez 2003). As Triceratium antediluvianum the species was recorded from Canary Islands ( Gil-Rodríguez et al. 2003; Moro et al. 2011), Adriatic Sea ( Viličić et al. 2002), Britain ( Hartley et al. 1986; Sims 1996), France ( Méléder et al. 2007), North Carolina ( Hustedt 1955), Colombia ( Lozano-Duque et al. 2010), Gambia ( Foged 1986), Egypt ( Zalat 2002), China ( Liu 2008). At last, it was mentioned as Biddulphia antediluviana in the samples from Britain ( Hendey 1954, 1974), Ireland ( Adams 1908), Portugal ( Moita & Vilarinho 1999), Romania ( Cărăus 2012), Brazil ( Eskinazi-Leça et al. 2010), Ghana ( Smith et al. 2015) and New Zealand ( Harper et al. 2012). However, the species is not mentioned in the sinopsis of marine benthic diatoms ( Witkowski et al. 2000).

In the Black Sea, A. antediluviana was rarely recorded, found as single specimens: in the Northwestern part of the Black Sea on silty-sandy soil off the coast of Romania ( Bodeanu 1987); in Ukraine, particularly in macrophyte algae fouling and on silty-sandy soil in the Tendrovsky and Jarylgach Bays, in Tuzlov and Shabolatsky estuaries ( Guslyakov et al. 1992); in Russia: in the area of the Zernov’s Phyllophora field ( Nevrova 2014a), in the Sevastopol Bay ( Kucherova 1973), in the bays of the southern coast of Crimea and the Caucasus ( Proshkina-Lavrenko 1963; Nevrova 2014b, 2015, 2016; Nevrova & Petrov 2019; Nevrova & Revkov 2003). In all the listed sources, the species is mentioned as Triceratium antediluvianum . According to the results of our recent research ( Nevrova 2022), in the last decade, the finds of Amphitetras antediluviana have become frequent, its macrocolonies in the form of chains have been found in many habitats off the Western, Southern and Eastern Black Sea coasts of Crimea, off the Caucasian coast, as well as off the Sea of Azov coast (see Table 2 View TABLE 2 ).

It can be concluded that despite significant morphometric (see Table 3 View TABLE 3 ) and morphological differences, clones of Amphitetras antediluviana from the Black and Azov Seas belong to the same species. Multiple detections of Amphitetras antediluviana colonies at various depths in the coastal habitats of the Black Sea and the Sea of Azov in the last decade indicate a change in environmental factors.As we suggest, the reason for that can be the recent climatic fluctuation and confounding of optimal factors for the survival and development of this species in the marine sublittoral ecosystems.