Halamphora hamiltoniana Pottiez, Bosak, Kanjer & Zidarova 2025, sp. nov.

Halamphora hamiltoniana Pottiez, Bosak, Kanjer & Zidarova, sp. nov. (Figs 13A–R) http://phycobank.org/105874 Description:— Valves narrowly semi-lanceolate with a moderately convex dorsal margin (Figs 13A–N, 13P–Q). Ventral margin slightly convex, flattened in the middle (Figs 13D, 13K, 13N). Apices protracted, rostrate to rostrate-subcapitate (Figs 13A–K, 13M, 13N, 13Q). Valve dimensions (n=19): length 10.0–24.0 µm, width 2.0–3.0 µm. Externally, valve face flat (Figs 13N, 13Q). Dorsal marginal ridge present, prominent (Figs 13M, 13N, 13Q), often visible in LM as a thin line crossing the striae close to the dorsal margin (Figs 13D, 13E, 13K). Raphe ledge moderately wide, running parallel to the raphe, slightly expanded in the middle and near the apices, then abruptly wedge-shaped narrowed at the apices (Figs 13M, 13N, 13Q). In LM, raphe ledge occasionally barely visible as a very thin line running along the length of the valve near the raphe (Fig. 13B). Raphe in LM straight to weakly arched (Figs 13A–L), in SEM weakly arched (Figs 13M, 13N), located halfway between the valve middle and ventral margin (Figs 13A, 13J, 13N, 13Q). Central raphe endings weakly dorsally bent, drop-like enlarged and relatively close together, closer in the smaller valves (Fig. 13O). Terminal raphe fissures dorsally hooked (Figs 13N, 13Q). Dorsal striae visible in LM, parallel in the middle, weakly radiate near the apices, usually 20–22, rarely up to 24 in 10 µm (Figs 13A–K). In SEM, dorsal striae biseriate, composed of two, mostly parallel rows of rounded to elliptic areolae, the latter 60–80 in 10 µm. Striae continuing areolate onto the mantle (Fig. 13P). Ventrally, axial area narrow, widening near the central area (Figs 13N, 13P). Ventral striae not discernible in LM, usually 30–32, sometimes more distantly spaced and only 23–24 in 10 µm, parallel to weakly radiate in the middle, weakly convergent at the apices (Figs 13L, 13Q). Each ventral stria composed of a single but relatively large, elongated areola (Figs 13L–N, 13P, 13Q). In the valve middle, ventral striae usually shortened to single, small, rounded areolae (Figs 13L, 13M, 13P), leaving a semi-elliptic central area ventrally, or sometimes ventral striae interrupted in the middle (Figs 13N, 13O) and central area then enlarged and reaching the margin. Internally (Fig. 13R), virgae raised and relatively wide. Central raphe endings finishing onto relatively elongated fused central helictoglossae, and terminal raphe endings finishing onto small helictoglossae. Axial silica band absent (Fig. 13R). Holotype:— CROATIA. The Adriatic Sea: Verudela, Istria Peninsula, 44°50’00”N, 13°49’27”E. Sample ENV14, Leg. L. Kanjer, 27 February 2022 (holotype: slide BR-4884! in BR Collection, Meise Botanic Garden; epitype: stub BR-4885! in BR Collection, Meise Botanic Garden; isotype: slide 468! at the University of Antwerp). Fig. 13D represents the holotype specimen, and Fig. 13M represents the epitype specimen. Etymology:— The new species is named after Mr Paul Hamilton (Canadian Museum of Nature, Ottawa, Canada) in honor of his contributions to diatom taxonomy and his recent work on the genus Halamphora. Taxonomic remarks:— The valves of H. hamiltoniana show an intermediate position between H. tenerrima (Aleem & Hustedt 1951: 16) Levkov (2009: 235) and H. nagumoi J.Stepanek et al. in Stepanek & Kociolek (2018: 53), both possessing comparable valve outline, dimensions and structure. When compared to the valves of H. tenerrima illustrated by Levkov (2009, Pl. 90, figures 51–59), Clavero et al. (2000, figures 1–14, as Amphora tenerrima) and Simonsen (1987, Pl. 551, figures 26–30, as A. tenerrima), the valves of H. hamiltoniana from the studied populations have more smoothly arched and usually not flattened in the middle dorsal margin, and more ventrally bent apices. They also present a slightly coarser striation (usually 20–22, rarely 24 in the smallest valves dorsal striae in 10 µm), compared to H. tenerrima, where the dorsal striae range 20–28 in 10 µm in LM, and even up to 30–34 dorsal striae in 10 µm were reported by Clavero et al. (2000) for the type. Ventral striae in H. hamiltoniana are also coarser, (23)30–32 vs> 32 in 10 in H. tenerrima (Clavero et al. 2000). In addition to these small differences, the valves of H. hamiltoniana have a more developed dorsal marginal ridge, and abruptly wedge-shaped narrowed raphe ledge at the apices, compared to H. tenerrima (Clavero et al. 2000, see also their figures 17–19). Its dorsal areolae are arranged in two nearly regular rows, the ventral striae are usually shortened (not usually interrupted in the middle as reported for H. tenerrima by Clavero et al. 2000). Its terminal fissures do not finish below the junction of the raphe ledge with the valve, in contrast to the type of H. tenerrima described in Clavero et al. (2000), but the terminal fissures of H. hamiltoniana finish above this junction (see Figs 13N, 13P, 13Q). Compared to H. nagumoi, H. hamiltoniana has slightly narrower valves, up to 3.0 µm (vs>3.0 µm) in width, with rostrate-subcapitate protracted apices (vs weakly protracted, rounded), a slightly finer dorsal striation (usually 20–22(24) vs 18–20 striae in 10 µm), and a slightly coarser ventral striation, of (23) 30–32 in 10 µm vs 31–34 ventral striae in 10 µm reported for H. nagumoi (Stepanek & Kociolek 2018). In addition, H. nagumoi has much shorter ventral striae, a wider axial area on the ventral side, and wider virgae between the two closely positioned rows of areolae in the striae (Stepanek & Kociolek 2018, Pl. 55, figure 1 & 2 vs Figs 13L–R). Distribution and habitat:— This taxon was only observed as an epiphyte on various macroalgae during winter months in the Northern Adriatic Sea, together with other Halamphora taxa, and several Navicula spp. and Cocconeis spp.

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