Microamblystegium saxicola Fedosov, Ignatova

Microamblystegium saxicola Fedosov, Ignatova View in CoL &

Jan Kučera gen. et spec. nov.

Type: Russia, Shikotan Island , Tymovo Distr., vicinity of Malokurilskoe Village, 43.88292°N, 146.83257°E, 230 m alt. In shaded niche of rock outcrops. 7 August 2021, coll. Fedosov V. E GoogleMaps . & Shkurko A. V. (holotype MW) .

Etymology: The generic name originates from Amblystegium (a genus of mosses) and refers to its small size. The specific epithet refers to the habitat where it was discovered.

Diagnosis: Microamblystegium differs from most oth- er genera of pleurocarpous mosses in combination of minute size of plants; stem not differentiated internally, composed of thin-walled cells; a weak single costa reaching above mid-leaf at least in several leaves; short and wide rhomboidal, smooth laminal cells and scarcely differentiated alar cells. It differs from the similar Arvernella pisarenkoi , Amblystegium fauriei and Platydictya hattorii in a longer single vs. forked costa, somewhat shorter leaf cells and scarcely developed groups of quadrate cells in leaf angles.

Sequences: Plastid, trn S- trn F OL689422 View Materials ; atp B- rbc L OL689420 View Materials ; nr ITS OL689127 View Materials .

Description. Plants small, in moderately dense, delicate mats, green, yellowish or olivaceous green. Shoots creeping, to 10 mm long, with terete foliation, loosely irregularly branched, stem in transverse section composed of homogeneous thin-walled cells, without central strand and sclerodermis, outermost cells somewhat smaller, with slightly thicker outer walls than cells inwards; paraphyllia absent. Axillary hairs few, pellucid, 3–4-celled, 30×6 µm, apical cell 10 µm long; proximal branch leaves of branch primordia variable in size, lanceolate or triangular. Leaves appressed when dry, spreading when moist, (0.22–)0.27–0.35(–0.40)×0.10–0.13(–0.16) mm, from ovate base gradually narrowed into lanceolate or narrowly triangular acumen, apex acute to subobtuse, some leaves with weak shoulders, slightly narrowed to base, not or shortly decurrent, concave; costa single, to 0.3–0.7 of leaf length, occasionally geniculate, gradually disappearing distally; margins plane, uneven to obtusely serrulate at shoulders; laminal cells rhomboidal or elongate-rhomboidal, (9–)15–17(–25)×5–6(–7) µm, with length to width ratio 1.5–2.5(–3):1, moderately thick-walled, smooth; cells along margin in 1–2 rows subquadrate to short-rectangular, 9–18×4–6 µm, alar cells not or weakly differentiated, subquadrate, transition to adjacent laminal cells gradual. Branch leaves somewhat smaller and narrower than stem leaves. Sexual condition, reproductive structures, and sporophytes unknown.

Differentiation. Microamblystegium saxicola resembles Arvernella pisarenkoi in its combination of small size, sparse branching, not differentiated stem cells in transverse section, rather weak costae, short leaf cells, and saxicolous habitat. It differs from it in single, somewhat longer costae, which typically reach mid-leaf or higher vs. usually forked costae extending to 0.2–0.4 the leaf length; leaves often with shoulders and narrower acumina vs. gradually tapered into wider acumina, and shorter cells with length/width ratio up to 2.5(–3):1 vs. usually 3–4: 1 in A. pisarenkoi . Microamblystegium saxicola also resembles the insufficiently known Japanese species Amblystegium fauriei , as illustrated by Kanda (1976) from type material, in its stem not differentiated in transverse section, but that species differs in having elongate, thick-walled, prorate laminal cells and subquadrate cells along basal leaf margin in 2–3 rows (in 1 row in Microamblystegium saxicola ). The likewise saxicolous Serpoleskea confervoides also lacks a central strand, but has a well-developed sclerodermis. Another minute saxicolous Far Eastern pleurocarpous species, Ignatovia microphylla , differs from Microamblystegium saxicola in having a strong costa nearly reaching the leaf apex, and smaller, rounded, thick-walled laminal cells. The insufficiently known Japanese species Hygroamblystegium calcareum Kanda resembles Microamblystegium saxicola in having a small size, a rather long single costa and weakly differentiated alar cells. However, H. calcareum has a stem with a differentiated sclerodermis and a central strand that differentiate it from M. saxicola . Anoth- er small and poorly known amblystegiaceous moss, which likely represents the C2 Amblystegiaceae clade (cf. Kučera & Hedenäs, 2020), Cratoneuron tenerrimum Kanda , according to illustration in Kanda (1976), also has a well differentiated sclerodermis and central strand in the stem, and its alar group is remarkably differentiat- ed, composed of strongly inflated cells.

Ecology and distribution. The new species is at the moment known from a single locality in the northern part of Shikotan Island. Shikotan differs from all other areas of Russia in a very mild and humid climate. The specimen was collected from shaded humid niche of an acidic rock, where it formed a pure mat. Among other epilithic species growing there, Bryoxiphium japonicum (Berggr.) E. Britton , Calohypnum plumiforme (Wilson) Jan Kučera & Ignatov , Dozya japonica Sande Lac. , and Forsstroemia yezoana (Besch.) S. Olsson, Enroth & D. Quandt are the most common ones.