Solitary herb with one flowering stem or as small tussocks with a few flowering stems. No runners. Stems simple, erect, up to 25 cm tall or more. Leaves basal and on the stems. Stems, petioles and leaves with sparse, white, villous hairs.
Leaves alternate. Basal leaves with long sheaths with strengthened veins, remaining as stiff fibres around the base of the stems for several years. Petioles 2.5–5(6) cm, 4–6 times as long as blade. Blades (0.9)1.2–3.0(3.5) × (0.9)1.2–3.0(3.5) cm, pedate or palmate with one broad central segment and two broad lateral segments well separated from the central segment and divided into several narrow or linear (finger-shaped) lobes. Stem leaves sessile, palmatisect with narrowly linear lobes.
A single terminal flower or 2–3 flowers in a short cyme.
Flower radially symmetric with 5 sepals and petals. Sepals ca. 4 × 4 mm, ovate or obovate, olive green or pale brown, with dense, white, villous hairs. Petals 6–9 × 8–11 mm (i.e., distinctly broader than long), broadly overlapping, plicate, yellow. Stamens 10–15, 2–4 mm long, dark yellow. Receptacle elongated, about 6 times as tall as broad, with dense, white, villous hairs. Carpels numerous, free.
The fruits are nutlets with villous hairs and with beaks curved 90° or more, in an elongated head.
Reproduction by seeds; no vegetative reproduction. The flowers are morphologically adapted to insect pollination (but see below). The plants regularly produces numerous nutlets which are assumed to ripen well. The Ranunculus auricomus complex, to which R. arcticus may be related, is pseudogamous agamospermic (needing pollen to stimulate the asexual seed production). An old study from Russia (Rozanova 1932) shows that R. arcticus (as R. "pedatifidus") does not set fruits after emasculation (castration) and pollination with R. auricomus pollen. This suggests that R. arcticus may be a sexual species as opposed to R. auricomus (see Ericsson 2001 and references therein). However, T. Engelskjøn (pers. comm. 2015) reports that he has counted the chromosome number to be 2n = 40. As the base number in Ranunculus is × = 8, Engelskjøn’s count represents a pentaploid number (the same as the major number in the agamospermous R. auricomus group) and strongly suggests agamospermy also in R. arcticus, something we have suspected from its very abundant seed production.
The nutlets are easily dispersed by animals due to their hooked beaks attaching to fur.
The tall, stout, native Ranunculus species in Svalbard are R. arcticus, R. nivalis, R. sulphureus, and R. wilanderi. In addition, R. repens, R. acris and R. subborealis are introduced (see these species). Ranuculus suphureus and R. nivalis both have dark brown hairs on the ‘sepals’, whereas R. arcticus and R. wilanderi have white hairs. The strange leaves of R. arcticus and R. wilanderi, with the broad middle lobe and the very narrow lateral lobes, immediately distinguish them from R. nivalis (with regularly, deeply lobed leaves), R. sulphureus (with broad, shallowly lobed leaves), and from the three introduced species, R. acris and R. subborealis with regularly deeply 5- og 3-lobed leaves and R. repens with three separate leaflets. The plicate petals are unique for R. arcticus in Svalbard.
The species most similar to R. arcticus is the local Svalbard endemic of the R. auricomus group: R. wilanderi. They have in common the strangely dissected leaves (albeit with some differences in details) but they differ in the following characters: In R. arcticus the sepals are densely pubescent, the pedicels elongate strongly during flowering, and the nutlets have strongly hooked beaks with tip pointing downwards; in R. wilanderi the sepals are sparsely pubescent, the pedicels are not elongating much, and the nutlets have evenly curved beaks not pointing downwards. Ranunculus wilanderi does not have the plicate petals of R. arcticus.
Found on slopes beneath bird cliff in climatically favourable sites. Medium dry heaths and meadows. The substrate is uniformly well-drained and with circumneutral to basic soil reaction (pH).
Thermophilous. Restricted to the fjord regions within the middle arctic tundra zone and the weakly and clearly continental sections. The species has been found in ca. 20 sites along Isfjorden, Kongsfjorden, Krossfjorden, and Wijdefjorden, all in the northern half of Spitsbergen.
The general range is circumpolar, mainly in the arctic zones.
Ericsson, S. 2001. Ranuculus arcticus Richardson. – In: Jonsell, B. (ed.), Flora Nordica II. Chenopodiaceae – Fumariaceae: 237–256.
Rozanova, M.A. 1932. Opyt anatiliczeskoy monografiy conspecies Ranunculus auricomus Koch. – Trudy Petergofskogo Estestvenno-Nauchnogo Instituta 8: 19–148.