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Mat-forming herb with extensive prostrate (or floating) stems, rooting at the nodes and with erect pedicels 1–2(3) cm from the nodes. Stems, petioles and leaves glabrous or rarely with a few white hairs at the leaf sheaths. Lateral shoots developing from axial buds within the leaf sheaths.
Leaves alternate, long-stalked, petioles usually (0.5)1–3 cm. Blades 0.3–1.0(2.0) × (0.3)0.5–1.0(1.5) cm, normally broader than long, palmately dissected to at least half their length into three main lobes. Leaf lobes narrow or broad, 2–4 mm broad, the lateral lobes often divided again making blades 5-lobed. Lateral lobes pointing outwards or only slightly forwards.
Single flowers in leaf axis, usually terminally on the shoot.
The flowers of Ranunculus and Coptidium apparently have green sepals and yellow or white petals; however, what appears to be the sepals are evolutionary the perianth, i.e., tepals, and what appears to be the petals are stamens transformed into staminodia or ‘honey-leaves’ with a nectary pit on the lower upper side. Below, these two kinds of floral leaves are denoted ‘sepals’ and ‘petals’.
Flower radially symmetric with 3 ‘sepals’ and ‘petals’. Sepals 2.5–3 × 2–3 mm, about as long as the ‘petals’, often broader than long, deflexed in mature stage, boat-shaped, glabrous, outer and inner surface yellowish green. ‘Petals’ 1–1.5 × 2–3 mm, obovate, patent, yellow. Stamens 6–10, with yellow anthers and greenish yellow filaments. Receptacle 1 mm tall, glabrous. Carpels numerous, free.
The fruits are nutlets, glabrous, and with shorts beaks curved 90–180° (most curved when ripe). Head of nutlets semiglobose, 2–4 mm long.
Sexual reproduction by seeds; local vegetative reproduction by the extensive, rooting stems. Seems to flower and ripen fruits quite regularly on Bjørnøya.
Dispersal assumed to be mainly by water and by birds (internal and external) of both fruits and shoot fragments.
The two most small-grown species of Ranunculus in Svalbard – R. hyperboreus and R. pygmaeus – are often confused. They are easily distinguished by R. hyperboreus having 3 ‘sepals’ and ‘petals’ and a prostrate growth form with rooting shoots, whereas R. pygmaeus has 5 ‘sepals’ and ‘petals’ and grows as solitary plants with no rooting shoots. They also differ ecologically: Ranunculus hyperboreus is the perhaps only true aquatic vascular plant in Spitsbergen, whereas R. pygmaeus is confined to dry or only occasionally moist ground.
The ssp. arnellii (Svalbard except for Bjørnøya) and ssp. hyperboreus (on Bjørnøya) are distinguished by the following characters (mainly from Nilsson 2001): Ssp. arnellii is smaller, more slender, and more richly branched, leaf blades are darker green and more deeply divided, leaf lobes are only slightly divergent, the mid-lobe is obovate or narrowly oblong and often divided again, lateral lobes almost straight and divided again, and the beaks of the nutlets are bending up to ca. 120°; ssp. hyperboreus is more coarse, less slender and often less richly branched, leaf blades are pure green and divided only to about the middle, leaf lobes are strongly divergent, the mid-lobe is ± elliptic-oblong and entire, the lateral lobes are often bending forwards and are entire or occasionally divided again, and the beaks of the nutlets are bending ca. 180° when ripe. Other characters reported to distinguish the taxa, based on material from other regions, are smaller flowers (3–5 mm vs. 4–7 mm); and smaller heads of nutlets (2–3 mm vs. 3–5 mm), see Nilsson (2001). These characters are not clear in the Svalbard material.
In small ponds, often associated with Hippuris lanceolata, or on wet moss mats, often with the moss genera Calliergon and Warnstorfia. Sometimes at the bottom of temporary lakes and ponds. Probably indifferent as to soil reaction (pH).
In Svalbard only on Bjørnøya.
For general range, see also Ranunculus hyperboreus ssp. arnellii. Subspecies hyperboreus is the common one in arctic North America, Greenland, Iceland, and mainland Europe.
The plants in Svalbard except Bjørnøya conform to ssp. arnellii as described by Nilsson (2001) but the length of the ‘petals’ relatively to ‘sepals’ is variable. Subspecies arnellii is reported to be mainly Eurasian, present in Svalbard, arctic European Russia and Siberia, but extending across the Bering Strait to Alaska. It is not reported from Greenland or NE North America. Subspecies hyperboreus is reported to be distributed in NW Europe, to be the only race in Greenland, and to be the predominant one in North America (Elven et al. 2011). However, a superficial survey of just a few Greenland sheets proved ssp. arnellii to be present in both E and W Greenland and to be sympatric with ssp. hyperboreum in these areas. This matter then needs to be re-investigated and the rank of the two to be re-evaluated (in view of the Greenland sympatry which might suggest two independent species rather than subspecies). We are in doubt whether the Spitsbergen and Bjørnøya plants deserve to be assigned to two different subspecies (or species).
Elven, R., Murray, D.F., Razzhivin, V. & Yurtsev, B.A. (eds.) 2011. Checklist of the Panarctic Flora (PAF).
Nilsson, Ö. 2001. Ranuculus hyperboreus Rottb. – In: Jonsell, B. (ed.), Flora Nordica II. Chenopodiaceae – Fumariaceae: 269–271.