Annual or biennial.
Herb with a single root and an erect stem 2–8 cm, often branched in its lower parts with several ascending branches shorter than or as long as the central stem. The entire plant is glabrous. Leaves concentrated in the lower parts of the stem and branches, opposite.
Leaves opposite, 3–10 × 1.5–4 mm, the basal ones spathulate, the cauline ones in 1–3 pairs, ovate, subacute, entire, green.
Flowers singly at the end of the main stem and branches, ebracteate; all branches ending in an erect flower on a conspicuously long pedicel up to 4–8 cm.
Flowers radially symmetric with a calyx of 4 patent, ovate sepals fused only at the very base and a corolla of 4 petals fused into a corolla tube with a few scales at its mouth and with patent, triangular, subacute lobes, 6–10 mm, pale blue or pale violet. Stamens 4 (2 pairs, with nectaries between the pairs). Gynoecium of 2 carpels, with one room, style nearly absent, 2 stigmas.
Fruit an ovoid capsule, 3.8–4.0 × 2.2–2.5, opening with two valves, with numerous, very small seeds.
Sexual reproduction by seeds; no vegetative reproduction. Flowers are perhaps adapted to insect pollination (colour, nectar) but are very small, and we assume self pollination to prevail. Seed production in Svalbard must be regular as there is no other way this species can upkeep populations, and we have seen last year’s fruits emptied of seeds in some sites.
The seeds have no special adaptation to dispersal but the dry stems and the apical opening of the erect capsule facilitate some ballistic spread (a few centimetres).
There is nothing similar in the Svalbard flora.
South-facing herb and grass slopes with partly stable vegetation, sometimes on moderately stable scree with forb and grass vegetation. On fine-grained substrates with a basic soil reaction (pH).
Thermophilous. Restricted to Spitsbergen and recorded from the middle arctic tundra zone and the weakly and clearly continental sections. Comastoma tenellum is known from two stands close together on Ossian Sarsfjellet at Kongsfjorden (Haakon VII Land) and from several stands in Flatøyrdalen and Ringhorndalen on the east side of Wijdefjorden (Ny-Friesland), the latter ones within the arctic steppe areas. The stands in Flatøyrdalen and Ringhorndalen are large in number of individuals and also with much evidence of efficient seed reproduction.
The species has an interrupted arctic–alpine circumpolar distribution, reaching far south in temperate mountains in Europe, Asia, and North America.
The few but vital stands in Svalbard are probably relics from the warmer parts of the Postglacial (the Hypsithermal). The confinement to some of the climatically most favourable localities in all of Svalbard suggests that long-time survival in favourable sites is a more feasible hypothesis than fortuitous bird dispersal to just these hotspots, especially as its site types (dry steep slopes and scree) are not favoured by birds.