Thallus brownish black to dark grey, or greenish black or green in moist shade, thin to moderately thick, 20–200 µm thick, cracks usually numerous, but in some specimens only rarely forming discrete islands; sterile areoles plane, mostly 120–400 µm diam, fertile areoles 200–850 µm diam, surface matt, smooth, or rough with ill-defined punctae. Prothallus not seen. Thallus in section with a basal layer which varies from mostly colourless to densely pigmented above, and with densely pigmented columns rising into the photobiont layer, sometimes reaching the surface; the columns either appearing isolated or joined below, depending on degree of pigmentation of the basal layer. Thallus with a pseudocortex, surface cells with dilute to dense, dull green to green-brown, K‒ pigment, often overlain by a thin, decolourized epinecral layer.
Anamorph: not known.
Teleomorph: Ascomata perithecial, forming low to moderate projections 400–600 µm diam but often too indistinct to measure accurately; surface smooth to roughened. Ascospores (12–) 13.5–16 (–17) × (6.5–) 7–8 µm, 1.7–2.2 (–2.5) times as long as wide, cylindric-ellipsoidal.
Compared with Hydropunctaria maura, H. aractina has a thinner and less cracked thallus with cortical cell walls that are dull green to greenish brown rather than brown.
In GB&I, known from Scotland (Wester Ross). A collection (illustrated) from Co Cork (Ireland) needs confirmation.
On rocks in the littoral zone, often growing with H. maura.