Thallus usually wide-spreading, cracked areolate, pale grey to (rarely) brown, areoles flat, 150–400 µm diam. Soralia 100–200 µm diam, mostly punctiform, arising from the centre of each areole, less often following cracks in the thallus and becoming stellate; soredia 20–25 µm diam, blue-grey (K--, N--). Photobiont chlorococcoid, cells thick-walled, 7–16(–19) x 8–12(–13) µm diam, often dividing to give 2–4 daughter cells (Chlorella type).
Anamorph: not known.
Teleomorph: ascomata apothecia, black, lecideine, initially innate with a white ring around the inner edge of the exciple but soon becoming sessile, 05–10 mm diam., proper exciple persistent, becoming flexuose; disc occasionally umbonate. Excipulum of branched, anastomosing hyphae, colourless with dark brown outer cells, reflexed below the apothecium. Hymenium 70–90 µm tall, upper third with brown pigment; hypothecium hyaline. Interascal tissue of simple paraphyses 2.5 µm diam, swelling to 5 µm at the apex, with a dark brown cap. Asci ca 45 x 20 µm, Teloschistes-type. Ascospores 7–9 x 5–6 µm, broadly ellipsoidal, hyaline, aseptate.
Chemistry. Thallus C--, K--, Pd--, UV--; soredia and medulla C--, K--, Pd--, UV+ white; divaricatic acid detected by TLC.
Fuscidea oceanica is unusual in regularly producing both sexual and asexual propagules. It is the only sorediate member of the genus occurring in the British Isles that is also usually fertile. However, as the soredia of F. oceanica are very small and inconspicuous they are easily overlooked in the field and this species may then be mistaken for other, non-sorediate, members of the genus (e.g. F. lygaea).
In GB&I, known principally from the NW Scottish Highlands (Wester Ross) and also occurs at scattered localities elsewhere in the Highlands including S Aberdeenshire, W Inverness-shire and Argyll Main.
Fuscidea oceanica is locally common on exposed, hard acidic rocks. It is particularly common on Cambrian quartzite, and on the Beinn Eighe NNR in West Ross it covers large areas and is the most frequent species.