Thallus: prothallus not seen. Thallus thin to moderately thick, 40–100 μm, dull mid green to dark greenish grey, thinner areas often without cracks, thicker parts with few to numerous cracks but rarely forming discrete ‘islands’ of thallus. Thallus surface minutely roughened with low projections ca 20–40 μm diam, occasionally forming minute ridges to 80 x 30 μm in size, projections concolorous but darker than surrounding surface in shaded specimens. Thallus comprising cells in vertical columns, in upper part of thallus 2–4 x 1.5–3.5 μm, walls sometimes slightly thickened. Pseudocortex present, but a surface layer ca 5 μm thick sometimes pigmented, pigment dull green, K--. Lower part of thallus with no or few living algal cells, cells often with large oil droplets; densely pigmented with ± discrete projections projecting upward from the basal layer into the green layer, occasionally reaching the thallus surface, pigment dark reddish brown, K+ dark greyish brown. Epinecral layer sometimes present, colourless, to 5 μm thick, comprising collapsed and scarcely recognizable cell remains. Photobiont cells 5.0–10.5 x 3.5–9 μm.
Anamorph: not known.
Teleomorph: ascomata perithecia, forming low to moderately projecting, occasionally rather prominent, warts in the thallus 300–840 μm diam, apex rounded, rarely depressed, ostiole inconspicuous. Exciple 260–270 μm diam. (few measured). Involucrellum well-developed, merging with dark basal tissue. Asci ca 43–52 x 21–26 μm, 8-spored. Ascospores (13–) 14.5–17.5 (–19.5) x (6–) 7–8 (–8.5) μm, (1.7–) 1.9–2.5 (–2.8) times as long as wide, cylindric-ellipsoidal, simple, colourless, filled with small oil droplets when mature.
Should be assessed as Data Deficient; almost certainly significantly under-recorded.
Indistinguishable morphologically from Hydropunctaria aractina but with a markedly different ITS sequence, and possibly differences in distribution/ecology. H. oceanica is similar but has brown rather than greenish cortical pigment, and the widespread species H. maura has a noticeably thicker thallus.
So far seemingly endemic to GB&I, with collections known from Wales (Pembrokeshire) and SW Ireland (Kerry and ? Cork).
On gently sloping to steep rocks on the seashore, especially where slightly irrigated by rain water, and on stone in freshwater stream shortly above the seashore.