Description: Wherever there are acid streams lined with overhanging trees, this species should be searched for. The crustose, granular-warted, grey-green thallus is scattered with red-brown discs which can seem almost black. The most distinctive feature is the prothallus: it is like a white halo around the thallus, as if a light were being shone from behind. B. inundata occurs low down in the water but always above the submerged zone, so is best looked for during times of settled weather and low flow. The species is scattered throughout Ireland in suitable habitats.

Key Identification Features:

• Flat, grey-green, granular-warted thallus with near black, rounded discs
• Thallus surrounded by a conspicuous white halo; always on acid rocks underlying shaded streams and rivers.

Original text submitted by Vince J. Giavarini

Distribution Map from NBN: Bacidina inundata at National Biodiversity Network mapping facility, data for UK.

iNaturalist: Bacidina inundata at iNaturalist World Species Observations database.