3050 dead molluscan shells and fragments of calcareous algae were collected from 104 benthic stations at depths from 2 to 160 m in the Firth of Clyde, the Firth of Lorne and on the Orkney shelf, British Isles. The following micro-algal borings were recognised and are described: (i) Rhodophyta: Conchocelis, (ii) Chlorophyta: Ostreobium quekettii, Eugomontia sacculata, Phaeophila sp. and (iii) Cyanophyta: Plectonema terebrans, Hyella spp.
These endolithic algae are bathymetrically zoned. Eugomontia, Phaeophila and the cyanophytes characterize the upper part of the photic zone (2–20 m). Shallow sheltered environments are typified by a high density of algal borings, most of them aligned perpendicular to shell surfaces. The lower photic zone is characterized by Ostreobium and Conchocelis whose borings are parallel to shell surfaces. The photic limit is about 40 m for the open Orkney shelf (59°N), and about 22 m for the partly enclosed firths (56°N).
The Orkney population is dominated by two chlorophyte taxa: rhodophytes and cyanophytes are absent. This may be due to higher tidal energy conditions.
