[2][3] The species was originally described by the Finnish lichenologist Edvard August Vainio in 1901.

[5] Fiona Mohr reclassified it in genus Collemopsidium based on morphological and phylogenetic analysis in 2004.

Collemopsidium ostrearum has an immersed thallus, which is occasionally partly bordered by a black prothallus.

[6] Collemopsidium ostrearum grows on calcareous rock and marine shells in the eulittoral zone.

The species has been recorded from Ireland, New Zealand, and São Tomé and Príncipe, though its apparent rarity may be due to under-collection rather than true scarcity.