The fungal portion develops an extremely thin thallus (body) that can either grow on the surface of the substrate or be completely embedded within it.

While this cyanobacterium normally grows as filaments when living freely on mollusk shells, within the lichen partnership it usually exists as single cells measuring 3–10 micrometres in diameter.

[6] The fungal reproductive structures (ascomata) can occur either singly or in groups and feature a black carbonaceous protective covering (involucrellum).

These lichens can be found in the eulittoral (intertidal) and littoral fringe zones, growing on both siliceous and calcareous rock as well as on the shells of various marine organisms.

However, Collemopsidium species often achieve higher coverage on limestone shores due to their ability to grow within the substrate, allowing them to persist despite erosion.