Collemopsidium ostrearum

This is a small species of lichen that lives on oysters, and on calcareous rock. Collemopsidium ostrearum has an immersed thallus, which is occasionally partly bordered by a black prothallus. Its perithecia (fruiting bodies) are immersed, densely clustered, and measure 0.34–0.60 mm in diameter, with ostioles up to 0.17 mm wide. The involucrellum is very wide-spreading and contains particles from the substrate. Multiple perithecia are often united within a single involucrellum. The ascospores measure 23–25 by 9–11 μm. The pycnidia appear similar to the perithecia and overlap with them in size.

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.

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