Collemopsidium foveolatum forme sur Ostrea.
Lecanactis ostrearum Vain. ?
Collemopsidium Cf. ostrearum (Vain.) F. Mohr.
Thallus endolithic, when young restricted in surface to minute and scattered scales, brownish (sometimes forming a brown scaly prothallus), usually round, 3-5 mm diam., but irregular in case of mosaics, adjacent conspecific colonies separated by a small whitish aragonite thickness. Perithecia numerous, embedded in minute holes digged in the oyster shell, at first flat, brownish, then deeper in the shell and finally at the bottom of holes, onion-like, black, sometimes with ostioles, margins torn when old scattered with aragonite crystals. This 'species' is common on wild Japanese oysters and oyster shells (Crassostrea gigas) where they may form large colonies and remarkable lace-like mosaics. This taxon is very close to Collemopsidium foveolatum that may be found together on barnacles and limpets attached to oyster shells. The latter has a chalky crustose thallus slightly prominent and slightly larger and more scattered perithecia; based on the current knowledge it is impossible to claim that these taxa are different species, morphological characteristics being not congruent with available molecular markers analyses. As for all Collemospidium 'species', crypto-species frequency seems rather high and additional research is required in order to clarify the taxonomy and phylogeny of this group. These studies are in progress and the species name given here is of course provisional. N.B. Based on some characteristics and its habitat, this form could be close to Collemopsidium Cf. ostrearum but the latter is poorly known, and described for specimen found in South Atlantic.
See: Evolution and taxonomy of the marine Collemopsidium species (lichenized Ascomycota) in north-west Europe. MORH Fiona, EKMAN Stefan, HEEGAARD Einar. Mycol. Res. 108 (5) : 515-532 (May 2004). The British Mycological Society.*