Collemopsidium foveolatum (A.L. Sm.) F. Mohr
Pyrenocollema halodytes ss. auct.
Arthropyrenia halodytes (Nyl.) Arnold p.p.
Thallus crustose, very thin (0.25 mm thick), immersed in the substratum, spreading out more or less in rounded patches, up to 0.5-1 cm diam., grey-beige, brown-yellow, often edged with a grey or whitish rim minutely embossed; scattered with perithecia, totally immersed in minute wells in the shell (not protruding, this characteristic being the most distinctive), hardly visible to the naked eyes as minute black dots , 0.1-0.2 mm, more or less regular in size and circular. Ascospores oblong, 1-septate, 15-21 x 5-9 µm (according to our measures). Pycnidia common to absent, black, immersed. Photobiont: cyanobacteria. Very common on shells, dead or alive, notably of barnacles, limpets (Patella, Balanus) and sometimes on oysters (Crassostrea japonica) if present, in the foreshore above the mean tide level, uncommon on calcareous rocks. This species has, for a long time, been called Pyrenocollema halodytes = Collemopsidium halodytes s.l. by many authors but this name is now restricted to a species with sessile perithecia and growing on rocks. As most of the Collemopsidium species, C. foveolatum cannot be identified unless it is found under its canonical morph, which obviously is not always the case. NB. On some pictures, limpets and oysters also harbour Collemopsidium sublitorale of which the perithecia ostioles are bigger and irregular, protruding over the thallus, when the perithecia fall down the holes are larger and irregular, those of Collemopsidium foveolatum comparatively are smaller, regular and sometimes with a smooth margin; these two species are commonly found together and intermediate specimen may be found, rather difficult to identify (currently under investigation). Non altered and living perithecia within wells are scarce and most of the time only empty wells can be found, notably on dead shells found on the foreshore.
N.B. The Collemopsidium genus is still poorly known and preliminary molecular results have shown that many cryptic species hardly are distinguishable using common macro and microscopic characteristics. In addition these preliminary molecular studies have suggested that the environmental conditions and the substrata might impact the morphological characteristics usually retained to identify these species. As a consequence, we present here the different forms found on various substrata, currently under investigation. See Collemopsidium foveolatum on Cirripedia, Collemopsidium foveolatum on Mytilus, Collemopsidium foveolatum on Patella, Collemopsidium foveolatum on Ostrea.
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 Briticsh Mycological Society.*