Collemopsidium pelvetiae (G. K. Sutherl.) Kohlm., D. Hawksw. & Volkm.-Kohlm.

    Thallus superficial, hardly visible on the substratum except when old and slightly scaly, dark brown brown-chocolate (visible mostly around the perithecia). Perithecia prominent, more or less rounded, 0.5-2 mm diam., blackish, notably towards the basis of branches (not on fertile fronds), and on the convex upper face of the algae, more rarely on the concave lower face, ostiole distinct. Ascospores oblong, shortly spathulate, 1-septate, distal cell larger, 11.5-16 x 5-6.5 µm. Photobiont: cyanobacteria. Rare, grows on the algae Pelvetia canaliculata but limited in our prospection area to coastal estuaries and in brackish waters. Perithecia bloom at the end of winter and during spring. This species is not visible all the year round.

N.B. Not to be confused with Stigmidium ascophylli (Cotton) Aptroot much more common, with much larger ascospores, smaller and less prominent 'perithecia' (pseudothecia in fact) and developed on fertile fronds of the hosts.