Ceramium diaphanum is a species of marine red algae (or Rhodophyta).
This small filamentous alga grows attached, or unattached, in masses reaching 20 cm across. The branches grow pseudochotomously the tips are strongly coiled in. It is corticated only at the nodes between the axial cells and form clear cortical bands. The rhizoids are multicellular.
The full life history is not yet confirmed. Tetraspores have been recorded in groups on the nodes.
Growing as attached or unattached clumps on other algae and Zostera at low littoral to 3 m deep.
Widely distributed in Ireland, England and Scotland.Isle of Man. Reported from Norway and United States of America.