• Collema dichotomum (With.) Coppins & J.R.Laundon
Collema dichotomum
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Known by both its popular name River Jelly Lichen and its acronym RJL, this submerged freshwater species has proved to be a reliable indicator of quality fresh water. Young forms of the lichen show up as an olive green, jelly textured umbilicus that soon divides into lobes. Later it subdivides into more flattened lobules often forming loose rosettes that resemble the fronds of brown-coloured seaweeds. The lobes are distinctive: fan-shaped from above, helmet-shaped from the side. Ledges of slate, mudstone and calcareous sandstones are the most favoured substrata while river bends and shallow river ledges are its major habitats. It is known from single sites in Counties Antrim and Laois. Formerly, it occurred on the Dingle Peninsula in Kerry.

Key characteristics

  • Olive green, submerged lichen with a jelly-like texture, fan-shaped lobes, helmet-shaped at the ends
  • It forms rosettes underwater where acid rivers flow over bedrock containing some lime (but not limestone).

Original text submitted by Vince J. Giavarini

 Simms, M. J., (2016). Collema dichotomum (With.) Coppins & J.R.Laundon. [In] LichenIreland. Accessed on 2020-07-14.