A red seaweed (Aglaothamnion diaphanum)

Distribution data supplied by the Ocean Biodiversity Information System (OBIS). To interrogate UK data visit the NBN Atlas.Map Help



Aglaothamnion diaphanum is a pale pink coloured red seaweed, 2-10 mm high, with a delicate texture. The fronds are branched in one plane, with a single principal axis and linear to fan-shaped (flabellate) outline. This species shows little variation except in the degree of development of branching. In some thalli, occasional branching become equal in length to the main axis, producing a fan-shape (flabellate) rather than linear frond outline.

Recorded distribution in Britain and Ireland

This rare species is known in the British Isles from only a few records from the Isles of Scilly.

Global distribution



Usually grows on large algae as an epiphyte but also on bedrock, and rarely found growing on pebbles. This subtidal species grows from 7-25 m depth and can tolerate moderately wave exposed sites.

Depth range


Identifying features

  • Uninucleate vegetative cells.
  • Zig-zag or U-shaped short side branches (carpogonial branches).
  • Thalli consisting of one to a few erect axes attached by small discoid filamentous holdfast and secondary creeping axes.
  • Hairs are abundant on female plants but absent during the spore-producing phase in the life cycle (bisporophytes).

Additional information

Could be confused with very small non-reproductive thalli of some growth forms of Compsothamnion gracillimum but Compsothamnion species are multinucleate in contrast to Aglaothamnion, which is uninucleate. Thalli reproductive from a small size and are usually easily recognisable by the reproductive spores (bisporangia). Identification of single specimens can be difficult, so if possible a number of thalli should be collected.

Listed by

- none -


  1. Howson, C.M. & Picton, B.E., 1997. The species directory of the marine fauna and flora of the British Isles and surrounding seas. Belfast: Ulster Museum. [Ulster Museum publication, no. 276.]

  2. Maggs, C.A. & Hommersand, M.H., 1993. Seaweeds of the British Isles: Volume 1 Rhodophycota Part 3A Ceramiales. London: Natural History Museum, Her Majesty's Stationary Office.


  1. NBN (National Biodiversity Network) Atlas. Available from: https://www.nbnatlas.org.

  2. OBIS (Ocean Biodiversity Information System),  2024. Global map of species distribution using gridded data. Available from: Ocean Biogeographic Information System. www.iobis.org. Accessed: 2024-06-15


This review can be cited as:

Edwards, R.V. 2003. Aglaothamnion diaphanum A red seaweed. In Tyler-Walters H. and Hiscock K. Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 15-06-2024]. Available from: https://www.marlin.ac.uk/species/detail/1929

Last Updated: 11/07/2003