White flatworm (Oligocladus sanguinolentus)

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



Oligocladus sanguinolentus is a small free-living flatworm that reaches up to 22 mm in length. It is under twice as long as it is wide. It has a flat and bilaterally symmetrical body. Flatworms are notoriously hard to identify in the field. However, Oligocladus sanguinolentus is whitish to yellowish-brown in colour with a marginal band of blue-grey, a mid-dorsal line and lateral brownish-red offshoots. It has two relatively long tentacles on its head. It has many eyes both at the base of the tentacles and on the underside of the head.

Recorded distribution in Britain and Ireland

Recorded at several locations off west Scotland, Wales and the Isles of Scilly but may occur throught British and Irish waters.

Global distribution



Often found intertidally on the smooth undersides of rocks at low tide or under red algae and Laminaria spp.. It has been found down to a depth of 60 m.

Depth range


Identifying features

  • Up to 22 mm in length, 13 mm in width.
  • A pair of anterior marginal tentacles.
  • Length of tentacles much greater than their basal width.
  • Ventral mouth in cerebral region, in front of a bell shaped pharynx.
  • About 50 eyes at the base of each tentacle and up to 30 with each cerebral group.
  • The mouth is arrow-shaped.
  • Three major branches to the gut, situated amongst multiple yolk glands.

Additional information

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Listed by

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  1. Cannon, L.R.G. (ed.), 1995. Biology of Turbellaria & some related Flatworms: Proceedings of the VIIth International Symposium on the Biology of the Turbellaria, Abo/ Turku, Finland. Kluwer Academic Publishers.

  2. Hayward, P.J. & Ryland, J.S. (ed.) 1995b. Handbook of the marine fauna of North-West Europe. Oxford: Oxford University Press.

  3. 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.]


  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: 2024-06-23

  3. South East Wales Biodiversity Records Centre, 2018. SEWBReC Worms (South East Wales). Occurrence dataset: https://doi.org/10.15468/5vh0w8 accessed via GBIF.org on 2018-10-02.


This review can be cited as:

Barnes, M.K.S. 2008. Oligocladus sanguinolentus White flatworm. In Tyler-Walters H. Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 23-06-2024]. Available from: https://www.marlin.ac.uk/species/detail/93

Last Updated: 27/03/2008