information on the biology of species and the ecology of habitats found around the coasts and seas of the British Isles

An isopod (Idotea granulosa)

Distribution data supplied by the Ocean Biogeographic Information System (OBIS). To interrogate UK data visit the NBN Atlas.
Only coastal and marine records shown



Idotea granulosa has an elongated body that is oval in shape, sharply narrowing posteriorly and dorsoventrally flattened. The antennae are less than one-fifth of the overall body length. Males are 5-20 mm in length, where females are 6-13 mm in length. It is uniformally brown, red or green with occasional white longitudal markings along its back. In adults pleotelson initially narrows quite sharply having concave lateral sides & forming a distinct median blunt point.

Recorded distribution in Britain and Ireland

Recorded throughout Britain & Ireland, but absent on most of the east coast of England. This species may be under recorded.

Global distribution



This is an species found feeding on algae, on open, but not overly-exposed coasts.

Depth range


Identifying features

  • Elongated oval shaped body, sharply narrowing and dorsoventrally flattened.
  • Body length is males 5-20 mm and 6-13 mm in females.
  • Uniformally brown, red or green in colour.
  • Occasional white markings along its back.
  • Antennae one-fifth of body length.

Additional information

Idotea granulosa colour varies depending on the nature of the algae inhabited. Larger specimens are found on the brown algae Ascophyllum nodosum and Fucus spp. Smaller specimens are found on green algae Cladophora spp. and Polysiphonia spp.

Listed by

- none -

Further information sources

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  1. Bruce, J.R., Colman, J.S. & Jones, N.S., 1963. Marine fauna of the Isle of Man. Liverpool: Liverpool University Press.

  2. Crothers, J.H. (ed.), 1966. Dale Fort Marine Fauna. London: Field Studies Council.

  3. Gibson, R., Hextall, B. & Rogers, A., 2001. Photographic guide to the sea and seashore life of Britain and north-west Europe. Oxford: Oxford University Press.

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

  5. 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.]

  6. JNCC (Joint Nature Conservation Committee), 1999. Marine Environment Resource Mapping And Information Database (MERMAID): Marine Nature Conservation Review Survey Database. [on-line]

  7. National Biodiversity Network (NBN) Atlas website. Available from:  Accessed 01 April 2017

  8. Naylor, E., 1972. British marine isopods. London: Academic Press. [Synopses of the British Fauna, no. 3.]

  9. Picton, B.E. & Costello, M.J., 1998. BioMar biotope viewer: a guide to marine habitats, fauna and flora of Britain and Ireland. [CD-ROM] Environmental Sciences Unit, Trinity College, Dublin.


This review can be cited as:

Rowley, S.J. 2005. Idotea granulosa An isopod. In Tyler-Walters H. and Hiscock K. (eds) Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 19-09-2018]. Available from:

Last Updated: 24/11/2005