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

Sea mat (Membranipora membranacea)

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



This epiphytic bryozoan forms encrusting lacy mat-like colonies of very small, rectangular autozooids, 0.42 x 0.13 mm, with tubercules or short spines in the corners. Each zooid has lateral calcified walls that contain ventral uncalcified bands providing flexibility. A frontal membrane completely roofs the space between the supporting side walls with a lophophore (feeding tentacles) visible within. Tower zooids may also be present; where the frontal membrane projects upwards in a columnar way.

Recorded distribution in Britain and Ireland

This species is found all around the coasts of Britain and Ireland.

Global distribution



Encrusting colonies are typically found on marine algae e.g. Laminaria digitata and Laminaria hyperborea. Occasionally small colonies can be found on Fucus serratus and in rock pools. Found on the lower shore to shallow water.

Depth range


Identifying features

  • Encrusting colonies of varying size with lacy appearance.
  • Pale greyish-white in colour.
  • Autozooids 0.42 x 0.13 mm.
  • Membrane covering whole frontal surface.
  • Commonly found on Laminaria digitata and Laminaria hyperborea.

Additional information

Colonies may vary in size. Membranipora membranacea grows quickly (several mm/day) in response to predation from sea slugs and in order to maintain its position on the kelp frond. Prolific growth is favoured by fast flowing water which provides food and oxygen. Egg production occurs once a year in the spring; the larva (cyphonautes) is planktotrophic and is the most abundant and largest coastal larva in Britain between June and August. The larva settles in late summer-early autumn.

Listed by

- none -

Further information sources

Search on:


  1. 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.

  2. Hayward, P. 1995. Antarctic cheilostomatous bryozoa. Oxford: Oxford University Press.

  3. Hayward, P., Nelson-Smith, T. & Shields, C. 1996. Collins pocket guide. Sea shore of Britain and northern Europe. London: HarperCollins.

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

  5. Hayward, P.J. & Ryland, J.S. 1979. British ascophoran bryozoans. London: Academic Press.

  6. Hayward, P.J. & Ryland, J.S. 1998. Cheilostomatous Bryozoa. Part 1. Aeteoidea - Cribrilinoidea. Shrewsbury: Field Studies Council. [Synopses of the British Fauna, no. 10. (2nd edition)]

  7. 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.]

  8. Ryland, J.S., 1970. Bryozoans. London: Hutchinson University Library.

  9. Stachowitsch, M., 1992. The invertebrates: an illustrated glossary. USA: Wiley-Liss.


This review can be cited as:

Rowley, S.J. 2004. Membranipora membranacea Sea mat. 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 22-09-2018]. Available from:

Last Updated: 05/08/2004