1. Seamats are small colonial creatures belonging to the group of animals called Bryozoa. 
  2. They look like lace and can encrust seaweeds, stones and shells.
  3. Each cell or compartment of the ‘lace’ is an individual animal or zooid, housed in a small box-like structure – which in turn is attached both to the underlying weed or stone surface and to its neighbours on each side.
  4. Basically, the animal within the box pokes its tentacles out of the opening when it is under water and uses them to filter food particles from the sea.
  5. The outline shape of the box, how it is constructed, and sculpturing features such as spines and knobs are used to verify the identity of the animals to species level under the microscope.
  6. However, the two most commonly occurring types found on the seashore – Electra pilosa (Linnaeus) and Membranipora membranacea (Linnaeus) – can usually be recocognised by characteristics visible to the naked eye or using a hand lens.

Electra pilosa (Linnaeus)

Bryozoa pictures: Bryozoan Sea Mat, Electra pilosa (Linnaeus), on dried Fucus seaweed at Port Eynon, Gower, South Wales, UK (P1020629aBlog1)

  • Electra pilosa is frequently found on large seaweeds or macro-algae such as Laminarians (kelps) or Fucoids. It particularly likes to grow on Fucus serratus.
  • The photograph above shows this type of seamat on a piece of dried seaweed.
  • The photograph below shows it on a piece of fresh Fucus serratus seaweed.
  • E. pilosa is distinguished from M. membranacea by the oval-shaped cells and its frequently ‘hairy’ appearance that results from one particular spine that decorates the boxes becoming very well developed.
  • You can see this hairiness in the two pictures; it is more noticeable around the edges of the seamat. 

You can find out more about this species of sea mat at the Marine Life Information Network of Britain and Ireland site.

Electra pilosa (Linnaeus) information on marLIN

Bryozoa pictures: The Bryozoan Sea Mat Electra pilosa (Linnaeus) on fresh Toothed Wrack (P1020718ablog2) 

Membranipora membranacea (Linnaeus)

Bryozoa pictures: The Bryozoan Sea Mat Membranipora membranacea (Linnaeus) as a white lace-like encrustation on strandline seaweed at Ringstead Bay, Dorset, UK on the Jurassic Coast World Heritage Site (P1110467sBlog3)

  • This species is also commonly found encrusting larger seaweeds or macro-algae – particularly Laminaria (or kelp) but also on Fucoids.
  • The picture above shows it on a frond of kelp.
  • You can tell the difference between M. membranacea and E. pilosa.
  • It forms extensive lacy sheets while E.pilosa creates more irregular colonies ar star-shaped patches.
  • The shape of the zooids or compartments is rectangular rather than oval as in E. pilosa.
  • It lacks the ‘hairy’ appearance of E. pilosa although under the lens you would be able to see short spines or tubercles at the corners. 
  • In the photograph below showing M. membranacea, the regularly spaced up-standing tufts are most likely to be part of the underlying seaweed surface (such as reproductive features) protruding through the sea mat rather than part of the colony structure its self. They are not the ‘hairs’ described as typical of E. pilosa.
  • Any comments or corrections to these descriptions are welcomed.

You can find out more about this species of sea mat at the Marine Life Information Network of Britain and Ireland site.

Click on : Membranipora membranacea (Linnaeus)

Bryozoa pictures: Bryozoan Sea Mat Membranipora membranacea (Linnaeus) as a lace-like encrustation on strandline kelp at Ringstead Bay, Dorset, UK on the Jurassic Coast World Heritage Site (P1110472aBlog4)

 Additional information can be found in the following books: 

Barrett, J. and Yonge C. M. (1958 but reprinted many times) Collins Pocket Guide to the Seas Shore, Collins, ISBN 0 0 219321 3, page 171.                         

Erwin, D. and Picton, B. (1990) Guide to Inshore Marine Life, Naturalists’ Handbooks 21,  The Marine Conservation Society, Immel Publishing, ISBN 0 907151 345, page 98.                           

Gibson, C. (2008) Pocket Nature Seashore, Dorling Kindersley, ISBN 978 1 4053 2862 3, page 94.                           

Hayward, P. J. (1988) Animals on seaweed, Naturalists’ Handbooks 9, Richmond Publishing, ISBN 0 85546 265, pages 6, 7, 19, 24, 33, 34, 36, 37, 84.                           

Hayward, P. J.  and Ryland J. S. (1995) Handbook of the Marine Fauna of North-West Europe, Oxford University Press, ISBN 0 19 854055 8 (Pbk)

Hayward, P., Nelson-Smith, A. and Shields, C. (1996) Sea shore of Britain and Europe, Collins Pocket Guide, , ISBN 0 00 21995, page 643.

[This is a revision of a post first published 10 February 2009]


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4 Replies to “Sea Mats – what are they?”

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