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

A hydroid (Hydractinia echinata)

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



Forms a horny mat, the hydrorhiza, about 3 mm thick on gastropod shells occupied by hermit crabs. The mat consists of thick jagged spines that reach 3 mm in height. Amongst the hydrorhiza mat there are three types of polyps: 1) a club like feeding polyp that grows up to 13 mm in length, with upper and lower circles of eight tentacles, the lower set being shorter than the upper; 2) male and female reproductive polyps (gonozoids) that have a few short terminal tentacles, and 3) specialized defensive stinging polyps (dactylozooids) that are long coils and look thread-like. The gonozoids liberate crawling planula larvae that can sense moving gastropod shells.

Recorded distribution in Britain and Ireland

Recorded from all round Britain but sparse on the east coast.

Global distribution

Common from Arctic Norway to north west Africa. Not recorded in the Mediterranean.


Lives on the shell of hermit crabs in the intertidal and subtidal.

Depth range


Identifying features

  • Forms a horny mat about 3 mm thick on the back of the gastropod shells occupied by a hermit crab.
  • Amongst the mat there are long feeding polyps of 13 mm in length and defensive polyps in a long coil that look thread like.

Additional information

No text entered

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

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

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


This review can be cited as:

MacDougall, C.A. 2003. Hydractinia echinata A hydroid. 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 15-08-2018]. Available from:

Last Updated: 28/01/2003