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

A hydroid (Obelia bidentata)

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



A colonial hydroid comprising erect, slightly zigzag but slender main stems that grow up to about 15 cm. The lateral branches tend to be in pairs, successively on opposite sides of the stem. The minor branches are delicate. The hydrothecae, a supporting, nonliving, protein-chitinous envelope surrounding the stem, is elongate and bell-shaped.

Recorded distribution in Britain and Ireland

Around British Isles from the Wash to near Portsmouth.

Global distribution



Found on inert substrata such as wood, shells, wrecks and on sandy bottoms, sometimes algae. Tolerant of brackish water, sublittoral to at least 200 m, rarely intertidal in pools. Also stranded.

Depth range


Identifying features

  • Colony comprising erect, polysiphonic, slender main stems.
  • Lateral branches in pairs, successively on opposite sides of stem.
  • Minor branches delicate, with downward curvature in life, pinnate; branchlets alternate, monosiphonic, flexuose, bearing hydrothecal pedicels at nodes.
  • Hydrothecae rim with 10-20 cusps, usually bimucronate.
  • Gonotheca long-obconical with slightly raised aperture.

Additional information

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Listed by

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Further information sources

Search on:


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

  2. 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.]


This review can be cited as:

Wilson, E. 2002. Obelia bidentata 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 22-09-2018]. Available from:

Last Updated: 17/07/2002