Thin-walled obelia (Obelia dichotoma)

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



Obelia dichotoma is generally a colonial hydroid although occasionally is unbranched and solitary. The colonial form varies from being large, erect and loosely fan-shaped or elongate up to 35 cm in height, to being short and either bushy or unbranched up to 5 cm in height. Fan-shaped colonies have stems with alternate first order branches that are almost the same lengths as the stems themselves and give rise to second order branches which, in turn, bear third order branches. The main stems are slender, initially monosiphonic, but thickening with age to become polysiphonic. The stems and branches have a slight zig-zagged appearance, and are marked at the nodes by three to four annulations. The number of annulations at the nodes increases to up 20 towards the base of the colony. The colour of the stems is usually light brown. The medusoid stage of the hydroid consists of a flat, circular, umbrella-shaped bell that has short marginal tentacles and is 0.25 - 0.6 cm wide.

Recorded distribution in Britain and Ireland

Found throughout the British Isles and Ireland.

Global distribution



Obelia dichotoma is usually found on floats, pilings, rocks, shells and other solid objects.

Depth range


Identifying features

  • Fan-shaped colony up to 35 cm in height or bush-like colony or solitary up to 5 cm in height.
  • Up to three orders of branching in fan-shaped colonies.
  • Stems and branches with slight zig-zagged appearance.
  • Nodes of stems and branches marked with three to four annulations.
  • Hydranths bell-shaped with 22-36 tentacles, and encased by hydrotheca.
  • Gonophores elongate, slightly flared with a small aperture and encased by a gonotheca with a short, terminal collar.
  • Pedicels with 3-18 annulations.
  • Stems light brown in colour.

Additional information

Obelia dichotoma reproduces from mid to late summer by releasing medusae from the gonophores. It has been reported that individual colonies release all of their medusae within a few hours. This species is similar to Obelia bidentata, Obelia longissima and Hartlaubella gelatinosa.

Listed by

- none -


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  8. Fraser, C.M., 1944. Hydroids of the Atlantic coast of North America. Toronto: University of Toronto Press.

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

  11. Medel, M.D. & Vervoort, W., 2000. Atlantic Haleciidae and Campanulariidae (Hydrozoa, Cnidaria) collected during the CANCAP and Mauritania-II expeditions of the National Museum of Natural History, Leiden, The Netherlands. Zoologische Verhandelingen, 330.

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  1. Bristol Regional Environmental Records Centre, 2017. BRERC species records recorded over 15 years ago. Occurrence dataset: accessed via on 2018-09-25.

  2. Centre for Environmental Data and Recording, 2018. Ulster Museum Marine Surveys of Northern Ireland Coastal Waters. Occurrence dataset accessed via on 2018-09-25.

  3. Lancashire Environment Record Network, 2018. LERN Records. Occurrence dataset: accessed via on 2018-10-01.

  4. Manx Biological Recording Partnership, 2018. Isle of Man historical wildlife records 1990 to 1994. Occurrence dataset: accessed via on 2018-10-01.

  5. NBN (National Biodiversity Network) Atlas. Available from:

  6. OBIS (Ocean Biodiversity Information System),  2024. Global map of species distribution using gridded data. Available from: Ocean Biogeographic Information System. Accessed: 2024-07-21

  7. South East Wales Biodiversity Records Centre, 2018. SEWBReC Marine and other Aquatic Invertebrates (South East Wales). Occurrence dataset: accessed via on 2018-10-02.


This review can be cited as:

Richards, S. 2007. Obelia dichotoma Thin-walled obelia. In Tyler-Walters H. and Hiscock K. Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 21-07-2024]. Available from:

Last Updated: 13/08/2007