MarLIN

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

A hydroid (Obelia dichotoma)

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

Summary

Description

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

-

Habitat

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 -

Further information sources

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Bibliography

  1. Bouillon, J., Medel, M.D., Pages, F., Gili, J.M., Boero, F. & Gravili, C. 2004. Fauna of the Mediterranean hydrozoa. Barcelona: Institut de Ciencies del Mar.

  2. Bruce, J.R., Colman, J.S. & Jones, N.S., 1963. Marine fauna of the Isle of Man. Liverpool: Liverpool University Press.

  3. Cornelius, P.F.S., 1990a. European Obelia (Cnidaria, Hydroida): systematics and identification. Journal of Natural History, 24, 535-578.

  4. Cornelius, P.F.S., 1995b. North-west European thecate hydroids and their medusae. Part 2. Sertulariidae to Campanulariidae. Shrewsbury: Field Studies Council. [Synopses of the British Fauna no. 50]

  5. Costello, M.J., Bouchet, P., Boxshall, G., Emblow, C. & Vanden Berghe, E., 2004. European Register of Marine Species [On-line]. http://www.marbef.org/data/erms.php,

  6. Fish, J.D. & Fish, S., 1996. A student's guide to the seashore. Cambridge: Cambridge University Press.

  7. Fraser, C.M., 1937. Hydroids of the Pacific Coast of Canada and the United States. Toronto: University of Toronto Press.

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

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

  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.

  12. National Biodiversity Network (NBN) Atlas website. Available from: http://www.nbnatlas.org.  Accessed 01 April 2017

  13. Picton, B.E. & Costello, M.J., 1998. BioMar biotope viewer: a guide to marine habitats, fauna and flora of Britain and Ireland. [CD-ROM] Environmental Sciences Unit, Trinity College, Dublin., http://www.itsligo.ie/biomar/

Citation

This review can be cited as:

Richards, S. 2007. Obelia dichotoma 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. Available from: http://www.marlin.ac.uk/species/detail/37

Last Updated: 13/08/2007