Double-toothed sea fir (Obelia bidentata)

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



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

No text entered

Listed by

- none -


  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.]


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

  2. OBIS (Ocean Biodiversity Information System),  2024. Global map of species distribution using gridded data. Available from: Ocean Biogeographic Information System. Accessed: 2024-05-21


This review can be cited as:

Wilson, E. 2002. Obelia bidentata Double-toothed sea fir. 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 21-05-2024]. Available from:

Last Updated: 17/07/2002