Thin-walled obelia (Obelia dichotoma)
|Researched by||Saskiya Richards||Refereed by||Admin|
|Other common names||-||Synonyms||-|
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 IrelandFound throughout the British Isles and Ireland.
HabitatObelia dichotoma is usually found on floats, pilings, rocks, shells and other solid objects.
- 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.
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.
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Last Updated: 13/08/2007