Obelia dichotoma Linnaeus, 1758)

Common name(s): Sea plume, Branched Obelia

Phylum Cnidaria
 Class Hydrozoa
  Order Hydroida
   Suborder Thecata (Leptomedusae)
    Family Campanulariidae
Obelia dichotoma, about 9 cm long.
(Photo by: Dave Cowles, August 2006)
Description:  This thecate hydroid has polyps enclosed by a true hydrotheca which is deeper than it is wide and large enough to cover the feeding polyp (hydranth) when the hydranth contracts.  The hydrotheca around each hydranth is bell or goblet-shaped, has no operculum, but does have a diaphragm near the base.  The rim of the hydrotheca is smooth or slightly toothed.  The perisarc of older parts of the colony turns brown.  The hydranths are on slender stalks.  The colony morphology is composed primarily of upright, extensively branching stems rather than stolons.  The hydranths alternate from one side to another along a branch.  The gonangia release medusae which swim freely in the water.  The released medusae are disk-shaped with 16 or more tentacles which are not highly extensile but bear conspicuous rings of nematocysts.  Up to 30 cm tall.

How to Distinguish from Similar Species:  Obelia bidentata has toothed rims on the hydrotheca and older parts of the colony do not turn brown.  Obelia geniculata is unbranched and usually lives on brown algae.  Clytia spp. have less than 16 tentacles in the medusa and have toothed margins on the hydrothecaGonothyraea spp retains the medusae attached to the polyps.

Geographical Range:  Alaska to San Diego, and worldwide in temperate zones

Depth Range:

Habitat:  Common in protected waters attached to pilings, eelgrass, mussels, barnacles, seaweed, or debris.  Uncommon on the open coast.

Biology/Natural History:  The reproductive polyps of this species are in urn or club-shaped cups in the angles of the branches.  They bud off small medusae (about 0.5 mm diameter when released, grow to about 5 mm).   The medusae produce eggs and sperm.  A fertilized egg grows into a planula larva.  The surface of the branches of the colony often harbors diatoms, the nudibranch Eubranchus olivaceus, the suctorian nudibranch Ephelota, other small hydroids, and caprellid amphipods.  The caprellid amphipods may eat diatoms, other creatures, or the Obelia polyps.  Eubranchus olivaceus eats Obelia polyps.  In Bodega Bay (CA) planula larvae from the medusae settle and begin growing into new colonies in winter, spring, and early summer.  Colonies normally grow for 2-3 months.

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Dichotomous Keys:
  Flora and Fairbanks, 1966
  Kozloff 1987, 1996

General References:
  Hinton, 1987
  Johnson and Snook, 1955
  Kozloff, 1993
  Morris et al., 1980
  Ricketts et al., 1985

Scientific Articles:

Web sites:

General Notes and Observations:  Locations, abundances, unusual behaviors:

A closeup of the colony, showing the brown perisarc on older sections and the bell-shaped hydrotheca around each hydranth.

Authors and Editors of Page:
Dave Cowles (2006):  Created original page