Obelia dichotoma Linnaeus, 1758: Medusa form (See also the Polyp Form)

Common name(s): Sea plume, branched Obelia

Synonyms: Medusa
Phylum Cnidaria 
Class Hydrozoa 
Suborder Leptomedusae (Thecata) 
Family Companulariidae 
Obelia dichotoma from a nocturnal surface plankton trawl. Diameter 2.5 mm.
(Photo by:  Dave Cowles, July 2017)

Description:  The Obelia dichotoma medusa is a small hydromedusa with the bell diameter greater than its height. It has no ocelli. The manubrium is short. More than 16 unbranched tentacles, all about the same size, are distributed evenly along the margin of the bell. The tentacles do not contain prominent rings of nematocysts. The 4 radial canals do not branch, nor do they have major diverticula. The gonads are nearly spherical and attached about halfway along the radial canals. They move farther out along the canal as the medusa matures. 8 statocysts. Diameter up to 4 mm.

How to Distinguish from Similar Species:  TheObelia geniculata medusa is about the same size and shape and has similar gonads, but the gonads are located closer to the mouth and move out to about halfway along the radial canal as the medusa grows.

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

Depth Range:  Pelagic

Habitat:  Pelagic

Biology/Natural History:  The medusae are about 0.5 mm diameter when they bud off the polyp. The change in morphology as the medusae grow makes correct identification and following the natural history difficult. They can be found along our coast in mid-spring through mid-automn. The medusae produce eggs and sperm.  A fertilized egg grows into a planula larva.  In Bodega Bay (CA) planula larvae from the medusae settle and begin growing into new colonies in winter, spring, and early summer.



Dichotomous Keys:
  Kozloff, 1987, 1996

General References:
  Morris et al., 1980
  Wrobel and Mills, 1998

Scientific Articles:

Web sites:

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

A slightly closer view of the same individual. See the short manubrium projecting from the central stomach, and the 4 radial canals with the gonads attached to them a bit over halfway out on the canals.

Authors and Editors of Page:
Dave Cowles (2017):  Created original page
CSS coding for page developed by Jonathan Cowles

Salish Sea Invertebrates web site provided courtesy of Walla Walla University