Monterey Bay Aquarium Research Institute
Marine Botany

Stephanocystis osmundacea bears reproductive fronds beginning in late spring that persist through the summer and are shed in late autumn. The life history of Stephanocystissimplified compared to some of the triphasic life histories of the red algae. Since Stephanocystis has a diplontic life history, it has only one free-living phase, and that phase is diploid. In other words, Stephanocystis has a life history very similar to our own! Get acquainted with the cast of characters below, then check out the animated cartoon illustrating Stephanocystis's life history by clicking here!


Cystoseira life history diagram

Stephanocystis osmundacea 's reproductive structures, called receptacles , are borne on the annual vegetative fronds of the apical region. The receptacles form small terminal branchlets distal to the pneumatocysts, these branchlets being spindle-shaped. Receptacle branches become swollen with the presence of conceptacles. Conceptacles are tiny, round chambers within the receptacle branch; conceptacles may form small bumps on the receptacle frond, and the pore, or ostiole, opening from the interior of the conceptacle to the surrounding water, is easily visible under a dissecting microscope. Conceptacles are the site of gametogenesis in Stephanocystis. BecauseStephanocystis is dioeceous, a conceptacle will bear either female or male gametes, but not both.


Oogonium:   In a female conceptacle, the first two divisions of the nucleus are meiotic and produce four haploid nuclei. These nuclei then divide mitotically to produce eight haploid nuclei. Stephanocystis's female oogonium displays a unique characteristic: only one of the eight haploid nuclei will be assimilated into the ovum. Thus only one ovum per female conceptacle is produced, and the remaining seven nuclei are extruded peripherally.

Antheridium: Stephanocystis's antheridia are produced in the same manner as a unilocular sporangium: the first two meiotic division of the original nucleus in the antheridium produce four haploid nuclei, which subsequently undergo a series of mitotic divisions to produce sixty-four haploid nuclei. All sixty-four haploid nuclei are incorporated into the production of sixty-four sperm.


The ova of Stephanocystis appear as one might expect from an oogamous alga. They are very large in relation to the sperm and are spherical in shape. The sperm of Stephanocystis, however, are unique to the taxonomic order Fucales. Each has two laterally inserted flagella. The anterior flagellum is short and pleuronematic, meaning that it has many hair-like appendages called mastigonemes. The posterior flagellum is longer and acronematic, or smooth. A large, nose-like anterior growth, called the proboscis, is present on the sperm, and this proboscis is thought to aid the sperm in binding to the egg.

Last updated: Mar. 16, 2015