Life cycle of hydrozoan Obelia sp.

This is a picture of a beautiful hydromedusa of the genus Obelia. In the center, you can see the manubrium - a little stalk at the tip of which the mouth opens. You can also see the four gonads. The medusa is the sexual stage of the hydrozoan life history! The four faint lines radiating from the manubrium toward the perimeter and underlying the gonads are the radial canals of the digestive system. They join the marginal ring canal and supply nutrients to the tentacles.

When the focal plane is just right, statocysts (balance organs) at the base of the tentacles are visible. You can see one of them at about 3 o'clock on this picture. Obelia is a classic example of alternation of generations in a hydrozoan life cycle. The medusa produces eggs and sperm. When fertilized, an egg develops into a simple non-feeding planula larva. The planktonic planula metamorphoses into a benthic polyp (hydroid), which buds off further polyps and forms a colony. Hydroid colonies have different kinds of zooids specialized for different functions.

This picture shows a small fragment of a colony, including a feeding zooid, its tentacles withdrawn into its goblet-shaped theca, at about 3 o’clock. A larger balloon-like structure at about 5 o’clock is the reproductive zooid, called
the gonangium. Gonangium includes gonophores (budding medusa-zooids), which you can see as round shapes inside the gonangium. When ready, the medusa buds off and swims away, as you can see in the bottom picture. Obelia's medusa was the most remarkable animal I have seen so far in this embryology class. I just wish to see its embryos someday.