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Stages of Obelia

The obelia is a cnidarian, which is a simple animal with a hollow body cavity. It lives in the ocean in colonies. Obelia captures prey and defends itself through stinging cells called cnidocytes. Despite this, obelia and other hydrozoans are prey to many animals, including fish, sea turtles, snails, worms and even other hydrozoans. Some predators, like nudibranchs, incorporate the stinging cells into their bodies for their own self-defense. But obelia are interesting because they seem to have an alternation of generations, which means there's an asexual phase of reproduction and a sexual phase of reproduction. The obelia has a polyp form, which is its asexual stage, and a medusa form, which is its sexual phase.

The Obelia Colony Animal
- The obelia colony has branched individuals that are attached to a hard substrate. Obelia colonies can become "fouling communities" when they attach in great numbers to the hulls of ships or submerged water pipes and interfere with them. Obelia has two zooids, or individual persons. In cnidarians there are only two types of zooids. The gastrozooids have mouths and tentacles and help the animal eat by trapping plankton in the tentacles. The other zooids are blastostyles, or gonangium.
Reproduction Part I
- The blastostyle has neither mouth nor tentacles and is a narrow elongated hollow tube. It produces medusa, a modified zooid, through asexual budding. Though it's produced asexually, the medusa, which is shaped like an umbrella, is part of the phase of sexual reproduction. The medusa contains four gonads -- sex organs -- in a radial canal.
Reproduction Part II
- The medusa has two sexes, or is dioecious, and testes and ovaries are in different individuals. Medusae are solitary and never give birth to other medusae. The sperm mature after the medusae leave the blastostyle. Sperm and eggs meet in the water and the fertilized eggs transform into balls of cells till a planula larva is formed. After a brief swimming stage, the planula larvae, which look like paramecia, settle down. They turn into club-shaped larvae and undergo a metamorphosis. One end attaches to the substratum and the other forms a mouth and tentacles so the larva looks like a hydra. Asexual budding continues until a new colony of obelia are formed.
Polymorphism
- Some scientists believe that the obelia doesn't have an alternating sexual and asexual phase as the gametes, the egg and sperm, don't actually come from the medusa but from the blastostyle that produces the medusa. These gametes then simply migrate into the medusa. The scientists believe this just demonstrates that different parts of the colony have different functions. For the gastrozooids it's catching prey and for the blastostyles it's reproduction. This is called polymorphism.

Nature