Life Cycle of Obelia

By Frank Whittemore

Obelia live in oceans throughout the world. Though they may appear to be plants they are, in fact, animals. Obelia are members of the Cnidaria phylum, which includes jellyfish, corals, and sea anemones. Obelia have two distinct stages to their life cycle. The first is the polyp stage. The second is the medusa stage.

Asexual Reproduction

Obelia live in colonies made up of stem-like polyps that are attached to rocks and other ocean substrate using filaments that resemble roots. The polyps can reproduce by budding, creating a tree-like structure as it grows, adding new polyps over time.

Types of Polyps

There are two types of polyps in an Obelia colony. Gastrozooids are used primarily for feeding and possess stinging tentacles around a mouth to gather and eat food. All gastrozooids within a colony are interconnected by a single gastrovascular cavity.

The colony also has polyps known as gonozooids, the sexual reproductive portion of the colony.

Sexual Reproduction

Each gonozooid will asexually produce tiny jellyfish known as medusae. They are shaped in the classic jellyfish form, with a swimming bell and trailing tentacles. The medusae will bud off from the parent. When mature they are dispersed into the open ocean.

Medusa Mating Stage

The medusae are free swimming. They may be either male or female, possessing reproductive organs that produce either eggs or sperm. Both eggs and sperm are released into the water. The sperm is motile, using flagella for locomotion. The sperm seek out the eggs and fuse to them, forming a zygote.

Zygote/Planula Stage

The zygote grows into an embryo, eventually becoming a planula, a ciliated, free-swimming larva. Each planula will eventually attach itself to rocks or other substrate and continue to grow, eventually developing into a polyp.

Polyp Development

The polyp will then produce more polyps as it grows. Some will differentiate into gastrozooids, allowing the colony to feed. Others will form gonozooids that will eventually produce more medusae that will be released to mate, completing the life cycle.