Hydrozoan Life Cycle

Hydrozoa is a class of invertebrate underwater creature that tends to be very small and predatory. One example of a hydrozoa would be a jellyfish. Many species spend their adult lives attached to a rock or the sea bed and grasp at any creature they can eat that drifts by. These creatures have a complex and interesting life cycle.

  1. Single Cell Stage

    • The hydrozoa starts out life as a single celled organism. This is the egg that is hatched after the adult hydrozoa is fertilized. This egg is simply the brief beginning of the complex life cycle of the hydrozoa. Their life cycle will take them through many diverse stages, which results in diversity in the hydrozoa's shape and size, diversity not found in many other creatures.

    Multi-Celluar Stage

    • The egg will soon begin dividing into multiple cells. This is known as the multiple cell stage. In this stage, the hydrozoa is no longer an egg but is not yet mobile. It is simply growing the way that any embryo does. During this stage, the parts for the next stage will grow and the hydrozoa will be prepared to begin moving and eating. The multi-cellular stage is simply a preparation stage.

    Planular Stage

    • The planular stage is the first important stage in the hydrozoa's life. In this stage, they will resemble an oval shaped piece of bacteria. During this stage, they try to avoid being eaten by other creatures and eat to prepare for their next transformation. This is the first stage that the hydrozoa is mobile. They will quickly lose their mobility in the next stage, however.

    Hydroid Colony Stage

    • The adult phase in the hydrozoa's life is the hydroid colony stage. This occurs after the planular stage has grown large enough. During this stage, the hydrozoa completely changes its size, shape, and nature. It plants itself into the ocean bed with a long thin stalk. On the end of this stalk is a mouth with which it feeds. It is capable of growing more than one mouth to feed on as it grows. During this stage, the hydrozoa can grow to cover huge areas of the ocean floor. They are also growing flower buds. In these flower buds is the next stage of their life: the medusa.

    Medusa Stage

    • After the flower buds grow big enough, they burst. From these buds emerge the medusa stage of the hydrozoa life cycle. The medusa is a jellyfish in shape. It is also the second time the hydrozoa becomes mobile. The medusa exists for one purpose: fertilization. It travels around the ocean floor, looking for other medusa to fertilize. They cannot mate with other medusa from their same colony as the sex of any one colony is always singular. Once it has found a medusa from another colony, they will mate and begin the cycle again.