Life Cycle of Hydrozoa

Hydrozoa are microscopic to tiny water animals that typically live in colonies and are related to the jellyfish family. Most hydrozoans live in saltwater, where they prey upon brine shrimps, insect larvae and annelid worms. Like jellyfish, hydrozoas also paralyze their prey by using their nematocysts, or stinging cells. Hydrozoa can reproduce both sexually and asexually.

  1. Hydrozoa Classification

    • According to classification, the bulk of hydrozoa live in salt-water.

      Hydrozoa are in the phylum Cnidaria and in the sub-phylum Medusozoa. The sub-phylum name refers to the long stinging tentacles trailing from the body of a hydrozoa, much like Medusa's hair in Greek mythology. Its class is Hydrozoa with two sub-classes of Leptolinae and Trachylinae. The former class contains the bulk of the stinging, salt-water hydrozoa while the latter includes the bulk of the fresh-water varieties.

    Sexual or Asexual Birth of a Hydrozoa

    • The dual nature of hydrozoan reproduction ensures the longevity of the species.

      In sexual reproduction, hydrozoan sperm floating in the water inseminates a female hydrozoa. The female hydrozoa then carries the fertilized egg, or buds, on her body column, where it develops into a polyp within two days.

      Asexual reproduction occurs when the female hydrozoa develops a budding of polyps without insemination. Asexual reproduction typically occurs when there is a reduction of available sperm.

    A Polyp is Born

    • The polyps are routed to the outside of the hydrozoan fan structure to gather food for the colony.

      Most hydrozoa live in a colony, so when the polyp detaches from its mother after three to five days, it immediately finds a space along the hydrocauli, or tubelike structure, which is formed by the other hydrozoa. Together, the hydrozoa connect to form a fan to find and capture prey. Polyps are specialized for feeding and gathering of food for the entire colony. They ingest food to be shared by others through the common bond called the hydrocaulus.

    The Polyp Develops into a Medusae

    • A medusae hydrozoa is named aptly for mythological Medusa because of the deadly tentacles.

      A medusae is a hydrozoa that has the ability to reproduce. Polyps may never develop into medusae hydrozoa and spend their lifetimes only gathering food. Should a polyp become a medusae, it will follow the lead of the sexual type present in the colony. All colonies are either female or male. Should it develop as a male, it will produce gametes that are released into the water, thereby fertilizing nearby female hydrozoa. Should it develop as a female, it will develop new buds either sexually or asexually as required.

    Numerous Orders of Hydrozoa

    • Hydrozoa are a diverse animal family.

      It is extremely difficult for biologists to classify hydrozoa beyond classes into orders because of their diversity in appearance and actions. Although hydrozoa can behave diffirently, especially depending on their locations, it is apparent from their life cycle that all types can be grouped together as one animal family.