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Hydroid Life Cycle

A hydroid or a hydrozoan are small, plant-like animals related to jellyfish, corals and sea anemones. There are about 2,000 species of hydroids that vary greatly in structure and appearance. All species do, however, have a polyp (a stalk-like structure) that is connected to tentacles and a mouth according to "Hydrozoa" at jrank.org. Hydroids are either medusa-like (free-swimming organisms) or polypoid-like (non free-swimming organisms).

  1. The Facts

    • An interesting feature of the polypoid hydroids is that some of them have a medusoid larval phase which settles onto a substrate, according to "Hydrozoa" at jrank.org. While some species are solitary, most live in colonies. These colonies come from a single root that stems from the substrate, from which the individual polyps arise.

    Life Cycle

    • Hydroids have three stages in their life cycle. In the beginning, ciliated planula larva (which are tiny, free swimming organisms of about 0.04 inches in length) originate and morph into an attached (sessile) polyp stage (typically making up colonies). These polyps then turn into a gamete-producing male or female medusa (sometimes referred to as jellyfish).

    Features

    • As a result of evolution, many hydroids suppress the medusa by keeping the sessile hydroid colony. These colonies range from 0.2 to 20 inches in length, according to "Hydroid" on britannica.com. Their branches result in zooids (hydroid polyps). Each zooid has a tubular body made up of two layers that are separated by a mesoglea (a thin, jelly-like layer of connective tissue). It also has a mouth and tentacles. The zooids are joined to a stolon (a tube that runs the entire length of the hydroid colony). This stolon allows for the exchange of food between the individuals of the colony and is protected by a perisarc (tough outer layer).

    Function

    • Hydroid colonies grow as a result of the increasing number of hydranths. Gonozooids (reproductive polyps) are found intermittently throughout the colony. These reproductive polyps release medusae or planula larvae (depending on the species). Furthermore, certain species retract their polyps within the perisarc while others lack this capability and structure, according to "Hydroids" on britannica.com.

    Considerations

    • While most hydroids are found in marine environments, some are also found in fresh water. Furthermore, an interesting feature of the species that form colonies is that the individual polyps fulfill different roles within the colonies, according to "Hydrozoa" at jrank.org. Some are specialized for feeding (gastrozooids) and defense, while other carry out reproduction (gonozooids).

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