Hydroid (zoology)

A hydroid is a life form belonging to the class Hydrozoa, a group within the phylum Cnidaria. They are typically colonial animals, meaning they consist of numerous individual polyps connected by a shared, branching structure called a hydrocaulus. This hydrocaulus, often resembling plant stems, allows for the distribution of nutrients and communication between the individual polyps.

Hydroids exhibit polymorphism, meaning the colony consists of different types of polyps specialized for various functions. Common types include:

• Gastrozooids (trophic polyps): Responsible for capturing and digesting prey. They typically possess tentacles armed with nematocysts (stinging cells).
• Gonozooids (reproductive polyps): Specialized for reproduction, often budding off medusae (jellyfish) or producing gametes directly. These polyps often lack tentacles.
• Dactylozooids: Defensive polyps that are often armed with numerous nematocysts. They may also be used for capturing prey or cleaning the colony.

The hydrocaulus is covered by a protective, non-living sheath called the perisarc. In some species, the perisarc extends to encase the individual polyps, while in others, the polyps are naked and protrude directly from the hydrocaulus.

Hydroids are primarily marine organisms, found in a wide range of habitats, from shallow intertidal zones to deep ocean environments. Some species are solitary, existing as individual polyps rather than colonial forms. Their life cycle may involve both a polyp stage (the hydroid) and a medusa stage (the jellyfish), with the relative importance of each stage varying among different species. Some hydroids, however, lack a medusa stage entirely. Reproduction can occur both sexually (through gametes produced by gonozooids) and asexually (through budding).