hydrozoans

Hydrozoans are a diverse group of small to colonial aquatic animals within the phylum Cnidaria. They include solitary freshwater species such as Hydra, colonial marine forms like hydroids, and pelagic colonies such as siphonophores (for example, Physalia physalis) and Velella velella. Most hydrozoans have a simple body plan with a gastrovascular cavity surrounded by tentacles bearing cnidocytes that contain venom-bearing nematocysts for prey capture and defense.

Life cycles in hydrozoans are variable. Many species display a polyp-dominant life cycle, with polyps budding to form colonies that may specialize into feeding, defensive, or reproductive zooids. In other species, a free-swimming medusa stage is present but often small or short-lived; sexual reproduction typically occurs in the medusa or in specialized reproductive polyps (gonophores), producing planula larvae that settle to form new polyps. In siphonophores, the individual zooids are highly specialized and function together as a single colonial organism.

Ecology and habitat: Hydrozoans are found in oceans worldwide, with freshwater Hydra also common in ponds and streams. They are carnivorous, feeding on small planktonic organisms using their tentacles to capture prey. Some species may form extensive colonies anchored to substrates, while others are free-floating or pelagic.

Taxonomy and diversity: Hydrozoa is a class within the phylum Cnidaria, encompassing a range of forms from solitary polyps to complex colonial assemblages. Notable groups include the true hydroids, hydromedusae, and the siphonophores, whose colonies illustrate a high degree of specialization among individual zooids.