Hydrozoans are a diverse group of aquatic animals, comprising approximately 3,700 known species within the phylum Cnidaria. These invertebrates share ancestry with creatures like jellyfish, sea anemones, and corals. While many are small, their forms, sizes, and habitats vary greatly. They are found in nearly all aquatic environments, including freshwater.
The Two Forms of Hydrozoans
Hydrozoans display two primary body plans: the polyp and the medusa. The polyp form is sessile, attached to a substrate. These polyps are cylindrical, with a mouth surrounded by tentacles at one end, and often form colonies where individuals are interconnected. Many colonial polyps exhibit polymorphism, with specialized individuals performing distinct functions such as feeding, defense, or reproduction.
The medusa form, in contrast, is free-swimming and characterized by a bell-shaped body with tentacles hanging downward. These medusae are smaller than true jellyfish, ranging from 0.5 to 6 centimeters in diameter, though some solitary species can reach up to 9 centimeters. Both polyp and medusa forms possess specialized stinging cells called cnidocytes, which contain nematocysts. These microscopic capsules, unique to Cnidarians, rapidly discharge a coiled, barbed tube, injecting venom to capture prey or deter predators.
A Complex Life Cycle
Many hydrozoan species exhibit a complex life cycle involving an “alternation of generations” between their asexual polyp stage and sexual medusa stage. The cycle begins with a colony of polyps, which reproduces asexually by budding off tiny medusae. These newly formed medusae are free-swimming and mature in the water column.
Once mature, the medusae reproduce sexually, releasing gametes into the surrounding water. Following fertilization, the zygote develops into a ciliated, free-swimming larva known as a planula. This planula larva settles onto a suitable substrate, such as a rock, where it undergoes metamorphosis and grows into a new polyp, completing the cycle. Some hydrozoan species have modified this cycle, by suppressing the medusa stage entirely, as seen in Hydra, or by having a reduced polyp stage.
Diverse and Famous Examples
The class Hydrozoa includes many well-known organisms, despite their often small size. The Portuguese man o’ war (Physalia physalis) is a notable example, frequently mistaken for a true jellyfish. This creature is actually a siphonophore, a colonial hydrozoan composed of multiple specialized polyps that work together for functions like floating, feeding, and reproduction. Its long, trailing tentacles contain potent nematocysts capable of delivering a painful sting.
Another hydrozoan is Hydra, a common freshwater species that exists solely as a solitary polyp. Unlike most hydrozoans, Hydra lacks a medusa stage in its life cycle. It is known for its regenerative abilities, where cut pieces can regrow into complete organisms. In marine environments, “fire corals” (Millepora species) represent colonial hydrozoans that secrete a hard, calcareous skeleton, resembling true corals. Despite their coral-like appearance, these hydrozoans are distinct and can inflict a burning sting due to their numerous cnidocytes.
Ecological Role and Habitat
Hydrozoans are widely distributed across nearly all aquatic environments, though most species are marine. They inhabit diverse aquatic ecosystems, from the deep sea to freshwater ponds and shallow coastal areas. Many hydrozoan colonies attach to substrates like rocks or ship hulls, while some, like siphonophores, float freely in the open ocean.
These animals play a role within aquatic food webs. As predators, they use their stinging tentacles to capture a variety of small organisms, including plankton, tiny crustaceans, and small fish larvae. Conversely, hydrozoan polyps, medusae, and eggs serve as a food source for other marine life, such as nudibranchs, fish, sea turtles, and crabs. Hydrozoan colonies also contribute to habitat complexity, providing shelter and surfaces for other small organisms to live.