Hydrozoa Aquatic Invertebrates

Hydrozoa is a class of aquatic invertebrates that are closely related to corals, jellyfish, and sea anemones.

They belong to the phylum Cnidaria, which also includes other gelatinous zooplankton like jellyfish and box jellies. Their closest relatives are the Scyphozoa, the true jellyfish. 

Hydrozoans can be identified by stinging cells called cnidocytes, which contain nematocysts used for capturing prey and defense. These organisms have two primary phases in their lifecycle – a sedentary polyp stage that attaches to a substrate and a free-swimming medusa stage. 

Most species exhibit an alternation between these two phases. Hydrozoans are found globally in both marine and freshwater habitats, ranging from shallow coastal areas to the abyssal depths of the ocean.

Evolutionary Marvels: Tracing the Origins of Hydrozoa 

Hydrozoa has an ancient evolutionary history, with fossils dating back to the Cambrian period over 500 million years ago. Over time, they have gone through major diversification into the thousands of species known today. Key evolutionary innovations include the development of cnidocytes for injecting venom, the medusoid body form for swimming, and radial symmetry optimal for a planktonic lifestyle.

Adaptive strategies like the polyp and medusa phases allow them to occupy both benthic and pelagic realms. Complex lifecycles aid in dispersal to new habitats. There is also an evolutionary trend toward colonial organisms with a division of labor between zooids.

Based on anatomical and fossil evidence, hydrozoans are closely tied to scyphozoan jellyfish, suggesting a common ancestor. But connections to anthozoan groups like anemones hint at a lineage tracing back to more sedentary, anemone-like forms. The fossil record illuminates the gradual diversification of hydrozoan species over hundreds of millions of years.


Body Structure

Hydrozoans have a very simple structure withHydra fresh-water animals of phylum Cnidaria and class Hydrozoa Under Microscope just two layers of tissue – the epidermis on the outside and the gastrodermis lining the gut. Between these two layers is a gelatinous layer called mesoglea. 

The epidermis contains nerve cells, muscular cells, gland cells, and stinging cells called cnidocytes. Cnidocytes contain stinging structures called nematocysts, which are used to capture prey.

In species with both polyp and medusa phases, the medusa is the sexual stage. Medusae have a bell-shaped body with tentacles hanging down from the margin of the bell. At the center of the underside is a muscular manubrium, which functions in feeding and digestion.

Reproduction and Life Cycle

The polyp stage of hydrozoans is usually sessile, resembling a miniature sea anemone with a stalk anchoring it to a substrate and tentacles surrounding an open-ended sac used for capturing prey. The medusa stage is free-swimming with a bell-shaped body that propels itself through pulsations. Hanging from the bell are tentacles armed with stinging cells for paralyzing prey.

Inside the bell, the main center of digestion is the manubrium, which also aids in circulation. Hydrozoans have a simple, two-layered body plan, with an outer epidermis and an inner gastrodermis separated by a gelatinous layer called mesoglea. The epidermis contains cells like muscles, nerves, glands, and cnidocytes with their stinging nematocysts. 

Most hydrozoan species alternate between the benthic polyp stage and planktonic medusa stage in their lifecycle. The polyp can also reproduce asexually, creating clones of itself through budding. Larval stages allow dispersal to new locations, where they then settle and transform into new polyps. 

New medusae are generally produced by budding directly off the polyp. The detached medusa contains the gonads and is responsible for sexual reproduction. However, some species have evolved to skip the medusa stage entirely, while others rely solely on asexual budding for propagation. There is an incredible variety of hydrozoan reproductive and larval strategies.


Hydrozoans are carnivorous predators that feed on a variety of small aquatic organisms, including crustaceans, mollusks, other cnidarians, protozoans, and even fish larvae. 

The nematocysts contained within their cnidocytes are used to sting and subdue prey. Polyps simply wait for prey to come within reach of their tentacles. Medusae swim through the water, capturing prey in their tentacles and manubrium.

Digestion begins extracellularly as the nematocyst venom paralyzes the prey. Further digestion occurs within the gastrovascular cavity. Nutrients are then distributed throughout the body via diffusion from the cavity.

Diversity in Hydrozoa: Discovering the Rich Tapestry of Species

Different hydrozoan groups occupy habitats ranging from coastal shallows all the way down to abyssal ocean depths beyond 8,000 meters. They can also be found in estuarine and tidal environments. The majority of species are marine, found globally from tropical to polar regions. They have adapted to various temperatures, pressures, and other conditions. 

There are also a smaller number of species adapted specifically to freshwater habitats like lakes, ponds, and streams. These freshwater groups have more restricted distributions centered on ancient lakes. Some hydrozoans form symbiotic relationships with other organisms like mollusks, crabs, or coral for dispersal and access to nutrients. 

The diversity of hydrozoan species is remarkable. Some of the most notable and fascinating examples include Hydra – a small, solitary freshwater polyp with an amazing ability to regenerate its entire body from tiny fragments. It is an important model organism in biological research. 

Major Groups


This large and diverse group contains over 3,000 species. The polyps are solitary or colonial, branching or unbranched, and either sessile or free-living. Medusae always develop directly from polyps without an intermediate bud stage. They can be solitary or colonial with decentralized gonads.

There are six main subgroups within Anthoathecata:

  • Filifera – Thread-like defensive structures in the polyps.
  • Capitata – Reduced tentacles and a hydroid head. 
  • Proboscoida – Single elongated polyp.
  • Aplanulata – Lack of a free-swimming larval stage.
  • Foraminifera -Colonies connected via stolons.
  • Tubulariidae – Unbranched colonies.

Well-known members of Anthoathecata include the freshwater Hydra and the Portuguese Man O’ War (Physalia physalis).


This is the largest group, with around 2,500 species. The polyps are solitary or colonial without a stem or branches. Many are microscopic. Medusae buds form either on stalks on the polyp body or on modified polyps called gonophores. The medusae separate and become free-swimming.

There are two main subgroups within Leptothecata:

  • Conica – Vase-shaped polyps.
  • Proboscoida – Single elongated polyp.

The marine “hydroids” found growing on docks, ships, and aquatic vegetation represent Leptothecata species like those in the genera Obelia and Clytia.


This small group of 60 species is limited to freshwater habitats. Some common names for these organisms include freshwater jellyfish and crystal jellies. They all have a fixed polyp stage. 

The medusae develop from polyp buds and detach to become free-swimming with decentralized gonads. Unique microtubular structures provide stability. Common genera include Craspedacusta and Limnocnida, which have a worldwide distribution.


This group of about 80 species has a typical polyp stage but an unusual medusa form. The medusa lacks tentacles or sense organs and has a highly muscular manubrium used for clinging onto prey.

Most narcomedusae inhabit the deep sea. One well-known genus is Aegina, which has medusae with wing-like lobes used for swimming. Species feed on other jellies by clinging to them with their manubrium. 


These 170 species have complex colonial structures consisting of many highly modified zooids with various specialized functions. Although they superficially resemble jellyfish, they are structurally very different.

Each colony has swimming zooids, feeding polyps, reproductive polyps, and polyps specialized for defense and capturing prey. All zooids remain functionally connected by a common gastrovascular system. 

The most infamous group of organisms in this order is the Portuguese Man O’ War (Physalia physalis), which can deliver extremely painful stings to humans. Other siphonophores like Praya dubia are bioluminescent, producing an eerie green glow.  


Most of the 500 species in this group inhabit shallow coastal waters. They are dioecious with separate male and female medusae. 

The polyp stage is either solitary or colonial, being found attached to various substrates like eelgrass or algae. The medusae develop from polyp buds and become free-swimming.

Common trachymedusan genera include the well-known moon jellyfish (Aurelia), sea nettles (Chrysaora), and Immortelles (Turritopsis). Turritopsis medusae have the ability to revert back to the polyp stage and achieve a form of biological immortality. 

Unique Species


Hydra fresh-water animals of phylum Cnidaria and class Hydrozoa
Hydra fresh-water animals of phylum Cnidaria and class Hydrozoa

Hydra belongs to the phylum Cnidaria, class Hydrozoa, and order Anthoathecata. These small freshwater polyps consist of a cylindrical body with one end attached to a substrate and the other end having a mouth surrounded by tentacles. 

Hydra displays remarkable powers of regeneration, able to regrow their entire body from tiny fragments. They are also able to reproduce through budding, allowing a single individual to spawn an entire colony of genetically identical polyps.

Research on Hydra has provided insights into the developmental pathways of all animals, including humans. The Hydra model system continues to help uncover the fundamentals of cell differentiation and morphogenesis.

Portuguese Man O’ War (Physalia physalis) 

This infamous siphonophore resembles a floating blue bubble with long tentacles below. It has a powerful sting, which can be extremely painful and even deadly to humans and large prey. 

This is not a single multicellular organism but a floating colonial structure of interconnected zooids, each with specialized functions. Polyp-like zooids handle feeding and digestion; others are specialized for locomotion, while some provide extremely potent venom. 

The Man O’War has no means of propulsion and is carried by winds and currents in warm ocean waters. Blooms of the Medusozoid colonies sometimes prevent beachgoers from entering the water. Their stings continue to be hazardous even after death when washed up on shore.

Turritopsis Dohrnii (Immortal Jellyfish)

This small hydrozoan species can transform its medusa stage back into the attached polyp stage, allowing it to bypass the death of old age. Most mortality comes from predation or disease rather than senescence.

Turritopsis populations appear to be expanding globally, a finding confirmed by genetic data. One hypothesis is that climate change may be providing this species with ideal conditions for rapid asexual reproduction.

Scientists are actively studying this unique medusa to uncover the secret of cellular transdifferentiation that allows morphological rejuvenation. The hope is it may provide clues applicable to renewing human cell types.

Human Interactions and Importance: Hydrozoa in Research and Industry 

Hydrozoan nematocyst venom contains a complex cocktail of proteins, neurotoxins, enzymes, and toxins that holds great promise for biomedicine and drug development. 

Researchers are just starting to tap into these compounds, which may have analgesic, anti-inflammatory, antibiotic, or even anti-cancer properties. Specific antivenoms can also potentially be developed from hydrozoan toxins as a treatment for stings. 

Certain hydrozoan species have become indispensable model organisms for genetics, developmental biology, and regenerative medicine due to their simple body plan and ease of cultivation in the lab. 

For example, Hydra has been critical for illuminating fundamental mechanisms like cell differentiation and morphogenesis, which are conserved even in humans. 

The unique ability of Turritopsis jellyfish to reverse aging through cellular transdifferentiation offers intriguing insights into longevity that may one day be applicable to humans. The diversity of hydrozoan reproductive strategies also makes them fascinating models for teasing apart evolutionary tradeoffs between sexual and asexual propagation.

Ecological Significance: Hydrozoa’s Role in the Ecosystem

As predators, hydrozoans capture a variety of small prey, including crustaceans, fish, other cnidarians, protozoans, and more, using their nematocysts. Digestion begins extracellularly, with toxins paralyzing the prey before finishing within their gastrovascular cavity. 

In turn, hydrozoan medusae, polyps, and eggs are consumed by diverse organisms ranging from sea turtles to sharks to crabs.

Their place both as predator and prey makes them an important link in marine and freshwater food chains. Hydrozoans also contribute to habitat heterogeneity – colonies attached to the sea floor provide surfaces where many other small organisms live. 

Meanwhile, hydrozoan partnerships with mollusks, crabs, and coral provide access to food and dispersal, benefiting their symbiotic hosts.

As predators and prey, hydrozoans exert both top-down and bottom-up forces that influence marine ecosystem structure. Population blooms of certain hydrozoan species can occur when polyps rapidly reproduce via budding, sometimes forming massive swarms of medusae. 

This can negatively impact fishing and tourism but also connects food web dynamics across habitats through nutrient cycling. 

Invasive hydrozoan species accidentally introduced through human activities can potentially damage native communities by overgrowing substrate, preying on endemic species, and altering energy flow through the food web.

Ecological Impacts of Climate Change

Hydrozoan species are expected to be highly vulnerable to the effects of climate change, particularly those limited to specialized habitats and narrow environmental tolerances. Distributions are projected to shift as warming oceans alter conditions.

Factors that may impact specific hydrozoans:

  • Changing water temperatures affect reproduction and growth.
  • Ocean acidification interferes with calcification.
  • Reduced dissolved oxygen affects respiration.
  • Altered current patterns disrupt reproduction.
  • Damage to symbiotic relationships like those with reef-building corals.
  • Changes in prey abundance due to shifting ecosystem dynamics.

However, some species may benefit or adapt to changing conditions:

  • Warming allows expansion of ranges poleward or into deeper waters.
  • Increased nutrients are favoring the growth of benthic colonies.
  • Changing currents transporting polyps and medusae into new areas.
  • Declines in competitors providing ecological opportunities.
  • Adaptation of reproductive timing and habitat selection.

More research is critically needed to project outcomes for the world’s incredible diversity of hydrozoans in the face of accelerating climate change. Maintaining biodiversity will require strategies informed by a detailed understanding of hydrozoan ecology and physiology.

Fascinating Hydrozoan Facts

Hydrozoans display many fascinating and unusual adaptations. Their ability to rapidly clone polyps via budding allows single individuals to spawn massive colonies of thousands of genetically identical clones. 

In colonial forms, polymorphism enables certain polyps to transform into specialized reproductive or defensive modules integrated for complex colony functions. And some larvae have evolved the strategy of forming mini-colonies that behave cooperatively, improving the chances of survival. 

Many hydrozoan species also exhibit bioluminescence, producing light either as a lure for prey or as camouflage from predators lurking below. Different flashing patterns and colors likely serve as communication signals for attracting mates or confusing enemies. There is still much to uncover about the nuanced ecological roles of bioluminescence in marine environments.

Hydrozoa Frequently Asked Questions

Is A Hydrozoa A Jellyfish?

Both jellyfish and hydrozoans belong to the Cnidaria phylum, but they have some important differences.

The jellyfish is a type cnidarian belonging to the Scyphozoa class. The jellyfish are usually free-swimming creatures with a bell-shaped shell and long tentacles containing stinging cells known as nematocysts. The pulsing movement of jellyfish propels it through the water.

Hydrozoans are a diverse class of cnidarians. These cnidarians can be found both in freshwater and in saltwater. They come in many different shapes and sizes. Some hydrozoans have colonies, which are composed of multiple zooids. Some are solitary, and look more like jellyfish with a smaller bell.

The life cycle of jellyfish is another key difference. Jellyfish have a life cycle complex that includes both a medusa, the free-swimming form of adult jellyfish, and a stationary polyp. The life cycle of Hydrozoans includes both the medusa stage and the polyp, although details vary between species.

While jellyfish and hydrozoans have some similarities, both are different types of cnidarians, with distinct characteristics and life cycles.

Is A Hydrozoa A Medusa?

In their life cycle, most hydrozoans go through both a medusa and polyp stage. The majority of hydrozoans are found in small colonies consisting of tiny polyps that are connected by hollow stems. (Called Hydrocauli). Colonies can produce calcium carbonate or be covered by a flexible exoskeleton of chitin.

Medusae are much smaller than jellyfish. They range from 0.5 centimetres to 6 centimetres. Some species that live alone can reach a size of 6 to 9 cm in many cases.

A hydrozoan has tentacles encircling its body, which is shaped like an umbrella. The mouth is located at the tip of a tube-like structure that hangs from the umbrella’s centre. The majority have four tentacles, with nematocysts around the mouth and on each.

What Are The Main Characteristics Of A Hydrozoa?

  1. Internal space for digestion is the gastrovascular cavity
  2. Gastrovascular cavity has one opening, the mouth
  3. Exoskeleton of chitin
  4. Are almoust entirely marine and predators
  5. Sexual reproduction produces the planula larvae
  6. Two body forms, a polyp and medusea
  7. Presence of stinging cells called Cnidocytes
  8. 7 orders consisting of 2700 species

Does Hydrozoa Lack Medusa Stage?

Biased transitions are common in the tree of life. The hydrozoa class is the same, having lost the feeding medusa stage over 70 times. The hydractiniidae family include one lineage with pelagic medusae (Podocoryna) and several without (e.g., Hydractinia).

Is Hydrozoa Asexual?

Hydrozoan polyps do reproduce asexually by using the act of budding, creating daughter polyps, medusae, or both. Some of the species medusae reproduce asexually as well, by fission or budding. Medusae (if present in the life cycle) or polyps produce gametes.

How Do Hydrozoa Swim?

They by jet propulsion. They do so but are weak and are carried by currents over long distances. 

They do this by contracting and relaxing a ring of muscles around the bell which pushes them up and they slowly float down. The muscles open and close the bell, drawing in water and then forcing it out again to push them along. Polyps are sedentary.

Is Hydrozoa Freshwater or Saltwater?

Majority of Hydrozoans are found in the ocean and live in saltwater but some species, like Hydra, are found in freshwater.


Hydrozoa displays a fascinating array of biological diversity, complexity, and adaptability. From the reversal of the aging process to integrated colonial systems, hydrozoans provide unique insights into the evolutionary outcomes possible even in simple diploblastic organisms. 

Environmental change represents a threat but also an opportunity for species that demonstrate phenotypic plasticity and ecological versatility. There is still much more to learn about these ancient creatures, which have persisted from the Cambrian period right up to today.

With thousands of species inhabiting all reaches of the Earth’s oceans, hydrozoans will undoubtedly continue to exhibit remarkable mechanisms for thriving in aquatic ecosystems around the world. Their future remains closely tied to our own as the stewards of the marine environments that generations of hydrozoans have called home.

Gordon Ramel

Gordon is an ecologist with two degrees from Exeter University. He's also a teacher, a poet and the owner of 1,152 books. Oh - and he wrote this website.

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