The Hydrozoa is a subgroup of cnidarians containging approximately 3700 species. It is a diverse group with a variety of life cycles, growth forms, and specialized structures. Like many cnidarians, hydrozoans have both polyp and medusa stages in their life cycle. They are distinguished from other groups by their complex life cycle, by the growth of medusae from buds rather than strobilae or from metamorphosis, by the presence of a velum inside the bell of the medusa, and by the production of gametes from ectodermal, rather than endodermal, tissue. Most hydrozoans are marine, and hydrozoan species are found in nearly every marine habitat type; a very few species live in freshwater. Most hydrozoans form colonies of asexual polyps and free-swimming sexual medusae. Colonies are usually benthic, but some, notably the siphonophores, are pelagic floaters. Colonial polyps often have some division of function, with certain polyps specialized for defense, feeding, or reproduction. Most hydrozoans are predators or filter-feeders, though a few have symbiotic algae (zooxanthellae), in the same way that other other groups of cnidarians do.

Better-known hydrozoans include Portuguese man-o-wars (Physalia physalis), the freshwater genus Hydra, fire coral (Milleporidae), and by-the-wind sailors (Velella velella). (Bouillon, et al., 2006; Brusca and Brusca, 2003; Jankowski, et al., 2008; Mills, 2009)

Geographic Range

Hydrozoans are found in all the oceans, at all latitudes. A few species occur in fresh and brackish water on all continents except Antarctica. (Bouillon, et al., 2006; Jankowski, et al., 2008)


Hydrozoans are found in nearly all marine habitats, except perhaps heavy surf zones. They are most abundant and diverse in warm shallow waters, probably as a reflection of food abundance. The small number of freshwater species occur in both lotic and lentic habitats, and are more abundant in eutrophic and mesotrophic waters. (Bouillon, et al., 2006; Jankowski, et al., 2008)

Physical Description

Most hydrozoan species have a planktonic larval stage called a planula. Planulae are radially symmetric ovoids, often covered with flagellate cells for swimming. They may be very simple embryos or have cells differentiated into several types. Planulae most often settle onto a benthic substrate and develop into a polyp.

Polyps are radially symmetric, and may be urn-shaped, conical, cylindrical, or club-shaped. In most species they are only a few millimeters tall, though the largest grow up to many centimeters, and one, Branchiocerianthus imperator can be 2 meters tall. At their base hydrozoan polyps have basal disks or elongate processes for attaching to substrate, or they may be attached to other polyps. Often there will also be connections here to hollow tubes (called stolons) that connect the polyp to others in its colony, and allow the exchange of food between polyps. Above the base is a ring of contractile cells called the sphincter. These can contract to isolate the contents of the polyp from the stolons, preventing undigested food from entering the stolons. Above this is the gastric column, which usually contains a digestive chamber with a single opening, a mouth at the apex of the column. A ring of tentacles is attached to the column below the apex and above the sphincter. The number, shape and size of tentacles varies greatly, but there are usually between and 8 and 50 on a single polyp (some have many more, and some specialized polyps may have fewer). Most colonial hydrozoans are polymorphic, with different structures reflecting different functions. Some are armed with large spines tentacles for defense but have no mouth, some have tentacles and functional mouths for feeding, and some are only reproductive, with no tentacles or mouth, and produce medusae (see below) or gametes.

Like all cnidarians, hydrozoans have special ectodermal cells called cnidocytes, each containing a single intracellular structure called a cnida (aka nematocyst). Cnidae are unique to the Cnidaria. Each cnida, when triggered by a mechanical or chemical stimulus, shoots out a tiny hollow tube at high speed. Some cnidae are is equipped with sharp spines, and/or venomous or acidic compounds, but some are adhesive and have neither spines nor toxins. Hydrozoans use different types of cnidae to capture prey, to repel predators, and to attach to substrate.

Most hydrozoan species are colonial. A founding polyp produces new polyps by budding, and these grow a network of interconnecting hollow tubes (stolons) formed of living tissue, collectively called the coenosarc. Colony growth forms vary between species, some may form a single layer of polyps spreading across the substrate, others growing as erect stems, with polyps growing off the stems. Polyps and the coenosarc may secrete chitinous sheaths, or stems, or calcareous coatings (the latter forming structures similar to the anthozoan Scleractinia, the stony corals). In many colonies, polyps are polymorphic, with different structures reflecting different functions. Some have no mouth, but are armed with large spines or cnidae-equipped tentacles for defense, some have tentacles and functional mouths for feeding, and some, with neither mouth nor tentacles, are strictly reproductive, and produce medusae (see below) or gametes.

The medusa is the sexually reproducing stage in most hydrozoans. They are often formed by budding from polyps, and are usually solitary free-swimming organisms. They are similar in structure to an inverted polyp, radially symmetric, and often have four-fold symmetry. Their main body part is the umbrella, a bell or cone shaped gelatin-filled structure, which floats with the opening down. Medusa are usually small, usually 1-50 mm in diameter, though a few are larger, the largest (genus g. Rhacostoma) grow to 400 mm in diameter. Around the inside of the opening is a muscular ring of tissue called the velum. The velum can contract and relax, changing the diameter of the opening, and playing an important role in swimming The presence of the velum is a diagnostic character for Hydrozoa, only one genus, Obelia, has lost it. Around the outside of the opening of the umbrella is a ring of tentacles, which vary greatly among species in number, shape, and degree of arming with cnidocytes. Inside the umbrella, suspended like the clapper of a bell, is the manubrium, which contains the gastric cavity, and ends in a mouth. Structures that produce gametes form on the sides of the manubrium. Most species have dioecious medusae, each individual producing only eggs or sperm. Some are monoecious, but usually not simultaneously hermaphrodite. In some species sex is determined by environmental conditions, mainly temperature.

Both polyps and medusae have networks of nerves, but no brain or central ganglion. Some have light-sensitive structures called ocelli, and many have statocysts that allow them to detect gravity and their orientation.

These structural patterns are common, but there is great variation in the life cycles of hydrozoans. Some have suppressed or reduced one or more stages. In the Siphonophora and a few other groups of hydrozoans, colonies of polyps are pelagic, and float at the surface by means of a gas-filled tissue. They often retain medusae as part of the colony. (Bouillon, et al., 2006; Brusca and Brusca, 2003; Jankowski, et al., 2008)


Hydrozoans have a complex life cycle, usually with two or three morphologically different stages. The classic cycle starts with fertilized eggs developing into small, free swimming larvae called planulae, which may be able to enter a dormant resting state to resist unsuitable environmental conditions. Planulae transform into sessile polyps, usually attached to substrate, but free-floating in some groups. Polyps duplicate themselves asexually by budding, often producing colonies of hundreds or thousands of polymorphic individual polyps. Polyps produce "adult" sexually-reproducing medusae by budding. Medusae are solitary, free-swimming, dieocious. They release sperm and eggs into the water, where fertilization occurs. This is the basic cycle, but there is an enormous range of variations. In nearly half of species (e.g. Hydra) the the medusa stage is entirely suppressed; polyps produce gametes directly. In others the medusa are formed, but never detach from the parent polyp, and produce gametes while still attached. In some cases these fused combinations form elaborate structures. In other taxa the polyp stage is suppressed, and planulae transform directly into tiny medusae, or form a polyp, produce a medusa, and resorb the polyp. Numerous taxa have suppressed the planula as well. (Bouillon, et al., 2006; Brusca and Brusca, 2003)


Hydrozoans are mostly broadcast spawners. In some species only sperm is shed, and eggs are retained on the parent. Eggs release sperm-attracting compounds.

Hydrozoan polyps reproduce asexually by budding, creating daughter polyps, medusae, or both. In some species medusae reproduce asexually as well, by fission or budding. Medusae (if present in the life cycle) or polyps produce gametes. Most hydrozoan species are dioecious, a few are sequential hermaphrodites. Eggs and sperm are most often released into the water column and fertilization is external. In some species eggs are retained and fertilized internally, in which case embryos may be releases as larvae or retained until even more developed. (Bouillon, et al., 2006; Brusca and Brusca, 2003)

Most hydrozoan species have minimal parental investment. Eggs and sperm are released into the water, and left to survive on their own. In a few species, eggs are retained in special structures on the parent, and the embryos are retained as brood, developing to the planula or even young polyp stage. In the latter case we have no information on whether the young are nourished by their parent, or just protected. (Bouillon, et al., 2006; Brusca and Brusca, 2003)

  • Parental Investment
  • no parental involvement
  • female parental care
  • pre-fertilization
    • provisioning
    • protecting
      • female
  • pre-hatching/birth
    • protecting
      • female


Hydrozoans combine sessile or sedentary polyp stages and free-swimming solitary stages in their life cycles. Polyps may be solitary or colonial. Some can move by crawling, but most are sessile. The Siphonophora form floating colonies of attached polyps medusae. Many free-swimming hydrozoans follow the diel migration pattern common to many pelagic marine organisms. They spend daylight hours in deep water where light does not penetrate, and rise to the surface after sunset. (Brusca and Brusca, 2003; Mills, 2009)

Communication and Perception

All hydrozoans have tactile and chemical sensing structures. Some also have eyespots that detect light, and/or statocysts that detect gravity. They communicate mainly by chemical signals. Some free-swimming hydrozoans, including many siphonophores have bioluminescent structures. It's not known what function these serve. It is unlikely that they communicate with other hydrozoans (their light sensors are too simple for this). Possibly they are lures for prey or have some predator defense function. (Bouillon, et al., 2006; Brusca and Brusca, 2003; Dunn, 2009; Mills, 2009)

Food Habits

Hydrozoans vary in their feeding habits. Many trap small zooplankton with their tentacles. Some filter suspended particles (such as fish eggs and fecal pellets) from the water column. Some consume phytoplankton. A few groups contain symbiotic algae, and may get most of their nutritional needs from their symbiotes.

Pelagic hydrozoans, including siphonophore colonies and medusae, are known to show some selectivity in prey types, some taking mainly fish larvae, others taking soft-bodied invertebrates, others micro-crustaceans. They are also sensitive to chemicals produced by prey, and will move towards higher concentrations of these chemicals.

Large populations of hydromedusae may be significant ecological factors in pelagic marine ecosystems. (Bouillon, et al., 2006; Brusca and Brusca, 2003; Dunn, 2009; Mills, 2009; Purcell, 1997)


Despite their protective stinging cells, hydrozoans are prey for many types of predators. A variety of snails and worms graze on polyps and stolons, as do some fish and crustaceans. Fish also consume medusae and pelagic colonial hydrozoans, as do some sea turtles (especially leatherbacks), ctenophores, and other cnidarians, including larger hydrozoans.

A variety of predators have the ability to consume the stinging cells of hydrozoans without triggering them. These predators then sequester the stinging cells in their body to defend them against their own predators. Nudibranchs are particularly well known for this ability, but some species of ctenophores, turbellarian flatworms, and priapulids can store cnidocysts as well.

Nearly all hydrozoans protect themselves with their cnidocysts. Some colonial species have specialized polyps that grow large tentacles armed with dense batteries of these stinging cells or grow large rigid spines. Many colonial polyps secrete a rigid protective layer over stolons and polyp tubes. This layer is often made of chitin, some groups produce a mineral skeleton. Free-swimming medusae cannot use rigid protection, but do defend themselves with stinging cells. There is evidence that some also contain toxic compounds that discourage predators from eating them. Most hydrozoan medusae also follow the diel migration pattern common to many planktonic organisms -- sinking below the limit of light penetration to avoid visual predators during the day, and then rising towards the surface at night in pursuit of prey. (Bouillon, et al., 2006; Brusca and Brusca, 2003; Dunn, 2009; Mills, 2009; Purcell, 1997)

Ecosystem Roles

Hydrozoans are both predators and prey for many marine organisms, and large seasonal blooms of medusae may strongly affect local fish and zooplankton populations. Some species of polyps are hosts for symbiotic algae, and many large pelagic forms have symbiotic hyperiid amphipods living on or in them. There is even a small species of fish, Nomeus gronovii, that lives in association with Portuguese man-o-wars. Some polyp colonies grow on the shells of hermit crabs, providing them protection. (Brusca and Brusca, 2003; Dunn, 2009; Mills, 2009; Purcell, 1997)

Economic Importance for Humans: Positive

Hydrozoans are important parts of many marine food chains, and so directly or indirectly support desirable food sources. Species in two families that have colonies with calcareous exoskeletons (Milleporidae, Stylasteridae) have been harvested commercially, and probably still are. There are now limits on their trade and some are considered endangered (see Conservation Status). (Brusca and Brusca, 2003; Mills, 2009; Purcell, 1997; Schuchert, 2009)

Economic Importance for Humans: Negative

The stings of some hydrozoans (most famously the Portuguese man-o-war, Physalis physalis), are dangerous to humans. Also, the fire corals (Milleporidae) get their name from the painful sensation divers get if they touch them.

Hydrozoans are common members of "fouling communities" -- the benthic organisms that naturally attach to hard substrates, and so grow on the hulls of ships and on submerged water pipes, interfering with their function. (Brusca and Brusca, 2003; Dunn, 2009; Mills, 2009)

  • Negative Impacts
  • injures humans

Conservation Status

The conservation status of the vast majority of hydrozoan species is unknown. Species in two families, the fire corals (Milleporidae) and the lace corals (Stylasteridae), have been commercially harvested, and in some places over-exploited. They are now listed in Appendix I of CITES, the international treaty limiting trade in wildlife. Also the IUCN has evaluated many species of Millepora and rated several of them Endangered. (Brusca and Brusca, 2003; Mills, 2009)

  • IUCN Red List [Link]
    Not Evaluated


George Hammond (author), Animal Diversity Web.


Arctic Ocean

the body of water between Europe, Asia, and North America which occurs mostly north of the Arctic circle.

Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

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Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

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living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.

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living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

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living in the southern part of the New World. In other words, Central and South America.

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Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.


living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

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on or near the ocean floor in the deep ocean. Abyssal regions are characterized by complete lack of light, extremely high water pressure, low nutrient availability, and continuous cold (3 degrees C).


reproduction that is not sexual; that is, reproduction that does not include recombining the genotypes of two parents


Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.

brackish water

areas with salty water, usually in coastal marshes and estuaries.


an animal that mainly eats meat


uses smells or other chemicals to communicate


the nearshore aquatic habitats near a coast, or shoreline.


used loosely to describe any group of organisms living together or in close proximity to each other - for example nesting shorebirds that live in large colonies. More specifically refers to a group of organisms in which members act as specialized subunits (a continuous, modular society) - as in clonal organisms.

colonial growth

animals that grow in groups of the same species, often refers to animals which are not mobile, such as corals.


having a worldwide distribution. Found on all continents (except maybe Antarctica) and in all biogeographic provinces; or in all the major oceans (Atlantic, Indian, and Pacific.


active at dawn and dusk


an animal that mainly eats decomposed plants and/or animals

  1. active during the day, 2. lasting for one day.

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature


an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.

external fertilization

fertilization takes place outside the female's body

female parental care

parental care is carried out by females


union of egg and spermatozoan


a method of feeding where small food particles are filtered from the surrounding water by various mechanisms. Used mainly by aquatic invertebrates, especially plankton, but also by baleen whales.


mainly lives in water that is not salty.


having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

internal fertilization

fertilization takes place within the female's body

intertidal or littoral

the area of shoreline influenced mainly by the tides, between the highest and lowest reaches of the tide. An aquatic habitat.


offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).


marshes are wetland areas often dominated by grasses and reeds.


A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.


having the capacity to move from one place to another.


specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.


active during the night

oceanic islands

islands that are not part of continental shelf areas, they are not, and have never been, connected to a continental land mass, most typically these are volcanic islands.

oceanic vent

Areas of the deep sea floor where continental plates are being pushed apart. Oceanic vents are places where hot sulfur-rich water is released from the ocean floor. An aquatic biome.


found in the oriental region of the world. In other words, India and southeast Asia.

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reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.


reproduction in which eggs develop within the maternal body without additional nourishment from the parent and hatch within the parent or immediately after laying.


An aquatic biome consisting of the open ocean, far from land, does not include sea bottom (benthic zone).


chemicals released into air or water that are detected by and responded to by other animals of the same species


generates and uses light to communicate


an animal that mainly eats plankton


the regions of the earth that surround the north and south poles, from the north pole to 60 degrees north and from the south pole to 60 degrees south.


the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.


"many forms." A species is polymorphic if its individuals can be divided into two or more easily recognized groups, based on structure, color, or other similar characteristics. The term only applies when the distinct groups can be found in the same area; graded or clinal variation throughout the range of a species (e.g. a north-to-south decrease in size) is not polymorphism. Polymorphic characteristics may be inherited because the differences have a genetic basis, or they may be the result of environmental influences. We do not consider sexual differences (i.e. sexual dimorphism), seasonal changes (e.g. change in fur color), or age-related changes to be polymorphic. Polymorphism in a local population can be an adaptation to prevent density-dependent predation, where predators preferentially prey on the most common morph.

radial symmetry

a form of body symmetry in which the parts of an animal are arranged concentrically around a central oral/aboral axis and more than one imaginary plane through this axis results in halves that are mirror-images of each other. Examples are cnidarians (Phylum Cnidaria, jellyfish, anemones, and corals).


structure produced by the calcium carbonate skeletons of coral polyps (Class Anthozoa). Coral reefs are found in warm, shallow oceans with low nutrient availability. They form the basis for rich communities of other invertebrates, plants, fish, and protists. The polyps live only on the reef surface. Because they depend on symbiotic photosynthetic algae, zooxanthellae, they cannot live where light does not penetrate.

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

seasonal breeding

breeding is confined to a particular season


remains in the same area


non-motile; permanently attached at the base.

Attached to substratum and moving little or not at all. Synapomorphy of the Anthozoa


reproduction that includes combining the genetic contribution of two individuals, a male and a female


lives alone


a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.


uses touch to communicate


that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).


the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.


an animal which has an organ capable of injecting a poisonous substance into a wound (for example, scorpions, jellyfish, and rattlesnakes).


uses sight to communicate

year-round breeding

breeding takes place throughout the year


Bouillon, J., C. Gravili, F. Pagès, J. Gili, F. Boero. 2006. An Introduction to Hydrozoa. Paris, France: Publications Scientifiques du Muséum.

Brusca, R., G. Brusca. 2003. Invertebrates. Sunderland, Massachusetts, USA: Sinauer Associates, Inc..

Dunn, C. 2009. "Siphonophores" (On-line). Accessed July 09, 2009 at http://www.siphonophores.org/index.php.

Jankowski, T., A. Collins, R. Campbell. 2008. Global diversity of inland water cnidarians. Pp. 35-40 in E Balian, C Lévêque, H Segers, K Martens, eds. Freshwater Animal Diversity Assessment. Dordrecht, The Netherlands: Springer Netherlands.

Mills, C. 2009. "Bioluminescence and other factoids about Aequorea, a hydromedusa" (On-line). Accessed July 10, 2009 at http://faculty.washington.edu/cemills/Aequorea.html.

Mills, C. 2008. "Hydromedusae" (On-line). Accessed July 10, 2009 at http://faculty.washington.edu/cemills/Hydromedusae.html.

Purcell, J. 1997. Pelagic cnidarians and ctenophores as predators: selective predation, feeding rates, and effects on prey populations. Annales de l'Institute Oceanographique, 72/2: 125-137.

Schuchert, P. 2009. "The Hydrozoa Directory" (On-line). Accessed July 09, 2009 at http://www.ville-ge.ch/mhng/hydrozoa/hydrozoa-directory.htm.

Wrobel, D. 2009. "The Jellies Zone - Jellyfish And Other Gelatinous Zooplankton" (On-line). Accessed July 10, 2009 at http://jellieszone.com/.