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Hydroids by Ronald L. Shimek October 1997 Aquarium.Net

This article by Ron Shimek is on hydroids, Aquarium Net has numerous articles written by the leading authors for the advanced aquarist


by Ronald L. Shimek

Animals in the PHYLUM CNIDARIA are some of the most commonly kept marine aquarium animals (Delbeek and Sprung, 1994). There are four classes, or major subdivisions, of this phylum and one of them, the Anthozoa, is commonly represented in reef aquaria with corals, soft corals, and sea anemones. Most cnidarians have two discretely different body forms, a polyp that lives on the

These are hydrocorals in the Order Stylasterina, which may look like fire corals, but don't have the nematocysts characteristic of them. These were photographed in Palau, and are about 4 inches (10 cm) high.

substrate and a swimming stage called a "medusa" or jellyfish (Kozloff, 1990; Ruppert and Barnes, 1994).

Unlike the majority of cnidarians, however, corals, sea anemones and their kin never are found as medusae. Two of the other three classes, the Cubozoa and the Scyphozoa, are characterized by having the medusa as the dominant stage and are not common in aquaria. Scyphozoans are occasionally found in aquarist tanks, either as polyp forms on live rock or as small medusa. Cubozoans, or sea wasps, can be exceptionally dangerous and as far as I know, no hobbyist has been dumb enough to try to keep these. The remaining class, the Hydrozoa contains most of the species in the phylum and is the subject of this article.

For a some information about Cnidarians and numerous links to other aspects of Cnidarian biology

The Class HYDROZOA contains roughly 10,000 species, accounting for about two thirds of the total number of species in the phylum. The common names for hydrozoans include hydroids, fire corals, and hydrocorals, but siphonophores, such as the Portuguese Man o' War are also members of this group. Hydrozoans are generally radially symmetrical with a basically cylindrical body having a ring of tentacles around the upper end.

Millepora dichotoma Colonies of fire coral, a Millepora species, are often from rapidly growing, early successional species and may form dense thickets where there has been significant reef damage. It is outcompeted by stony corals and replaced over a period of years. This area in Ponape had been badly damaged by a hurricane the previous year.

A few of the colonial and planktonic members of the group are bilaterally symmetrical, and these colonies have defined front and back ends. All hydroids are found in water, most are marine, but a few species are common in fresh water, and are the only fresh-water cnidarians (Pennak, 1992).

These are simple animals. They are fundamentally a sac consisting of an outside layer and inside layer with a secreted, "non-living" layer in between. They only have one opening into digestive region - a "mouth" - usually surrounded by tentacles. Consequently, undigested food has to be expelled out the mouth. Most of them are predatory. The outer surface on the side opposite the mouth opening is either specialized to attach to a substrate (in polyps) or to swim (in medusae).

The two layers that comprise the body are covering tissues called epithelia, an outer epidermis and an inner gastrodermis. In between these layers is a thin, but tough, lamellar layer without cells, known as the "mesoglea" (this name means "middle jelly" and refers to its position in the sandwich). The thin lamellar mesoglea helps differentiate the hydrozoans from the other classes which have thicker and more fibrous mesoglea. Hydroid mesoglea, in the polyps, is basically the glue holding the internal and external tissues together.

Figure 1 The basic morphology of a hydrozoan shown by diagrams showing slices through the animals. In the diagram of the colony, polyp and medusa, the plane of the slice is through the center of the animal. In the section showing the tissues, mesoglea, and skeleton, the plane is across the stalk. The epidermis (outside covering tissue) is shown in red, the gastrodermis (digestive tissue) is shown in blue. The mesoglea is represented by the white line between them. The exoskeleton is shown ending below the polyp body, but in some species it may enclose the polyp.

Although hydrozoans have good tissues, organs in the sense of structures composed of two or more tissues are rare. And they do not have "systems" in the sense of a digestive system or nervous system. They have a digestive region or gut (also sometimes called a "gastrovascular cavity" or "coelenteron") but lack digestive organs. Although they have nerves and sensory cells, these are arranged in a diffuse arrangement called a nerve net. There are no large evident nerves and there are few clusters of cells or ganglia. A brain is totally lacking, and there is no defined associative region of the nervous system. Unlike most "higher" animals, nervous impulses can run both ways in each nerve cell, and the rate of nervous conduction is slow. Nervous responses and reflexes are relatively few and are generally simple. They do have behaviors, but these also tend to be rather simple. The only complex behavior patterns are found in some of the pelagic colonies (Mackie, 1978; Thomas and Edwards, 1991).

A major and unifying characteristic of the Cnidaria is the presence of nematocysts. In fact, the name Cnidaria is derived from the Greek " knide ," meaning "nettle," which refers to the stinging properties of the nematocysts. Often called "stinging cells," nematocysts are NOT cells at all, but are proteinaceous capsules secreted by specialized epithelial cells called cnidocytes. The capsule has a hyper-coiled internal thread.

This Caribbean species, possibly in the genus Agalophenia , had potent nematocysts. The sting was very noticeable through a pair of cotton work gloves. The brown central skeletal stalk of scleroprotein is clearly visible.

These capsules contain a high internal osmotic pressure, roughly equivalent to a pressure of about 2000 psi (140.6 kg/sq. cm). Nematocysts appear to discharge due to osmotic changes and the discharge is very rapid; the threads are blown out as the pressure is released (Thomas and Edwards, 1991).

Measured by ultra-high speed cinematography, the tip of the thread of some of the nematocysts in fresh-water Hydra have been shown to move at about 2 m/sec, which requires accelerating force of about 40,000g. Even with the small mass of the tip of the nematocyst thread, the tip of the thread with that acceleration can punch through and imbed themselves in just about any biological substance. The tip of the thread may be open or closed. If the extruded thread has an open end, the fluid contents of the capsule generally exit through that perforation. These contents may include adhesive or viscid substances or toxins. Nematocyst firing generally appears to require multiple stimuli; often both mechanical and chemical stimuli are necessary. Hydroids are good cnidarians and possess nematocysts and many have toxins that are exceptionally potent. Fire corals are hydroids, and the "bite" of their "sting" is legendary. Most nematocysts are used for prey capture. Some are probably specifically used for defense; however, there are no good data on this aspect (Thomas and Edwards, 1991; Gitter and Thurm, 1996)

For a discussion of basic hydrozoan biology plus a picture of fresh-water Hydra follow this link

After they are secreted, the nematocysts may be transferred to another cell, and moved throughout the animal. In hydrozoans the nematocyst-secreting cells are much more common in the external epidermis rather than in the internal gastrodermis, and may not actually be found the latter tissue. Nematocysts, however, are common in the gastrodermis, and if so, are transferred there from the epidermis (Weber, 1995).

For many illustrations of hydrozoans follow this link and note some of the specific files.

For example:

For illustrations of Nematocysts or cnidocytes:

Undischarged nematocyst:

Discharged nematocyst

For illustrations (drawings) of:

These colonies of a temperate species of Plumularia are about 3 inches (7.5 cm) in height. These animals were found in shallow water on the coast of British Columbia. The gonozooids are the yellowish structures near the center of the colony stalk. Gastrozooids are found lining the branches.

Generalized polyps

Generalized medusa 2

The gut or gastrovascular cavity in hydrozoan polyps is a simple sac without any internal septa, pouches, or folds. The gut in the medusa may be rather highly branched or very simple. After prey are killed by nematocysts and brought into the gut, the body stretches over the prey and the gastrodermal cells lining the gut secrete digestive enzymes directly into the prey. Digested food is simply absorbed into the adjacent cells, or moved in a slurry throughout the colony.

Hydrozoans have complex life cycles where the animal typically passes through two or more different stages. Polyps generally live on the bottom and are considered primitively to reproduce only asexually by budding off either new polyps or medusae. Medusae are primitively produced by buds off the polyp, and are pelagic, swimming with and feeding on the plankton. These swimmers may visualized as an upside-down polyp with an inflated end opposite the mouth. They have the appearance of an umbrella or dome with the mouth in the center pointing down with the tentacles typically as fringe along the edge. They move by swimming with pulsating beats which contract tissues around the circumference of the bell forcing water out the bottom. Medusa reproduce sexually by producing gametes from gonads on the undersurface of the bell. The gametes fuse to form a zygote (fertilized egg) which grows up to be a planula larva (as described below) which changes into a polyp which grows up and buds off medusae, which.... Recently, an interesting and possibly unique example where the medusa itself was able dedifferentiate and settle to grow up as a polyp has been documented (Piraino, et al., 1996).

Figure 2 A diagram of the basic life history found in hydrozoan animals. The medusa are sexually reproducing and have separate sexes, both male and female are shown. The colony buds off medusa, which is asexual reproduction. Only one colony is shown, but as all the medusae budded off one colony are the same gender, the colonies also must be genetically male and female. The zygote is the fertilized egg.

Hydrozoans have separate sexes; hermaphroditism, as found in some of the hermatypic corals, is rare. Typically sperm are shed into water by the male, while ova are retained in female. The basic developmental pattern leads to a typical larva of the cnidarians, called a planula. Planulae are basically small, mobile, flatworm-like blobs of cells covered in cilia or beating hairlike extensions of the cells. They are released from the mother and swim in the plankton. Eventually they settle to the bottom and metamorphose into a polyp.

Photograph of a hydroid planula

Hydrozoans asexually reproduce by budding, particularly in the colonial forms. In most of these, budded individuals remain connected together to form a colony. In such a colony there may be many different types of polyps, which are often called "zooids." Although there may be other kinds of zooids, the basic kinds are the "gastrozooids" used for feeding, the "gonozooids" which are used for reproduction, and which can produce medusa, and the food catching, and defensive "dactylozooids" (Kozloff, 1990; Ruppert and Barnes, 1994). In a number of species, there can be dispersal of asexually produced fragments or buds, as well. In these species, the polyp body is pinched off and floats away.

Garveia annulata , temperate hydroid from the N. E. Pacific. The feeding polyps are clearly visible at the tips of the branches and are about 1/8th inch (3 mm) across. The large spherical objects near the center of the colony are gonozooids. This species does not release free-swimming medusae.

It is regenerated on the parent colony, and in due course may land, fasten down and grow as a dispersed clone of the parent (Gravier-Bonnet, 1992).

The basic cnidarian is a simply constructed sac surrounding a simple undivided digestive cavity with a single opening serving as the mouth. This body plan works well as a small animal. It apparently doesn't work at all as a large animal and there are no large (bigger than about an inch (2-3 cm) in diameter) solitary hydroid polyps. It is well at this point to briefly discuss the problem that attaining large size presents. Apparently for organisms to get large, parts of them have to be functionally specialized. At least on earth, there are no large animals without some sort of specialized organs or structures.

Large and complicated hydrozoans have evolved. Unlike other animals, however, they have not solved the problem of functional specialization by developing specialized tissues or organs, but instead have become colonial organisms created by budding many connected specialized individuals off a common basal or origin point. These individuals do not separate from the rest of the colony, rather, they may have an altered morphology that allows them to specialize in function. All of them share a common gut, but only a few may feed. In essence, cnidarians get large and complex by using specialized budded, connected individuals to form what is called a "colony." The diversity of polyps or zooids is called polymorphism (poly = many; morph = shapes). All other animals get large and complex by developing specialized tissues and organs. Morphological complexity based on budded individuals also serves to truly separate the cnidarians from most other animals and it is best developed in the Class Hydrozoa. Colony polymorphism is almost absent in the Classes Cubozoa and Scyphozoa, and expressed in an intermediate manner in the Anthozoa.

Figure 3 The basic morphology of a hydrozoan colony showing the basic polyp types. Gastrozooids are the feeding individuals, gonozooids bud off jellyfish. The stolon grows over the substrate to send up separate stalks.

The Class Hydrozoa contains the most structurally diverse and complicated animals in the phylum, as well as those that are probably the most simple. Most hydroids have both polypoid and medusoid stages - but either stage can be reduced or lacking. Some contrasting extremes ares indicated below, but the references give many other examples.

This class is comprised of about 10 subdivisions referred to as orders; each of these orders differ from the other orders by having some prominent morphological or developmental differences. The species in the Order Hydroida are considered to the "archetypical" hydrozoans, and have the common name "hydroid." These species generally have a prominent polyp; most form benthic colonies of polyps in marine ecosystems.

This temperate species has individuals arising from a stolons that are on the substrate. It lacks gonozooids and medusae, and the bumps visible on the sides of some individuals are gonads.

Typically species in the Order Hydroida are considered to have both polyp and medusa forms. However, every conceivable condition between fully functional polyps and medusae and animals lacking the medusa stage altogether are found. Many species, such as those marine species in the genus Hydractinia , which often grow on hermit crab shells, and the common fresh-water genus, Hydra , lack medusa altogether, while others such as the common marine Obelia have fully functional medusae.

The classical "typical" hydrozoan illustrated in texts and seen in classrooms, and sometimes found in fresh-water aquaria, is Hydra . Most hydrozoans are marine, and are colonial. Hydra is not typical in that it is found in fresh water, has no medusa, and is solitary rather than colonial. So much for "typical" representatives...

There are species in the Order Hydroida that have

Polyps which bud off normal feeding medusae.

Polyps which bud off medusa with complete gonads, but an incomplete gut. These medusae live for a short period and exist only to liberate gametes into the water column.

Polyps which create medusa buds with functional guts, tentacles, and gonads, but which never release the medusa. Essentially, these medusa are gamete factories attached to the polyp colony.

Polyps which create medusa buds with non-functional guts, no tentacles, and but gonads. These are not released. These structures are effectively gonads on medusoid-like bumps.

Polyps which have no recognizable medusa or medusoid buds, but which produce gonads which liberate gametes into the water.

On the other hand, species in the Order Trachylina, such as the individuals of Gonionemus often examined in biology classes as hydrozoan medusa, typically have prominent medusa and some lack polyps altogether. Their life cycle is from swimming medusa to swimming planulae and then the planula metamorphoses

This is a close up of a temperate hydrocoral, Allopora californica . The irregular holes in the pink surface of the coral are where the zooids emerge. There is a central gastrozooid found in each hole and very fine, hair-like dactylozooids found around the edges. Dactylozooids are visible around some of the holes in the upper left.

into a swimming medusa. There are only a few fresh-water medusa and these species are all in the Order Trachylina; they do have polyps but they are small and reduced in complexity. The most common fresh-water jellyfish in the Northern Hemisphere is Craspedacusta (Pennack, 1992). Although widespread, fresh-water jellyfish are relatively rare in space and time; the only place where I know that they can be found regularly is in the Willamette River in Portland, Oregon, where they can be regularly found in late spring and early summer.

Polyp Structure

Hydrozoan polyps are typically small, only a few species get larger than about 1 cm across the tentacles and most are much smaller than that. The tentacles may be arranged in a one or two whorls around the circumference of the polyp, or scattered over the surface. The mouth typically is found on a projection called the hypostome in the center of the body or hydranth. The flesh of most species is colorless or tan, but strikingly-colored species also exist, and some species, particularly tropical marine forms, have zooxanthellae, zoochlorellae or other algal symbionts. Although some species, such as in the fresh-water genus, Hydra , are solitary and mobile, most are colonial and securely fastened to the substrate in one spot.

Photograph of Pennaria , a hydroid polyp with odd tentacles.

Each hydrozoan species which forms colonies typically has a determinate branching pattern for the position of the polyps. This results in colony morphology that is specific for given groups of species and this can be utilized as an aid in identification of the species. The gonozooids or specialized reproductive zooids on these colonies are often visible, particularly in the spring or other periods with predictable plankton, as the budded off medusa feed on the plankton.

This large temperate hydroid species, either in the genus Hybocodon or Tubularia , is about 2/5ths inch (1 cm) in diameter. There are two whorls of tentacles, one around the outside edge and one around the central mouth on the stalk. The pinkish structures are gonozooids, which in this case produce medusae.

The polyps or colonies often have an exoskeleton composed of colorless, yellow, tan or black material made of a scleroprotein. These colonies are often encrusting, but some may send up stalks or branches that may be as much as 6 to 12 inches (15 to 30 cm) off the substrate. Particularly in high current areas in nature, these branches may be oriented to provide maximum filtration opportunities for the polyps found on them (Svoboda, 1976; Gili, and Hughes, 1995).

Scleroproteins are structural proteins, and most animal groups have ones specific to the group. A typical human scleroprotein is the keratin which forms hair and fingernails. Hydrozoan exoskeletons are often said to contain an ammonianated polysaccharide called chitin, but few, if any, of these occurrences have been confirmed. Chitin is essentially a sugar polymer similar to cellulose or starch, with ammonia residues attached to each of the sugar units. Chitin is an amazing material! The old test for chitin was to boil the suspected material for 24 hours in concentrated sulfuric acid; after which any residue was boiled for 24 hours in concentrated lye, sodium hydroxide. Anything that was left after this treatment was chitin... Now, chemical stains specific for chitin and X-ray crystallography are used to confirm its presence. Chitin is transparent, colorless, and flexible; and until the early 1970's, it was customary (and exceedingly sloppy...) to assume the chemical composition of all structures that had those characteristics was chitin. Many have since been determined to be protein instead.

Hydrocorals are hydrozoans which secrete an internal skeleton of calcium carbonate. The body in these hydrozoans is constructed of a complicated mass of tubes containing both epidermis and gastrodermis; the calcium carbonate lies outside these tubes, but there is also a surface layer of epidermis covering it; so technically they have an endo- (or internal) skeleton. Essentially, the polyps are buried a mass of calcium carbonate and protrude only through small holes in the surface.

Fire coral, a Millepora species, showing the typical colony formation.

The two, rather closely related, groups of hydrocorals are the Orders Milleporina and Stylasterina. The former is found exclusively in the tropics, contains only a few species and all are called "fire coral" from the potent nematocysts found in the dactylozooids. The latter has both temperate and tropical forms and does not seem to have particularly virulent nematocysts (Lewis, 1992).

Hydrozoan polyps may be quite long-lived. No cnidarian polyp goes through an old age or senescence and conceptually may be regarded as immortal. Unless they get a disease, environmental conditions change, or they loose in an ecological struggle (competition or predation), they can live a very long time, indeed. However, as they are immobile flesh fastened to the ocean bottom, they are often the primary foods for many benthic carnivores, for example the grazing snails such as some top shells, and nudibranchs. Consequently, their realized life span is often very much shortened (Gili, and Hughes, 1995).

Medusa Structure

Hydrozoan medusa are typically small, only a few exceed 6 inches (15 cm) in diameter. Generally the hydrozoan medusa are ephemeral, living only a few months at the most. Their function is dispersal and sexual reproduction. Unlike the cnidarian jellyfish, hydrozoan jellies are often transparent and clear. Many are also brilliantly bioluminescent, glowing a bright blue-green when disturbed. Medusae from the other cnidarian groups, the Scyphozoa and Cubozoa are typically translucent or opaque. Hydrozoan medusae also have a velum or tissue shelf projecting inward around the inside edge of the bell.

This large hydrozoan medusa, Aequorea victoria , gets to be about 4 inches (10 cm) across. It is brilliantly bioluminescent, and the chemicals that cause that bioluminescence are harvested from it and used in cellular physiological research, particularly cancer research.

This also serves to differentiate them from the scyphozoans which lack a velum. A similar structure found in cubozoan medusae, is called a velarium in one of the many absurd nomenclatural distinctions found in invertebrate morphology. Apparently velums can only be found in hydrozoans... The velum (or the velarium in cubozoans) restricts the aperture to the medusa bell when it contracts forcing the water to exit through a smaller aperture. This causes an increase in water velocity, and increases locomotory efficiency in these medusa.

Medusa may have statocysts (balance organs) and some of them have ocelli or eyespots. The statocysts allow the animals to note changes of orientation. The ocelli (singular = an ocellus) are photoreceptors that detect changes in light intensity, but which are incapable of forming an image. In the case of the common Polyorchis found along the Pacific Coast of North America, both the statocysts and the eyespots appear to be fundamental in its normal behavior. During the day this medusa sits on the bottom in eel-grass beds or under benthic kelp. After night fall it starts to swim; but generally does not swim very far above the bottom.

This medusa, Polyorchis penicillatus , has large red ocelli or eyespots around the bottom rim of the bell between the bases of the tentacles. The animal is about 1 inch, (2.5 cm) across.

It then stops swimming and spreads its tentacles and drifts slowly down the sediment surface under it, where it captures small crustaceans and worms moving on the surface. It will continue to hunt all night, but when the day comes, it descends to the bottom coming to rest in a shaded area until the next evening. Presumably the ocelli tell it when it is night or day.

In contrast to the morphologically simple hydrozoans in the Order Hydroida mentioned above, the structurally most complicated cnidarians are in the Order Siphonophora. Siphonophores are typically large, highly polymorphic, colonial and planktonic; most individuals are developed from modified medusae.

The "Portuguese Man o' War," Physalia , is a siphonophore, but is simpler in structure than many. An individual is basically composed of four types of modified medusoids. During growth and development the planula changes into a medusoid which forms a tube called the primary pneumatogen (or gas float, which uses carbon monoxide gas for floatation). From this structure swimming bells, dactylozooids, and gonozoids are formed. In a mature colony, the dactylozooids (called tentacles) can reach 20 m.

Diagram of Portuguese Man o' War polyps

Photograph of a Portuguese Man o' War

Photo of a chondrophore, a different kind of pelagic floating colony similar to, but not related to the siphonophores

Other siphonophores, such as Muggiaea , use oil droplets for flotation and have complicated swimming bells. These animals are entirely pelagic, and swim beneath the surface. Little is know about their biology as they are amazingly difficult to collect. A mature colony in some of them can be over 30 m long, made of thousands of zooids is exceptionally fragile. Even collecting one intact is exceedingly difficult (Kozloff, 1990; Ruppert and Barnes, 1994).

Hydrozoans are generally considered to be primarily heterotrophic, obtaining their nutrition by preying on small zooplankton. Although a number of the species including fire corals ( Milleporina species), and fresh-water Hydra contain algal symbionts, these species are still considered to be primarily predatory. In natural situations, both the benthic and pelagic forms may be important predators on zooplankton (Svoboda, 1976; Lewis, 1992; Arai, et al., 1993; Gili and Hughes, 1995).

This Caribbean hydroid, possibly a Sertularella species, is about 3/4ths inch (2 cm) high. The gastrozooid polyps are found on the bumps on the branches.

References Cited:

Arai, M. N., G. A. McFarlane, M. W. Saunders and G. M. Mapstone. 1993. Spring abundance of medusae, ctenophores, and siphonophores off southwest Vancouver Island: Possible competition or predation on sablefish larvae. Canadian Technical Report of Fisheries and Aquatic Sciences. 1939,I-IV:I-IV,1-37.

Delbeek, J. C. and J. Sprung. 1994. The reef aquarium. Ricordea Publishing. Coconut Grove, FL. 544 pp.

Gravier-Bonnet, N. 1992. Cloning and dispersal by buoyant autotomised hydranths of a Thecate hydroid (Cnidaria:Hydrozoa). Scientia Marina. 56:229-236.

Gili, J.-P. and R. G. Hughes. 1995. The ecology of benthic hydroids. Oceanography and Marine Biology: an Annual Review. 33:351-426.

Gitter, A. H. and U. Thurm. 1996. Rapid exocytosis of stenotele nematocysts in Hydra vulgaris. Journal of Comparative Physiology a Sensory Neural and Behavioral Physiology. 178:117-124.


Lewis, J. B. 1992. Heterotrophy in corals: zooplankton predation by the hydrocoral Millepora complanata. Marine Ecology Progress Series. 90:251-256.

Mackie, G. O. 1978. Coordination in physonectid siphonophores. Marine Behavioral Physiology. 5:325-346.

Pennak, R. W. 1992. Freshwater invertebrates of the United States. 3rd edition. John Wiley and Sons. New York.

Piraino, S., F. Boermo, B. Aeschbach and V. Schmid. 1996. Reversing the life cycle: medusae transforming into polyps and cell transdifferentiation in Turritopsis nutricula (Cnidaria: Hydrozoa). Biological Bulletin. 190:302-312.


Svoboda, A. 1976. The orientation of Agalophenia fans to current in laboratory conditions (Hydrozoa, Coelenterata). In: G. O. Mackie. Ed. Coelenterate Ecology and Behavior. Plenum Press. New York. pp. 41-48.

Thomas, M. B. and N. C. Edwards. 1991. Cnidaria: Hydrozoa. In: Harrison, F. W. and J. A. Westfall. Eds. Placozoa, Porifera, Cnidaria, and Ctenophora Wiley-Liss. New York. pp. 91-183.

Weber, J. 1995. The development of cnidarian stinging cells: Maturation and migration of stenoteles of Hydra vulgaris . Roux's Archives of Developmental Biology. 205:171-181.

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Last modified 2006-11-20 03:20