Hydrozoa Sea firs (hydroids), hydras and siphonophores

Sea firs (hydroids), hydras and siphonophores (Class Hydrozoa)

The class Hydrozoa comprises over 2,700 species and is subdivided into seven orders: Athecata, Thecata, Limnomedusae, Trachymedusae, Narco medusae, Siphonophora and Hydrocorallinae. Both polyp and medusa are generally present, but the latter may be reduced and then does not always leave the parent colony. Many members of the class live between tidemarks or in shallow coastal waters, while others are deep-water forms rarely seen.

Most members are small, both polyps and medusae measuring only a few millimetres. The class con tains both colonial and solitary forms, is marine with the exception of the few species of freshwater hydras and jellyfishes, and shows the most extreme polymorphism.

Many are able to lay down a skeleton, either a chitinous secretion (termed the ‘perisarc’) of the ectodermal cells round the stolons and polyps of the colony, or a calcareous material as in the hydrocorallines.

The hydrozoan polyp is the simplest in the phylum; the two body layers enclose a coelenteron which lacks septa, and the nervous system is a nerve-net whose properties are, as yet, scarcely investigated.

The medusa generally has four radial canals, a variable number of solid or hollow tentacles, as well as sense-organs and light-sensitive ocelli which appear as red, brown or black spots at the bases of the tentacles.

In addition a velum is generally present; this is a circular shelf which projects inwards from the bell margin. According to the order to which they belong, some medusae are referred to as anthomedusae, others as lepto medusac. Anthomedusae (order Athecata) are thimble-shaped and bear gonads on the manubrium while the leptomedusae (order Thecata) are saucer shaped and the gonads develop on the radial canals.

In addition, leptomedusae possess statocysts, special cells at the base of the velum and considered to be organs of equilibrium. Each contains a cal careous concretion or statolith which, pulled in various directions by gravity according to the tilt of the medusa, touches against sensory ‘hairs’ and so indicates orientation with respect to gravity.

In the medusa there is evidence of two distinct nerve-nets. One, situated at the bell margin and overlying the ring of muscle fibres, is a through conducting system of bipolar nerve cells which causes the contraction of the whole bell. It probably evolved with the use of the bell as a swimming organ. The second nerve-net consists of multipolar cells spread over the exumbrella and subumbrella and is responsible for local contractions of the bell.

Order Athecata

Most of the hydrozoans belong to this order or to the Thecata. The Athecata include the sea firs or hydroids of the intertidal zones as well as the fresh water hydras and By-the-wind-sailor (Velella). Generally there are both polyp and medusa forms, although the medusa is often reduced and is not always free-living. In colonial forms the chitinous perisare surrounds the stolons and continues to just below the head of the polyp. In solitary forms the perisare is often lacking.

Medusae begin their growth as gonophores pro duced as asexual buds. The gonophores are oval, stalked bodies lacking a perisarc and are produced from various parts of the polyp colony according to the species. In only about a third of the Athecata do the gonophores produce free-living antho medusae. In the remaining two-thirds the gono phores remain attached and the eggs are fertilised in situ. In these instances the planula often develops into an actinula before its release into the sea.

Classification in Hydrozoa

Classification into families is usually based on such details as the type of tentacles borne on the feeding polyp.

Tubularia (family Tubulariidae) is a colonial hydroid with a set of long, thin tentacles round the mouth and another set at the base of the polyp. Between the two sets are gonophores resembling bunches of grapes. There are no free-living medusae; the eggs are fertilised in situ and develop into actinulae.

Each polyp, often rose-pink in colour, rises from a long stem originating in a mass of rooting stolons. Species of Tubularia are common in the British Isles and America, especially in strong currents, and are found at low tide levels on rocks and seaweeds like the kelps or oarweeds, Laminaria.

Other genera are often found between tide marks. Clava (family Clavidae) forms pinkish colonies on seaweeds, particularly the wracks. Coryne (family Corynidae) has rose-coloured polyps bearing knobbed tentacles and is found on larger seaweeds or rocks. Gonophores produced on the polyp head in Coryne release anthomedusae with four long tentacles on the bell margin, four radial canals and a long manubrium.

Hydractinia (family Bougainvilliidae)

Hydractinia (family Bougainvilliidae) forms a thick mat of stolons on shells occupied by hermit crabs. It is polymorphic, the main forms being feeding polyps, protective spines, reproductive polyps and defensive polyps with tentacles only.

Freshwater Hydras

The freshwater hydras (family Hydridae) are probably the most familiar of the hydrozoans, numerous species occurring in ponds, lakes and streams throughout the world. The best-known species in Britain and the United States are the Green Hydra (Chlorohydra viridissima) and the Brown Hydra (Pelmatohydra oligactis). Hydras are atypical of the phylum in that they inhabit fresh water, and must be capable of controlling the amount of water entering the body due to differences in the ionic concentration of cells and freshwater.

As a phylum cnidarians seem unable to make this adjustment and are confined to the sea. Hydras are also atypical hydrozoans in that they lack any evidence of a medusa stage; in existing as solitary individuals rather than colonies; in some species being hermaphrodite and in all lacking a perisarc

The polyp is elongate with a mouth surrounded by five to six tentacles, a columnar region varying from a few millimetres in some species to fifteen mm. in others, and a basal disc. There is no medusoid phase in the life-history.

Instead, eggs produced as bulges on the column are fertilised in situ, usually in the autumn, and the developing embryo secretes a chitinous material on its surface. The resistant embryo then drops off the parent and lies dormant for a period, before hatching in the spring as a small hydra. This phase in the life-history is thought to be a secondary adaptation to life in freshwater ponds liable to dry up. When ample food is available hydras reproduce asexually by budding. A small bulge appears on the column and develops tentacles at the end furthest from the parent. The continuous coelenterons of parent and bud become separated by constriction at the base of the young polyp, which then drops off.

Hydras, like other members of the Athecata, are able to regenerate lost parts. The colours of the hydras are given by the presence of symbiotic algae in the endodermal cells.

Genera of the order Athecata

Two unusual genera of the order Athecata are

  1. By-the-wind-sailor (Velella)
  2. Porpita.

They resemble an enlarged, inverted and solitary version of a polyp such as Tubularia, floating on the surface. They have a round or oval float, which in Velella has an extension or sail, and under this is a central mouth surrounded by gonophores and with tentacles round the margin. The float contains air chambers and it is claimed that air is pumped through, constituting a respir atory system.

The small medusae released by Velella are known as Chrysomitra, since they were originally thought to be a separate genus. They have no mouth and die after releasing gametes. Porpita has a yellow float and Velella a purple sail. Both are widely distributed. Velella is often washed.

By-the-wind Sailor (Velella) is a pelagic animal inhabiting warmer seas, but shoals are often swept northwards in the Atlantic to become stranded on British and American Atlantic shores.

A colony of the Oaten-pipes Hydroid (Tubularia larynx) showing the two sets of tentacles separated by bunches of gonophores. (Life size).

Pink colonies of Clava squamata on the knotted wrack (Ascophyllum nodosum). There is a marked similarity of form between the individual polyps seen here and the solitary Pelmatohydra oligactis below.

The Brown Hydra (Pelmatohydra oligactis) is a solitary freshwater polyp which can reproduce asexually by budding. Two buds, at different stages in development, can be seen.

Sertularia pumila (Dyamena pumila). Each hydroid lives in a protective case and is joined to the other members of the colony by an extension of the polyp body.

Thecate hydroids (Order Thecata)

This order comprises the sea firs in which the chitinous perisare surrounds the stolons and forms a protective cup, the theca, round the head of the polyp. Medusae are produced asexually by budding from a modified feeding polyp, the blastostyle, around which the perisarc forms a protective case or gonotheca. Blastostyles may be produced from the stolons or polyp stems. The free-swimming medusae of the order, when released, are termed lepto medusae.

Only a fifth of the thecate hydroids pro duce these. In the rest the medusae may become very much modified and are difficult to identify. Thecates are widely distributed geographically. They and hydroids in general flourish especially in temperate and cold waters. Thecates are found on algae and stones (many prefer red algae) or between tidemarks, while others prefer deep water.

The order includes Obelia (family Campanu lariidae), common on oarweeds, with polyps in open goblet-shaped thecae and releasing free medusae; Clytia (family Campanulariidae), delicate a branching colony on red weeds, releasing free medusae; Lafoea (family Lafoeidae) with colonies formed of parallel stolons giving mechanical sup port and polyps growing at right angles to the stolons in elongated thecae without lids. Sertularia (family Sertulariidae) is common on wracks, with polyps growing in pairs opposite each other in thecae with lids.

Known commercially as ‘white weed’, many species of Sertularia growing offshore are dried, stained and sold for decorative purposes.

Order Limnomedusae

This order comprises a collection of saucer-shaped medusae with four to six radial canals and numerous tentacles in sets round the bell margin. In Olindias, found in shallow tropical waters, some of the tentacles bear suckers, enabling it to walk on seaweeds. Gonionemus also has suckers and is recorded from the British Isles and America. An uncommon freshwater species, Craspedacusta sowerbii, is a large hydrozoan medusa up to twenty mm. in diameter.

It was first found in London in 1880 in a pond containing an Amazonian water lily brought from Brazil and has since been found in certain ponds and lakes in America. It has a minute polyp, called Microhydra before its relation to the medusa was discovered, which buds off small medusae.

Order Trachymedusae

In this small group of medusae the mouth is borne at the end of a long pseudomanubrium which is an extension of the subumbrella surface. There are four to six radial canals and few tentacles. There is no polyp phase, the planula developing into an actinula, which then expands radially to become a medusa. Some authorities consider this type of life history to be ancestral. Examples of the order which live in warmer waters are Geryonia and Liriope.

Order Narcomedusae

This is another small group of medusae which have no polyp phase. They have broad flat bells and lack a manubrium. The mouth opens directly into the stomach region. Tentacles around the bell margin

actually arise above the margin and ectodermal extensions (peronia) of the tentacles run along the outer surface of the bell to indentations in its margin, giving the bell a scalloped edge. In Coina the planula becomes parasitic, attaching itself to the trachymedusa Geryonia and developing as an elongated stolon which then buds off medusae.

Siphonophores (Order Siphonophora) Siphonophores form swimming or floating colonies composed of both polypoid and medusoid forms. They display the highest degree of polymorphism found in the cnidarians. They are found in all seas, but prefer warmer waters. Relatively small and transparent, they are often unnoticed.

Two examples are Halistemma and Agalma. Forms derived from polyps include feeding polyps with at single long tentacle bearing nematocysts, palpons or ‘feeling’ polyps which have a tentacle but no mouth, and gonozooids which bear gonophores. Medusoid forms include swimming bells, floats and bracts, in addition to gonophores.

The float is usually the only part of the colony visible above water-level and often contains a gland which secretes gas into the float, enabling the colony to rise and fall beneath the waves. Below the float are numerous swimming bells, medusae which are unable to feed or reproduce and act only as swim ming organelles. And below them are clusters of feeding and reproductive individuals on a long central axis.

Some colonies have no float and the summit of the colony consists of swimming bells. Muggiara, a genus with a single bell, often occurs off the south west coasts of the British Isles.

The Portuguese Man o’ War (Physalia physalis) is a familiar siphonophore. Its long axis is shortened so that the feeding and reproductive forms lie under neath a large, oval contractile float. The float is generally blue in colour and may be very large, from ten to thirty cm. long, with the tentacles of the feeding polyps extending for many yards. The nematocysts of this species contain powerful toxins and their sting is dangerous to man.

A small fish Nomeus, however, habitually swims about among the tentacles without coming to harm and never lives independently. Normally an inhabitant of the open Atlantic, Physalia may appear around the south and west coasts of the British Isles after prolonged southwesterly winds and along the Atlantic coast of the United States.

Order Hydrocorallinae

This order consists of a few genera which lay down a calcareous skeleton. There are two suborders. The Milleporina consists of one genus, Millepara,

found in tropical shallow seas, where it often forms

extensive reefs or contributes to true coral reefs. It forms upright, white or whitish-yellow leaf-like or branching growths.

The surface of the calcareous skeleton is pitted with a ring of small pores surround ing each of the larger pores. When alive, the feeding polyp occupies the centre pore and is connected by stolons to other feeding polyps and to defensive dactylozooids which occupy the smaller pores. The dactylozooids are feeding polyps reduced to ten tacled forms with powerful nematocysts which produce a burning sensation on contact with the skin. The small reduced medusae of this suborder live only for a few hours and shed their gametes.

The Stylasterina resemble the Milleporina, but the distribution of feeding and defensive polyps is different and there are no free medusae. They form upright, branching, calcareous growths, often pink or purple, and inhabit tropical and subtropical seas.


Gonionemus murbachii is a medusa that can walk on seaweed by using the small suckers borne on the numerous tentacles round the bell margin.

The Portuguese Man o’ War (Physalia physalis) is an inhabitant of the open seas. It possesses a large oval float from which hang long, trailing tentacles that bear powerful nematocysts.

Leave a Comment