Obelia & Metagenesis


Obelia is a marine colonial trimorphic hydrozoan having polyp, medusa and blastostyle stages in its life history. It is abundant in both Atlantic and Pacific coastal waters.

The small branched hydroid colony is found attached to rocks, stones, shells of animals or on large seaweeds and appears like a delicate whitish or light brownish fur-like growth.

The sexual form is a bell-like medusa which possesses gonads and disperses sperms and ova in water.


Obelia colony consists of a horizontally branching root-like stolon or hydrorhiza from which emerge many vertical branching stems called hydrocaulus. Branches emerging from the stem terminate either in nutritive zooids, called the polyps or hydranths or in reproductive zooids called blastostyles or gonangia, which produce medusa. Thus, Obelia colony is trimorphic, exhibiting two types of zooids attached to the sedentary colony and a free swimming zooid or medusa.

The colony is made of an inner, tubular and living tissue, the coenosarc, which encloses a canal, called the gastrovascular cavity, whose walls consist of two layers, an outer epidermis and an inner gasrodermis and a gelatinous mesogloea in between. Coenosarc is surrounded externally by a yellowish or brownish, non-living chitinous layer, called perisarc that protects the colony and serves as an exoskeleton. Annuli on perisarc permit limited swaying movements and bending of the zooids.

The body wall

The body wall of the colony is composed of two layers, the outer epidermis and inner gastrodermis and a thin, transparent, non-cellular gelatinous layer called mesogloea sandwiched in between. Epidermis carries stinging cells or nematocysts, which are especially abundant on tentacles forming batteries. A nerve-net composed of branched nerve cells is present on each side of mesogloea. Gastrodermis consists of large nutritive-muscle cells and narrower gland cells that secrete digestive enzymes and help in digestion, which is both extracellular and intracellular.

There are three types of zooids in Obelia colony as follows:

Polyp or Hydranth

It is the nutritive zooid of the colony and is also called gastrozooid or trophozooid. Its distal end is produced into a conical elevation called the manubrium or hypostome, in the middle of which is located the mouth. From the base of hypostome arises a circle of up to 30 filiform tentacles containing nematocysts. Perisarc around the hydranth dilates to form a loose cup-like, transparent protective sheath, the hydrotheca into which the polyp withdraws when in danger. 

Blastostyle or gonangium

When the colony has reached full development, it produces special club-shaped bodies called blastostyles or blastozooids or gonozooids. The perisarc, covering the blastostyle, forms a loose, transparent, vase-like capsule called the gonotheca. The blastostyle, by lateral asexual budding, produces sexual zooids called medusae or gonophores. Fully formed medusae detach from blastostyle and escape through an aperture called the gonopore. Gonotheca, together with blastostyle and medusae is referred to as gonangium.

Medusa or Nectophore

Large number of medusae is produced by budding on blastostyle. They detach and escape from gonangium through its opening called gonopore and freely swim and feed in water.

A fully grown Obelia medusa is a radially symmetrical tiny umbrella, measuring 1 or 2 mm in diameter. The subumbrellar surface has four radial canals and a circular canal on the margin. A mature medusa bears four gonads, one in the middle of each radial canal.

From the centre of the sub-umbrellar surface projects down a hollow process, the manubrium that bears a four-sided mouth in the centre leading into a gastrovascular cavity. Circular edge of umbrella is produced inwards into a fold called velum, which does not carry any canal in it. Margin of the umbrella bears numerous short tentacles with nematocysts.

Nervous system in medusa is a diffused network of neurons.

Medusa floats in water and is drifted by water currents with manubrium hanging downward and tentacles swaying freely. It also swims actively by muscular contractions and velum. Medusa is carnivorous and feeds on planktons. Medusa has eight marginal statocysts, situated at the bases of tentacles on the subumbrellar surface for providing a sense of equilibrium to the animal. 

Medusae are reproductive zooids or gonozooids possessing four gonads. They are dioecious as testes and ovaries are derived from different individuals. Each medusa bears only four gonads situated on the subumbrellar surface, over each radial canal. Although medusa bears gonads and sex cells, it does not produce them. Spermatogenesis and oogenesis takes place inside the blastostyle and the resulting sperms and ova migrate and fill the gonads of the medusa. So the function of medusa is to disperse them in water.


Life History of Obelia includes both asexual and sexual generations that alternate with each other to complete the life cycle. The sedentary hydroid colony alternates with the pelagic medusa phase. Hydroid colony reproduces by asexual budding to produce hydranths and blastostyles. Medusae are also produced by budding from blastostyle but they are free swimming sexual forms that carry ova and sperms in four gonads and disperse them in water far and wide. Such alternation of generation in which the asexual polypoid generation appears to alternate regularly with the sexual medusoid generation but both forms are diploid, is called Metagenesis. In true alternation ofgeneration one of the stages in life cycle is haploid, whereas in metagenesis both stages are diploid.

Medusae are the sexual zooids or gonozooids possessing gonads. They are dioecious, i.e. testes and ovaries are borne by separate individuals. Each medusa bears four gonads on the subumbrellar surface, one in the middle of each radial canal. Each gonad is an ovoid, knob-like body having an outer epidermis and an inner lining of gastrodermis. Gametes are housed between the two layers. Outer wall of mature gonads ruptures to release the gametes in water.

Fertilization occurs in sea water where the gametes are set free. Parent medusae die soon after liberating the gametes.

Fertilized egg or zygote undergoes holoblastic cleavage, resulting in a solid morula, which then transforms into a hollow ball of cells called blastula. Blastula is made of blastomeres and encloses a cavity called blastocoel.

Gastrulation takes place by delamination of the endodermal cells from one end of blastula, eventually filling the blastocoel completely. Such solid gastrula is called stereogastrula in which the outer layer of cells is the ectoderm and the inner mass of cells filling the blastocoel is endoderm.

Gastrula elongates and forms an elongated free swimming ciliated planula larva that swims about in search of a suitable substratum for fixing to form a sedentary colony. Soon the solid endoderm splits and develops a cavity, the enteron. Now the fixed planula becomes a truly two-layered larva with an outer ciliated ectoderm and an inner endoderm. The larva now closely resembles a simple polyp and is called hydrula. By budding, hydrula gradually grows into a branching Obelia colony.