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Obelia – Definition, Structure, Metagenesis, Reproduction

Sea fur is an alternative term for Obelia. Obelia has a delicate, semitransparent, pale to light-brown hydroid colony. It consists of stems with vertical branches called hydrocauli and root-like branches called hydrorhiza.

In order to complete its life cycle, Obelia’s asexual and sexual generations alternate. The life cycle of Obelia regularly alternates between hydroid and medusoid phases.

Medusae lay eggs that, when fertilised, evolve into hydroids. Through asexual reproduction, hydroids create medusae. This phenomenon was formerly known as “generational succession.”

Definition of Obelia

Obelia is a colonial, sessile marine organism that adheres to seaweeds, rocks, lumber piles, and mollusk shells in shallow water.

Obelia begins as little, immobile creatures with stalks and tentacles resembling sea anemones. These microscopic creatures are also known as hydroid polyps.

The common name for Obelia is marine fur. Moreover, this animal’s reproductive process contains two distinct phases. Moreover, it requires two generations to complete.

The common species of Obelia are: a) Obelia geniculata (Knotted thread hydroid), b) Obelia longissima (Sessile hydroid), c) Obelia dichotoma (Sea thread hydroid), d) Obelia bidentata (Double toothed hydroid)

Classification of Obelia

Obelia belongs to the family of creatures known as hydrozoa and encompasses various species. The obelia has the following biological classifications:


Habitat of Obelia

Obelia is a marine colonial form found clinging to seaweed, mollusk shells, rocks, and timber piles in shallow water up to 80 metres deep. The international spread of Obelia, which forms a white or light-brown plant-like fur in the sea, has earned it the common name sea-fur.

  • The sole exceptions to Obelia’s worldwide distribution are the high-arctic and Antarctic seas.
  • They develop in shallow water, in intertidal rock pools, and are often found between 80 and 100 metres below the surface.
  • The medusa stage of Obelia species is often observed in coastal and offshore plankton worldwide. 
  • On rocks, stones, mollusk shells, seaweed, wooden pilings, and wharves, colonies of Obelia are frequently observed as a fragile, fur-like growth.
  • Typically, Obelia geniculata grows on kelp fronds.

Morphology of Obelia

Obelia is a tiny aquatic hydroid. It resembles a little, branching tree that is white or brown in colour. The height of Obelia varies by at least 2 centimetres. The body of Obelia is composed of horizontal hydrorhiza and vertical hydrocaulus threads. Fig. : An Obelia geniculata colony

Morphology of Obelia
Morphology of Obelia

A. Hydrorhiza (Root of a hydroid)

  • Hydrorhiza is the portion of the colony consisting of stolons, which are tubular structures.
  • It covers the surface of the substratum and aids in the colony’s adhesion.

B. Hydrocaulus (Stem of a hydroid)

  • A few small vertical filaments, approximately 2 to 3 centimetres in length, emerge from the hydrorhizas. This is known as hydrocauli (Sing., Hydrocaulus).
  • Each branch of a hydrocaulus divides alternately and finishes in a polyp. The aggregate word for polyps is zooids.
  • These zooids have a nutritional purpose and aid in feeding. They are known as gastrozooids.
  • The axils of proximal branches contain reproductive zooids that are cylindrical. These structures are known as gonozooids, blastozooids, and blastostyles.

Living Tissue of Obelia – Coenosarc

  • Colonies of Obelia, including their hydrorhiza, hydrocaulus, and zooids, include living tissue known as coenenchyme or coenosarc.
  • The coenosarc is composed of two strata;
    • Inner gastrodermis and outer epidermis.
    • Between the epidermis and the gastrodermis is a non-cellular layer called the mesoglea.
  • The entire colony of Obelia is traversed by a thin canal known as the coenosarcal canal, which is connected with the gastrovascular cavity of the zooids.
  • The canal system’s continuity aids in transporting digested food throughout the colony.
Living Tissue of Obelia – Coenosarc
Living Tissue of Obelia – Coenosarc


  • The epidermis is thin and composed of Cnidaria-typical cells. They include epithelio-muscular cells, cells that secrete mucus, interstitial cells, nerve cells, and nematoblasts.
  • The nematocysts are isorhizas that are basitrichous. They are comprised of an oval capsule, a long thread with spines, and an open tip.


  • It is made up of endothelial-muscular cells, nutritive cells, gland cells, and nerve cells and forms the lining of the gastrovascular cavity.

Protective Covering – Perisarc

  • The entire Obelia colony is encircled by a protective coating known as perisarc. Perisarc or periderm is a noncellular, stiff, translucent, yellowish-brown, and cuticular substance.
  • It makes the colony’s vertical portion stiff and solid. The epidermis secretes the perisarc, which is separated from the coenosarc by a fluid-filled gap.
  • Yet, the contact between the coenosarc and perisarc makes the colony more stiff.
  • At certain spots, the perisarc is organised into annuli, which are flexible rings. They allow for the swaying movements caused by water currents.
  • The perisarc of hydranth is known as hydrotheca, whereas the perisarc of gonozooid is known as gonotheca.

Morphology of A Gastrozooid

  • The Obelia Gastrozooid is a feeding polyp. Its purpose is to nourish the entire colony.
  • The Gastrozooid is a tubular, diplobastic zooid with a continuous central gastrovascular cavity and coenosarcal canal.
  • The polyp is connected to the hydrocaulus by a hollow stalk, while its distal end forms a manubrium or hypostome.
  • The apical region of the manubrium has a terminal mouth surrounded by multiple long, firm tentacles, frequently 24 in number, containing nematoblasts.
  • The perisarc of the gastrozooid, known as the hydrotheca, is translucent and cup-shaped, serving as a platform or shelf for the polyp to rest on.
  • The combination of the gastrozooid and hydrotheca constitutes hydranth.
  • In the event of an emergency, the polyp can withdraw inside its hydrotheca and fold its tentacles over the manubrium covering its mouth. The presence of shelf prevents the polyp from retreating into the hydrocaulus
  • The annuli of the perisarc surrounding the stalk of the polyp allow the stalk to sway in response to the power of the water current.
Morphology of A Gastrozooid
Morphology of A Gastrozooid

Morphology of A Gonozooid (Polyp Stage)

  • The gonozooids, also known as blastozooids or blastostyles, are cylindrical reproductive bodies found in the axils of hydrocaulus and the stalks of gastrozooids.
  • Gonozooids are fewer in number than gastrozooids since the latter are exclusively found in the colony’s proximal portion. It lacks a mouth and tentacles and possesses a decreased gastrovascular cavity. Hence, it is incapable of feeding and instead gets food processed by gastrozooids and transmitted through the gastrovascular cavity.
  • Gonozooids, like other portions of the colony, are surrounded by a perisarc termed gonotheca. It is restricted both distally and proximally by annuli. The opening in the apical portion of the gonotheca is known as the gonopore.
  • Gonozooid develops a large number of tiny medusae or gonophores through an asexual process called budding.
  • Via the gonopore, mature medusae detach from the gonozooids and escape into the surrounding water.
  • The gonozooids, gonophores, and gonotheca constitute the gonangium as a whole.
Morphology of A Gonozooid (Polyp Stage)
Morphology of A Gonozooid (Polyp Stage)

Morphology of A Medusa

  • The Medusa of Obelia is an umbrella-shaped, radially symmetrical zooid measuring around 6-7 mm in diameter.
  • The outside convex surface of a medusa is known as the ex-umbrellar surface, whereas the inner concave surface is known as the sub-umbrellar surface.
  • From the middle of the subumbrellar surface hangs a short manubrium with a quadrangular mouth at its distal end.
  • The medusa is of the craspedote type because its edge forms a tiny rudimentary velum.
  • Initially, the edges of the medusa have sixteen short, contractile tentacles, which grow in number over time.
  • The mouth opens into a short gullet that leads to a large, expanded stomach, from which four narrow, radial canals mark the four major per-radii. The radial canals reach the umbrella’s margin and open into a circular canal that runs parallel to the boundary.
  • The radius that bisects two per-radii is known as an inter-radius, and the radius that bisects a per-radius and an adjacent inter-radius is known as an ad-radius (eight numbers).
  • Thus, the tentacles found at the ends of these radii are referred to as per-radial tentacles, inter-radial tentacles, etc.
  • The inner layer of gastrodermis lines the entire system of canals, and both the ex-umbrellar and sub-umbrellar surfaces are covered with epidermis.
  • Nervous system consists of two scattered nerve nets which are concentrated around the margins of the umbrella and create two circular nerve rings.
  • At the base of ad-radial tentacles are eight statocysts, which serve as receptor organs. They are the organs of equilibrium, muscle coordination, and balance.
  • On the sub-umbrellar surface, Medusa bears four gonads. They are located in the middle of each radial canal and are per-radial.
  • These medusae are dioecious, with separate male and female individuals.
Morphology of A Medusa
Morphology of A Medusa

Locomotion In Obelia

A. Movement in Polyps

  • The polypoid colony of Obelia is connected to the substrate and sessile. It does not travel from location to location.
  • Due to the presence of annuli in the perisarc, polyps exhibit specific movements in response to the force of water currents.
  • Because to the presence of longitudinal and circular muscles in their body wall, the polyps are also capable of contraction and extension.

B. Locomotion in Medusa

  • Hydropropulsion: Medusae are free-swimming creatures. Typically, they swim in the water using jet propulsion. The contraction and expansion of bell muscles alternately close and open the bell, forcing water out of the sub-umbrellar chamber and propelling the body upwards. The contraction of the epidermal muscle tails of the sub-umbrellar surface aids in the closure of the bell cavity, whereas the elastic mesoglea and contraction of the muscle tails in the middle of the top surface are responsible for the opening of the bell. This kind of jet propulsion is known as hydro propulsion.
  • Passive drifting: Under the influence of strong water currents and wind, Medusae also drift and float passively in the ocean. Thick mesoglea gives medusae buoyancy and enables them to float.

Nutrition In Obelia

Nutrition in Polyps

  • The gastrozooids are the Obelia colony’s nutritional zooids.
  • The majority of their diet consists of small crustaceans, tadpoles, worms, insect larvae, etc.
  • The gastrozooids collect food with the assistance of nematocysts located on their tentacles.
  • The food is forced into the gastrovascular cavity through the mouth, where it is partially digested by proteolytic enzymes generated by the gastrodermal gland cells.
  • The semidigested food is completely digested by the food vacuoles of the nutritive cells. Hence, digestion is extracellular and intracellular.
  • Through cell-to-cell diffusion, aided by the beating flagella of gastrodermal cells, the digested components of food are transported throughout the body; the gastrovascular cavity serves both digestion and transportation of food.
  • The undigested food particles is expelled from the gastrozooids’ mouths.

Nutrition in Medusa

  • The feeding process of medusa is comparable to that of polyps.
  • Medusa is a severe carnivore whose nematocyst-infested tentacles are used to grab prey.
  • As with polyps, food is digested both extracellularly and intracellularly in the stomach, but not in other organs.
  • By the network of radial and circular tubes present in medusa, digested food is delivered to the entire body.

Respiration In Obelia

  • Obelia lacks respiratory organs; hence, gas exchange occurs by diffusion across the entire body surface. Oxygen diffuses straight into the epidermal cells from the surrounding water, while carbon dioxide diffuses out.
  • As there is a continual flow of water into the gastrovascular cavity of a polyp or medusa, diffusion of gases can also occur during water circulation.
  • Here, water and gastrodermal cells exchange gases, from which oxygen diffuses to each Obelia cell.

Excretion And Osmoregulation In Obelia

  • Obelia lacks specialised excretory and osmoregulatory organs.
  • It eliminates nitrogenous waste as ammonia, which diffuses through the body wall.
  • Excess water is expelled through the mouth from the gastrovascular cavity. As the only aperture, the mouth also functions as a contractile vacuole.

Sense Organs – Statocyst

  • Obelia polyps are sessile zooids that do not require sensory organs. Yet, medusae are free-swimming zooids whose bodies may tilt and lose balance while swimming. Hence, they possess balancing organs, statocysts, which allow them to restore their position.
  • Structure A statocyst is a sac containing fluid and lined with sensory epithelial cells. The cell’s basal portion is related to nerve cells, whereas the inner ends are responsible for sensory processes. The cavity of statocyst contains a calcium carbonate particle known as statolith or otolith. The particle is mobile and is secreted by lithocyte, a big cell.
  • Function Statocysts contribute to the balance and equilibrium of the medusa. If the medusa tilts while swimming, the statolith particle rolls over the tilted side and presses against the sensory processes. The activated cells transfer the nerve impulse to the nerve ring, which is associated with the muscle tails.
  • The nerve impulse triggers the fast contraction of the muscle tails on the stimulated side, which restores the medusa to its former posture.

Structure of Obelia

  • Obelia has two forms throughout its life cycle: polyp and medusa. The first type has two genuine tissue layers – an epidermis and a dermis (ectodermis).
  • In contrast, the second type of tissue is the gastrodermis (endodermis), with jelly-like mesoglia filling the space between the two tissue layers.
  • These creatures possess a neural net but lack a brain and ganglia. In addition, the gastrovascular cavity is present, which is where digestion begins. This hole eventually becomes intracellular.
  • These creatures possess incomplete digestive systems. Moreover, the food that enters is digested and then ejected via the same aperture. The mouth is located at the top of the body during the polyp stage. This mouth is additionally ringed by tentacles.
  • At the medusa stage, the mouth is situated near the distal end of the main body structure. Moreover, there are four gonads in the manubrium. Manubrium also refers to the primary body structure.
  • Initially, food enters the body through the mouth.
  • Then it definitely enters the manubrium. A network of canals is utilised for the distribution of food. In addition, this canal system has four radial canals and an outer ring.

Life Cycle of Obelia

The fundamental function of the medusa is reproduction. Obelia is dioecious, which implies that each medusa contains only one set of reproductive organs. Male and female medusae are identical in every way. The gonads (ovaries or testis) show as knobs beneath the radial canals on the sub-umbrella. As a result, gonads are radial in nature.

  • Fertilisation: Fertilization occurs when the outer wall of the gonad ruptures, releasing the ova and sperm into the ocean. Water is utilised in fertilisation. Occasionally, flagellated sperms swim in the water and fertilise the ova of female medusae. As a mobile form, the medusa provides two essential functions for the colony: reproduction and gamete distribution.
  • Cleavage: The fertilised egg experiences an equal and holoblastic cleavage (complete). The blastula is a hollow sphere that encases the blastocoel in a single cell layer. This cavity is completely populated by cells that have emerged from the blastula’s wall. The embryo’s new term is Stereo gastrula, formerly known as solid gastrula. The embryo is liberated from the egg membrane as the planula, a free-swimming larva. The larva swims for an extended period of time, resulting in the species’ vast distribution. As a hollow emerges in the endoderm cell mass, the enteron forms.
  • Hydrula: The Gastrula transforms into a free-swimming, ciliated planula larva that searches for a suitable substratum to cling to and establish a sedentary colony. The solid endoderm fractures and an enteron cavity forms. The fixed planula now possesses an outer ciliated ectoderm and an inner endoderm, making it a genuine two-layered larva. Near the open or distal end, a dilatation occurs. Short tentacle buds emerge in a circular from this area. At the end of the hypostome, a mouth or aperture forms rapidly. The young hydranth, known as hydrula, resembles a hydra-like polyp and undergoes repeated asexual budding to form a complex Obelia structure.
  • Alternation of Generation: The process of alternation of generation is referred to as metagenesis. Obelia alternates between asexual reproduction and sexual reproduction. Polyps create medusae asexually, while medusae produce polyps sexually.

Reproduction In Obelia

Obelia’s life cycle consists of both polyp and medusa phases. The polyp is asexual and reproduces asexually, whereas the medusa is sexual and reproduces sexually.

a) Asexual Reproduction – polyps

  • The polyps proliferate asexually by bud formation.
  • As the colony matures, blastostyles emerge from the axils of the proximal gastrozooids and the hydrocaulus.
  • In the spring and summer, each blastostyle produces a great number of medusa buds.
  • These medusa buds develop and mature over time. They separate from the blastostyles and enter the water through the gonopore once they have reached maturity.

B) Sexual Reproduction – Medusa

Sexual Reproduction – Medusa
Sexual Reproduction – Medusa
  • The sexual reproduction of Obelia occurs during the medusa stage, with separate male and female medusa.
  • The medusae create ova and sperm, which they discharge into the water for fertilisation.
  • Sperms can enter the female medusa with the water current, and fertilisation can occur within the female medusa’s body.
  • The fertilised egg undergoes complete and equal cleavage, which results in the creation of a solid ball of cells known as the morula. Morula produces a central cavity, blastocoel, which is surrounded by loosely organised blastomeres. This blastula is known as a coeloblastula.
  • New cells gradually separate from the blastomeres and migrate into the blastocoel from one end of the coeloblastula. Slowly, the entire blastocoel is filled with cells, and the hollow blastula transforms into the stereogastrula, a solid gastrula, by delamination.
  • The embryo’s outer surface gets ciliated, producing a ciliated larva, planula larva. It consists of an exterior layer of ciliated ectoderm and an inner layer of endodermal cells. It vigorously swims in the water and aids in species distribution.
  • After a brief period of time, the larva settles down and attaches one of its ends to the substratum.
  • The connected end creates a basal disc, while the distal end produces a mouth encircled by tentacles.
  • This sessile phase is called the hydrula phase because it resembles a hydra.
  • Gradually, hydrula undergoes repeated asexual reproduction and transforms into an Obelia adult colony.


  • The life cycle of Obelia is a notable example of alternation of generation, as the asexual and sessile phase (polyp) of Obelia reproduces asexually through budding and gives rise to the sexual and free-swimming medusa.
  • The sexually reproducing and free-swimming medusa develops new polyps by sexual reproduction.
  • Consequently, a diploid asexual hydroid phase and a diploid sexual medusoid phase alternate.
  • This oscillation between two diploid phases is referred to as metagenesis.

Importance of Obelia

  • Obelia is a genus of colonial hydrozoans that are widespread in marine ecosystems. These creatures are significant for numerous ecological and scientific reasons.
  • Obelia and other colonial hydrozoans are ecologically significant members of marine communities. They provide habitat and refuge for other organisms, as well as food for a variety of marine animals. Obelia contributes to nutrient cycling by filtering and consuming planktonic organisms and transferring energy from primary producers to higher trophic levels.
  • Obelia and other colonial hydrozoans are scientifically significant model organisms for the study of a number of biological processes. They feature a simple body plan consisting of a polyp stage and a medusa stage, enabling researchers to examine their development, behaviour, and physiology in depth. Obelia is especially useful for investigating the genetics and molecular biology of coloniality, as well as the relationships between marine creatures.
  • In addition, Obelia and other colonial hydrozoans are employed in biomedical research, namely the study of regeneration and stem cell biology. The ability of these animals to regenerate their bodies and appendages has significant implications for the development of medicines for human tissue repair.
  • Obelia is a crucial organism in marine ecosystems and scientific research, yielding insights into biological processes, genetics, and medical therapies.


What is Obelia?
a) A species of jellyfish
b) A genus of colonial hydrozoans
c) A type of coral
Answer: b) A genus of colonial hydrozoans

Where can Obelia be found?
a) In freshwater environments
b) In terrestrial habitats
c) In marine environments
Answer: c) In marine environments

What is the body plan of Obelia?
a) Only a polyp stage
b) Only a medusa stage
c) Both a polyp stage and a medusa stage
Answer: c) Both a polyp stage and a medusa stage

What is the function of Obelia in marine communities?
a) They provide habitat and shelter for other organisms
b) They serve as a food source for many marine animals
c) They filter and consume planktonic organisms and contribute to nutrient cycling
d) All of the above
Answer: d) All of the above

What is the role of Obelia in scientific research?
a) To study the development, behavior, and physiology of colonial hydrozoans
b) To study genetics and molecular biology of coloniality
c) To study interactions between different organisms in marine communities
d) All of the above
Answer: d) All of the above

What is the importance of Obelia in biomedical research?
a) To study the ability of colonial hydrozoans to regenerate their bodies and appendages
b) To study stem cell biology
c) To develop therapies for human tissue repair
d) All of the above
Answer: d) All of the above

What is the structure of Obelia colonies?
a) They consist of a single polyp
b) They consist of multiple polyps connected by stolons
c) They consist of a single medusa
Answer: b) They consist of multiple polyps connected by stolons

What is the function of the gastrozooids in an Obelia colony?
a) To capture food
b) To digest food
c) To provide support for the colony
Answer: b) To digest food

What is the function of the gonozooids in an Obelia colony?
a) To capture food
b) To digest food
c) To produce gametes
Answer: c) To produce gametes

What is the reproductive strategy of Obelia?
a) Sexual reproduction only
b) Asexual reproduction only
c) Both sexual and asexual reproduction
Answer: c) Both sexual and asexual reproduction


What is Metagenesis with an example?

Metagenesis is a biological phenomena in which an organism undergoes a series of transformations or life cycle phases in which sexual and asexual reproduction alternate. These modifications may encompass morphological, physiological, and behavioural changes.
Jellyfish life cycles are a classic illustration of metamorphosis. Jellyfish have a complex life cycle consisting of polyp and medusa stages. The polyp stage is the asexual stage in which the jellyfish assumes the form of a tiny, tube-like, stationary creature. The polyp reproduces asexually by producing identical clones.
The polyp will eventually develop into a medusa, the sexual stage of the jellyfish life cycle. The medusa is the form of jellyfish with which the majority of people are most familiar. Jellyfish reproduce sexually during this phase by releasing eggs and sperm into the sea. The fertilised eggs transform into a larval stage, which finally settles on the ocean floor and gives rise to a new polyp.
This cycle of alternating asexual and sexual reproduction is an example of jellyfish metamorphosis. Other organisms that undergo metagenesis include certain cnidarians, flatworms, and algae.

Which stage is found in the life cycle of Obelia?

Obelia is a genus of hydrozoans, which belong to the phylum Cnidaria and are aquatic invertebrates. The life cycle of Obelia consists of two separate stages: the polyp stage and the medusa stage.
The polyp stage is the asexual stage of the life cycle of Obelia, during which the organism assumes a tiny, stalk-like shape. The polyps replicate asexually by producing identical clones.
The sexual stage of the life cycle of Obelia is the medusa stage. The medusa is a free-swimming, bell-shaped organism that reproduces sexually by releasing sperm and eggs into the water. The fertilised eggs transform into larvae, which eventually land on a substrate and form a new polyp.
Hence, the polyp and medusa stages are both present in the life cycle of Obelia.

How does the medusa stage fit into the life cycle of Obelia?

The medusa stage is one of the two stages in the life cycle of the hydrozoan genus Obelia, which is a member of the phylum Cnidaria and consists of aquatic invertebrates. The medusa stage is the sexual stage and fits within the Obelia life cycle as follows:
During the asexual polyp phase of its life cycle, the Obelia organism resembles a tiny stalk. The polyps replicate asexually by producing identical clones.
The polyps will eventually give rise to the sexual medusa stage. The medusa is a free-swimming, bell-shaped organism that reproduces sexually by releasing sperm and eggs into the water. These fertilised eggs eventually transform into a larval stage that settles on a substrate and creates a new polyp.
Hence, the medusa stage of Obelia produces sexually generated offspring that will mature into a new asexual polyp stage. Sexual and asexual reproduction alternate throughout the life cycles of many cnidarians, including Obelia.



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