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Obelia – Habit & habitat, Colony Structure, Reproduction, Life Cycle.

In this article of Obelia, We are going to learn about:- 1. Systematic position, 2. Special features, 3. Habit and Habitat, 4. Morphology, 5. Histology, 6. Movement, 7. Nutrition, 8. Respiration & excretion, 9. Reproduction, 10. Sense Organ, 11. Life cycle.


Systematic position

Kingdom: Animalia

   Phylum: Cnidaria

      Class: Hydrozoa

         Order: Leptothecata

             Family: Campanulariidae

                Genus: Obelia

                    Species: Obelia geniculata


Special Features of Obelia

1. Obelia is a very tiny marine animal with a height of 2 cm or a little more.

2. Its body has two kinds of filaments; hydrocaulus and hydrorhiza.

3. Its life cycle begins as hydroid polyps.

4. Hydroid polyps are small, immobile animals that have tentacles and stalks. 

5. Their reproduction cycle has two distinct stages. Asexual Polyp stage and sexual Medusa stage.

6. Polyp stage is immobile but the Medusa stage is motile.

7. Their body wall consists of two basic layers. The outer is the epidermis and the inner is the gastrodermis.

8. There is a layer between epidermis and gastrodermis, called Mesoglea. Mesoglea is a jelly-like material.

9. They have tentacles surrounding their mouth which help them in preying and capturing food. Tentacles in medusa help them in locomotion also.

10. Obelia has an incomplete digestive tract.

11. The life history of Obelia exhibits alternation of generations between sexual and asexual and is termed as metagenesis.


Habit and Habitat

1. Obelia is typically a marine, colonial, and sedentary animal.

2. They have a cosmopolitan distribution.

3. Abundant in both Pacific and Atlantic coastal waters.

4. It is found up to a depth of 80 meters.

5. It occurs in asexual and sexual forms.

6. The asexual form is a hydroid colony attached to rocks, stones, shells of animals, wooden piling, etc. and the sexual form is an umbrella-like free-swimming stage.


Morphology of Obelia

To start with Obelia is a monomorphic form having a polyp stage only. But due to the development of blastostyle later, it becomes a dimorphic colony. In an Obelia colony, there are two zooids, Polyp, and Blastostyle. After the development of medusa, the dimorphic colony becomes trimorphic. Obelia is a trimorphic colony having three different kinds of zooids.

1. Polyp or Hydranth (Nutritive zooid)

2. Blastostyle or Gonangia (Asexual budding zooid)

3. Medusa (Sexual zooid)


A. Morphology of Hydroid Colony

Polyp of Obelia

Fig: Obelia colony

1. External Features 

  • The hydroid colony of Obelia is delicate, semi-transparent, and whitish to light brown in color.
  • It consists of vertical branching stems called hydrocauli, and root-like stems called hydrorhiza.
  • Both hydrocauli and hydrorhiza are of the same thickness.
  • The growth of the colony is sympodial.
  • The ultimate branch terminates in a nutritive zooid called polyp or hydranth.
  • In the older polyps, cylindrical nutritive zooids are placed called blastostyle or gonangia.


2. Perisarc

  • Coenosarc is surrounded externally by perisarc.
  • It is yellowish or brown in color.
  • It is a tough, transparent, and non-living chitinous layer.
  • It protects the colony and serves as a supporting exoskeleton.
  • At the base of each zooid, the perisarc bears annular constrictions are called perisarcal annuli.


3. Coenosarc

  • Branches and zooids of the colony consist of an inner, tubular, and living portion called coenosarc.
  • It consists of a cellular wall that encloses the gastrovascular cavity.
  • The cellular wall consists of two layers; epidermis and gastrodermis and mesoglea in between.

4. Zooids

The term zooid is used for an individual form of a coelenterate colony. Hydroid colony exhibits two types of individuals or zooids and they are polyp and blastostyle.

a) Polyp: 
  • The polyps are like a miniature hydra.
  • It has a cylindrical body attached to the hydrocaulus by its proximal end.
  • It is covered by a cup-shaped hydrotheca.
  • The hypostome or manubrium is situated at the free distal end of the polyp.
  • Hypostome is surrounded by numerous tentacles.
  • Tentacles are longer than the hypostome, tapering and filiform.
  • The apex of the hypostome bears mouth. The mouth is capable of great dilation and contraction.
  • Below the hypostome is the stomach region of the polyp.
  • The body of polyp encloses a spacious cavity called gastrovascular cavity.
  • The polyp is protected in hydrotheca, which is a prolongation of the perisarc.
b) Blastostyle:
  • When hydrocaulus has reached full development, it produces special club-shaped bodies called blastostyle or blastozooid or gonozooid or gonangium.
  • It is covered by a transparent gonotheca.
  • It has no mouth, no tentacles.
  • These are less numerous than the polyps and occur in the axis of older polyps.
  • The blastostyle, by lateral asexual budding, produces sexual individuals called medusae or gonophores.
  • Fully formed medusae detach from the blastostyle to escape into the surrounding water through an aperture, the gonopore, formed by the rupture of gonotheca at its distal end.


B. Morphology of Medusa

Medusa of obelia

fig: Medusa


  • Medusa of Obelia is radially symmetrical, umbrella-like zooid which measures approximately 6-7 mm in diameter.
  • The outer surface of medusa is convex and the inner surface is concave. The convex surface is known as Ex-umbrellar surface and the concave surface is known as sub-umbrellar surface.
  • Manubrium of medusa hangs from the centre of the sub-umbrellar surface.
  • The mouth is quadrangular and present at the distal end of the manubrium.
  • The margins of the medusa bear initially 16 short, contractile tentacles; which gradually increase in number.
  • The tentacles are highly contractile and beset with nematocysts. Their bases are somewhat swollen to form a tentacular bulb that may lodge the sense organs statocysts and serve as sites for nematocyst formation.
  • The mouth leads into a narrow passage running through the manubrium, called gullet. It is followed by a dilated stomach, lying at the base of the manubrium and occupying the central part of the umbrella.
  • Four narrow radial canals extend from the stomach to the margin of the umbrella.
  • Radial canals open into a circular canal or ring canal, running close and parallel to the free margin of umbrella.
  • Whole system of canals is lined by gastrodermis and both ex-umbrellar and sub-umbrellar surface is covered by epidermis.


Histology of Obelia

1. Histology of colony

A. Epidermis

  • The outermost layer of the body wall is the epidermis.
  • Epidermis is thin and consists of chiefly large, conical and columnar epithelio-muscle cells.
  • Interstitial cells, nerve cells, and cnidoblasts with nematocysts are present in the epidermal layer.
  • The nematocysts are abundant on the tentacles and manubrium only. They help in preying.
  • Nematocysts of Obelia are of penetrant type. The nematocyst’s capsule is oval, butt is absent, thread is open at the tip and has spines on its base.
  • A nerve net, composed of large and branched nerve cells, is present on each side of the mesoglea.

B. Gastrodermis

  • The inner layer of the body wall is the gastrodermis.
  • Gastrodermis lines the gastrovascular colony throughout the colony.
  • It consists of chiefly long, granular muscle cells.
  • The inner free ends of gastroderm cells have flagella which produce current in the enteron. They can form pseudopodia to engulf and digest food particles like Amoeba.
  • Digestive juice is secreted from the gland cells of gastrodermis. 
  • Gastrodermis of tentacles having a gastrodermal core made of a single row of small, cylindrical and greatly vacuolated cells with thick cell-walls.


2. Histology of Medusa

  • The basic histological structure closely resembles that of the hydroid colony.
  • The epidermis covers the ex-umbrellar and the sub-umbrella surface on all sides.
  • The entire gastrovascular canal system (gullet, stomach, radial canal, and circular canal or ring canal) is lined by the gastrodermis.
  • Both epidermis and gastrodermis are continuous along the margin of the mouth.
  • Gelatinous mesoglea, lying between epidermis and gastrodermis is much thickened towards the ex-umbrellar surface.
  • Mesoglea is devoid of cells but contains fibers, secreted by both epidermis and gastrodermis. It consists of about 95% of water.
  • Interstitial cells give rise to cnidoblasts which are abundant on the margin of the umbrella, tentacles, and around the mouth.


Movement / Locomotion


  • The colony is sessile. Does not exhibit bodily movement.
  • Polyps can contract and expand their body and bend their tentacles.


  • They are free-swimming forms.
  • Medusa usually floats passively in water. It simply drifts here and there by water currents.
  • They generally swim in the water by the jet propulsion method.
  • The contraction and expansion of bell muscles alternatively close and open the bell which forces water out of the sub-umbrella cavity downwards and propels the body in the upward direction.
  • Thick mesoglea of medusa helps them in floating.


Nutrition of Obelia


  • Polyps are the nutritive zooids of the colony.
  • They are carnivorous and prey upon small aquatic crustaceans, nematodes, and other worms.
  • They prey and capture their food with the help of their nematocysts present in the tentacles.
  • The food particles are pushed into the gastrovascular cavity through the mouth. The food is digested partially in the gastrovascular cavity with the help of digestive juices secreted by gland cells.
  • Food vacuoles engulf the partially digested food and digest them intracellularly. Thus, the digestion is both extracellular and intracellular.
  • Digested food diffuses into the cells of the entire colony.
  • As there is no anus, undigested food is expelled out through the mouth.


  • They are strictly carnivorous.
  • Food consists of small aquatic animals or animal bodies.
  • Preying mechanism is the same as a polyp.
  • Digestion is both extracellular and intracellular.
  • Extracellular digestion takes place in the gastrovascular cavity.
  • Intracellular digestion takes place mostly in the manubrium, stomach and tentacular bulbs.
  • Digested food is distributed to the entire body through the radial and circular canal.


Respiration of Obelia

1. Obelia does not have any specific respiratory organ.
2. The gaseous exchange takes place by diffusion through the body surface.
3. Oxygen from the surrounding water diffuses into the body and carbon dioxide diffuses out the surrounding water.


Excretion of Obelia

1. The nitrogenous waste material of Obelia is ammonia.
2. Obelia does not have any special excretory organ.
3. It excretes the waste materials by diffusion through the body wall.


Reproduction of Obelia

The life history of Obelia exhibits an alternation of generations between sexual and asexual. Blastostyle is the asexual form and reproduces asexually. Medusa is the sexual form and reproduces sexually.

Asexual reproduction

1. Blastostyles are the asexual reproductive zooids.
2. The asexual reproduction process is budding.
3. Blastostyles rise from the axis of each mature polyp.
4. Each blastostyle produces a large number of medusa buds inside of them.
5. Reproduction season is Spring and Summer.
6. The buds inside the blastostyles gradually develop and when they are ready, they detach from the blastostyle and escape in the surrounding water.

Sexual reproduction

1. Medusae are the sexual reproductive zooids.
2. They are dioecious.
3. There are four gonads in the middle of each radial canal.
4. Each gonad produces a mass of sex cells.
5. In the male medusa, the sex cells give rise to sperm and in the female medusa, the sex cells give rise to the ovum.
6. The male medusae release sperms in the water where the ova of the female medusa remains inside the ovary.
7. Sperms of the male medusa enter into the female medusa by water currents.
8. Both male and female medusa die after fertilization.


Sense organs of Obelia

1. As polyps are sessile, they do not require any sense organs.
2. Medusae have a specified sense organ named statocyst. They have eight marginal statocysts.
3. While swimming, medusa may lose their body balance and they can regain their balance with the help of statocyst.
4. Statocyst is a fluid-filled sac lined by sensory epithelial cells.
5. There is a round particle of calcium carbonate in the cavity of the statocyst. The round particle is called statolith or otolith.
6. Statolith is movable and secreted by lithocyte.
7. Statocysts are considered to be the organs of equilibrium and muscular coordination.
8. During swimming, if the body becomes tilted, the statolith falls over the tilted side against the process of sensory cells which become stimulated.
9. In this way, a nerve impulse is created and transmitted to the nerve ring and with some other processes, the medusa gets back to the normal position.


Life Cycle of Obelia

1. The hydroid colony facilitates asexual reproduction.

2. Hydroid colony has polyps or gastrozooids. After maturing the polyps, gonozooids or blastostyles arise from the axis of the polyps.

3. Inside the blastostyles, the medusa buds develop gradually.

4. After maturing the medusa buds inside the blastostyles, they detach from the blastostyle and escape into the surrounding water through gonopore.


life cycle of obelia

Fig: life cycle of obelia


5. Then the young medusa gets matured.

6. After maturing, both the male and female medusae release gametes in the water.

7. Fertilization is external and the parent medusae die soon after releasing the gametes.

8. After fertilization, the egg and sperm form a zygote.

9. Zygote then develops into 2 celled stage and then 4 celled stage.

10. 4 celled stage later turns into a blastula. The blastula then turns into a free-swimming planula larva. The planula larva is a special character of Obelia.

11. Planula larva then swim freely searching substrates like rocks, logs, stones etc. 

12. After finding the substrate, the planula larva settled down. 

13. Then the larva turns into a young polyp.

14. Young polyp develops gradually and turns into a new obelia colony.


 ——————- THE END ——————–

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