Obelia - Introduction

You might have seen pictures of the ocean floor where you can spot anemone-like colonies, or you might have seen a jellyfish in life or online. These are all creatures that belong to the obelia genus. Though they might appear like they are different organisms to an observer, they are all forms of the same organism as they change forms in their different stages of a very complex life cycle.

The Obelia genus belongs to the invertebrates marine animals and can be found in all the oceans of planet earth. It is a sedentary marine colonial form that can be seen attached to the surface of seaweeds, rocks, wooden piles, and molluscan shells in shallow waters (till 80 meters deep). It grows like a tree branch in an upright fashion and has many specialized reproductive and feeding polyps. It exists in many stages and varieties like sessile, asexual, sexual, free-swimming medusoid phase, polypoid stage, etc.

The scientific name of obelia is obelia (though it has many species with different names), and obelia common name is sea fur as it forms a light-brown or whitish plant-like fur in the seas. The distribution of obelia is cosmopolitan except for Antarctic seas and high-arctics.

In this article, we will get some common information about obelias like their class, order, etc., along with details on obelia colony, obelia medusa, and other information related to its life cycle.

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Classification And Other Basic Details of Obelia

Obelia belongs to the animal kingdom within the hydrozoa class and has many species. The list below outlines how the obelia is classified in biology:

  • Kingdom - Animalia (animals)

  • Phylum - Cnidaria (sea anemones, corals, jellyfishes, and other relatives)

  • Class - Hydrozoa (characterized by bodies that are radially symmetrical)

  • Order - Leptothecata (Hydrozoans whose hydranths are covered with gonophores and perisarc).

  • Family - Campanulariidae (stinging celled animals composed totally of hydroids)

  • Genus  Obelia

  • Common Species - Obelia bidentata (double toothed hydroid), Obelia castellaneta, Obelia dichotoma (sea thread hydroid), Obelia fimbriata, Obelia geniculata (knotted thread hydroid), Obelia irregularis, Obelia logic yatha, Obelia longissima (Sessile hydroid).

  • Geographical distribution - widely distributed from the arctic region down to the Gulf of Mexico and the Pacific coast. Also, from southern California to Oregon.

  • Sense organ - Statocyst


Obelia Characteristics

Obelias are often brilliantly colored animals that exhibit radial symmetry. Their phylum Cnidarian gives them two basic metazoa characteristics, such as gastrovascular cavity (internal space for digestion) and a circle of tentacles, representing the extension of the body wall. Here are few other key obelia characteristics:

  • Obelia is a very tiny marine hydroid with a height of 2 cm or a little more.

  • The obelia’s body has two kinds of filaments which are vertical hydrocaulus and horizontal hydrorhiza.

  • Obelia’s life cycle begins as hydroid polyps. Hydroid polyps are small, immobile animals that have tentacles and stalks. Their structure resembles sea anemones; hence they are also called sea furs.

  • Their reproduction cycle has two distinct stages (Polyp stage and medusa stage), and it takes two generations to complete the cycle.

  • Their body wall consists of 3 basic layers: the outer layer or epidermis, an inner layer made up of endodermal cells that line the gastrovascular cavity, and a layer in between the two called mesoglea. Mesoglea is a jelly-like mucoid material that fills the area between the two tissues.

  • The gastrovascular cavity lies along the polar axis of the obelia and has an opening to the outside at the end, which forms the mouth.

  • Since obelia has a mouth and digestive cavity, it allows them to eat a much bigger range of food sizes which are not common in other sponges and protozoans. 

  • They have a circle of tentacles surrounding their mouth, which aids them in capturing and ingesting food.

  • Food enters the body of an obelia through the mouth and enters the manubrium. Food is distributed all through the body of an obelia through a canal system that comprises 4 radial canals and an outer ring. In the end, the food is expelled through the mouth.

  • Obelias have an incomplete digestive tract.

  • These animals have a nerve net but do not possess a brain or ganglia.

  • The life history of obelia exhibits alternation of generations (between sexual and asexual) and is termed as metagenesis.


Obelia Reproduction

One of the most interesting aspects of obelia is its reproduction strategy. In this respect, obelias are fascinating and strange species that use both sexual and asexual methods of reproduction in a single reproductive strategy. Further to this unique phenomenon, the reproductive strategy of an obelia requires two generations (polyp and medusa generations) to complete one life cycle. Let us delve into the two obelia reproduction stages below:

  • The Polyp Reproduction Stage - The polyp stage is the first stage in the life cycle of an obelia. All obelia begin as polyps that are connected to a solid surface such as the ocean floor. With time the polyp grows and forms a colony which is made up of hydranth and gonangium units. The colony feeds through the hydranth portion that consists of mouths and stomachs, enabling the colony to feed. The reproductive units of the colony are its gonangium portions. The gonangiums units reproduce asexually by budding and releasing free-swimming medusa.

  • The Medusa Reproduction Stage - Medusa is a jellyfish with its characteristic bell shape and tentacles. In this stage, the free-swimming medusa reproduces sexually by releasing either sperm or eggs into the water. When the sperm and egg fertilize, it results in a zygote that develops in a free-swimming larva covered in tiny hairs or cilia. The cilia present in the larva is used for swimming for the larva’s development. Finally, the animal reaches the floor of the ocean and gives rise to a polyp. After that, a new life cycle begins.


Obelia Colony

Obelia colony is a mesh of highly branched structure where horizontal hydrorhiza is attached to the substratum and has the vertical branches or hydrocauli (about 2.5 cm in length) that bear zooids. Both hydrocaulus and hydrorhiza are hollow tubes. Polyps or zooids surround hydrocaulus on both sides in a cymose formation.

The Obelia colony is trimorphic (having three distinct forms), and some of its salient features are described below:

The Colony is Made up of:

  • Hydranths or polyps are nutritive zooids. They have a body shaped like a vase with mouths and tentacles. Enclosing the polyp is a pericardial covering which is called hydrotheca. Hydranths are feeding polyps that capture very small larvae and insects to feed themselves.

  • Blasto Styles are reproductive polyps that are club-shaped and have tentacles. They are enclosed within gonotheca, which is the pericardial covering. There are buds arising from its axis, which form the medusa.

  • Medusa is in the shape of a saucer (having convex and concave sides) and bears gonads. Medusa has marginal tentacles, a ring canal, 4 radial canals with 4 gonads born on them, and a central manubrium hanging on the concave side.

  • Obelia colonies appear like a delicate fur-like growth on top of stones, rocks, seaweeds, mollusk shells, and wooden pilings.

  • The colony of obelia contains living tissues called coenosarc or coenenchyme. The blast styles, polyps, and their tubular connections are made up of ectoderm, mesoglea, and endoderm. All these layers are together referred to as coenosarc.

  • Obelias colonies can have different colors ranging from cream to light brown.

  • The form of an obelia colony depends upon obelia species, and there are deviations from usual patterns in many species. A typical pattern of obelia colony is sympodial growth, which means that there are many units added on top of each other that make up the main axis rather than a single main growing axis. Each module or unit is in the shape of an arching stalk that terminates in a hydrant and then branches off to the next unit above it.

  • Hydrants in a colony alternate, and one can see gonangia developing in the axis. 

  • The apical or terminal bud is a developing hydranth.

  • The entire colony has a tough covering of yellow chitin, which is secreted by the ectoderm. This covering is called the perisarc.


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Obelia Medusa

The obelia medusa is a zooid in the shape of an umbrella. It is radially symmetrical and measures around 6 to 7 mm in diameter. It is a free-swimming reproductive zooid of the obelia colony.

 Some of its Chief Characteristics are:

  • The medusa is of craspedote type since its edges are protruding inwards in an insignificant velum.

  • The outer surface of medusa is known as the ex-umbrellar surface and is convex in shape. 

  • The inner surface of the medusa is known as the sub-umbrellar surface and is concave in shape. 

  • There is a short manubrium that has a quadrangular mouth at its distal end. This manubrium hangs from the center of the inner concave surface.

  • Initially, the margins of a medusa have 16 contractile short tentacles. These tentacles gradually increase in number.

  • Medusa’s mouth opens in a short gullet which goes into an expanded wide stomach. From the stomach, 4 radial canals (narrow) arise that make up the four principal per-radii.

  • The radial canals go up to the margins of the umbrella and finally open into a circular canal that runs parallel to the margin.

  • The radius which bisects two per-radii is known as inter-radius (which are 4 in number).

  • The radius that bisects a per-radius and an adjacent inter-radius is called an ad-radius (8 in number).

  • The tentacles that are present at the end of these radii are named as per the radii they are present on. For example, inter-radial tentacles, pre-radial tentacles, etc.

  • This whole system of canals has an inner layer of gastrodermis.

  • The epidermis covers both the umbrellas (ex-umbrella and sun-umbrellas).

  • Two diffused nerve sets make up the nervous system of a medusa. These nerve sets are concentrated around the umbrella’s margins forming two circular rings of nerves.

  • There are 8 receptor organs known as statocysts present at the base of the ad-radial tentacles. These organs provide the medusa with balance, equilibrium, and muscular coordination.

  • There are 4 gonads on the sub-umbrellar surface of a medusa, on the radial canals.

  • The gonads are pre-radial in position, and each of these gonads is present in the center of each radial canal.

  • Medusae are dioecious, i.e., male and female medusae are different individuals.


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Respiration in Obelia

Obelia lacks respiratory organs; hence the gas exchange in this animal occurs by diffusion through the body surface.

  • Oxygen from the surrounding water diffuses directly into Ophelia’s epidermal cells, and carbon dioxide diffuses out similarly.

  • The circulation of water in the gastrovascular cavity of a medusa or polyp can also lead to the diffusion of gases since there is a continuous influx of water when this happens.

  • In this case, gases exchange between gastrodermal cells and water from where oxygen diffuses to each and every cell of obelia.

FAQs (Frequently Asked Questions)

1. Explain What is Bioluminescence With Respect to Obelia.

Ans: Some obelia species (for example, Obelia geniculata) can cause bioluminescence or flashes of light from their colonies. When the colonies are stimulated, it can lead to the emission of light from several points within the obelia colony with the same spots flashing repeatedly. Photocytes or light-producing cells are located in the endoderm of obelia. These photocytes are more prevalent in the nodes and pedicles and are very frequently found at the tip of the upright colony. The release of light takes place from the photoprotein molecules within photocytes when they are activated. They are usually activated when there is an increase of calcium ions within the photocyte (when the colony is stimulated, phagocytes respond this way). Some of this energy gets transferred to a secondary fluorescent protein which causes secondary fluorescence and emits light of a different color (usually green). The locations of primary phagocytes and the secondary fluorescent proteins then seem to coincide and work together as a system. A few phagocytes present in the gonangia and the medusa have 16 green-colored fluorescent spots located around the ring canal in the shape of a ring. As the medusa ages, these fluorescent spots grow in number. Medusae do not appear to luminesce when the colony to which they are attached is stimulated. Instead, the tentacle bulbs of free medusae luminesce. 

2. What is the Locomotion System in a Medusa?

Ans: Medusae are free-swimming organisms. Locomotion in a medusa happens in two ways:

  • Hydro or Jet Propulsion - By contracting and expanding bell muscles, a medusa keeps closing and opening the bell which forces water downwards from the sub-umbrellar cavity resulting in a propelling force that makes the medusa move in the upward direction. 

  • Passive Drifting - Under the force of wind or strong water currents, medusae float and drift passively in water. The buoyancy and floating capacity of a medusa comes from its thick mesoglea.

3. Elaborate on the Structure and Function of the Sense Organ of an Obelia.

Ans: The first reproduction stage of obelia, i.e., polyps, does not require any sense organs since they are sessile zooids. But a medusa is a free-swimming organism, and their bodies might tilt or lose balance while swimming. That is why they have sense organs called statocysts which enable medusae to regain their balance during movement. The structure of a statocyst is a sac filled with fluid and lined by sensory epithelial cells. The two parts of the cells are:

  • The basal part, which is connected to the nerve cells.

  • Inner ends that bear the sensory processes.

In the cavity of the statocyst, there is a round-shaped particle made of calcium carbonate, which is termed otolith or statolith. The large cell lithocyte secretes this particle, and it is movable. 


Statocyst helps in maintaining equilibrium and balance in obelias in the following manner:

  • When the medusa tilts, the movable particle rolls over to the tilted side and presses on the sensory processes.

  • The nerve impulse is then transmitted to the nerve ring (connected to the muscle tail) by the stimulated cell.

  • Due to the nerve impulse, there is a rapid contraction of the muscle tail present on the stimulated side, which allows in regaining the obelia’s position.