Life Cycle of Obelia – Diagram, Structure and Classification

The life cycle of Obelia shows the alteration of generation. Obelia’s common name is sea fur. It is a group of aquatic organisms. Obelia classification shows it belongs to Class Hydrozoa and Phylum Cnidaria. Obelia are diploblastic, primitive organisms with a unique life cycle in the Animal Kingdom. The Obelia life cycle alternates between two phases, each having a different structure and function. This is called the alteration of generation. In this article, we will look into the life cycle obelia – diagram, obelia classification, characteristics, and obelia structure.

What is Obelia?

Obelia’s common name is sea fur. It is a group of hydrozoans belonging to the phylum Cnidaria, including jellyfish, corals, and sea anemones. These aquatic animals, except the Poles, are found in marine and freshwater environments worldwide. Obelia characteristics are that they are colonial organisms with a relatively simple diploblastic body.

Obelia under a microscope, look like a complex structure, with many small polyps connected by branching tubes. Obelia structure of body resembles sea anemones that remain attached to the substrate at the bottom, like rock or sea bed, and have tentacles that they use for feeding and reproduction. However, during the life cycle of Obelia, they also have a free-moving stage of life.

Definition of Obelia: Obelia is a sedentary marine colonial organism that sticks to seaweeds, rocks, lumber piles, and molluscan shells in shallow water.

Obelia Classification

The scientific classification of Obelia is:

Kingdom	Animalia
Phylum	Cnidaria
Class	Hydrozoa
Order	Leptothecata
Family	Campanulariidae
Genus	Obelia

Life Cycle of Obelia

Obelia is a dioecious organism with distinct sexual and asexual life stages. The life cycle of Obelia can be studied in various stages. The different stages of the life cycle of Cnidarians are also called zooids. The life cycle of Obelia is characterized by alternation of generations, involving both asexual and sexual reproduction with different body structures and functions.

1. Fertilization

The cycle begins with the release of sperm and egg from male and female reproductive medusa, respectively, into the water, where external fertilization occurs. The sperm are flagellated and swim around to find and fertilize with the female medusa’s egg.

2. Cleavage and Larva Formation

Once fertilized, the zygote undergoes complete cleavage to form the blastocysts that develop into ciliated, motile, free-swimming larvae known as planulae. Planulae are oval-shaped larvae that swim in the water, settle on substrates, and grow into sessile, feeding polyps known as hydranths through metamorphosis.

3. Obelia Polyp or Hydranth Stage

The obelia polyps or hydranths are the sessile phases in the life cycle of Obelia, with the primary function of feeding and propagating the colony. This stage is also called the gastrozooid or trophozooid. They reproduce asexually by budding, giving rise to additional feeding polyps and extending the colony. In Obelia, certain specialized polyps called gonozooids produce reproductive structures called gonangia, which give rise to the next stage in the life cycle.

4. Medusae Stage

Meanwhile, certain hydranths transform through a process known as strobilation to form the medusae stage. These are the sexual zooids and are also called the gonophores. Medusae are jellyfish-like structures equipped with tentacles and capable of sexual reproduction. The medusae release gametes (sperm and eggs) into the water, where fertilization occurs externally.

In this way, the life cycle of Obelia continues.

Life Cycle of Obelia Diagram

Lets have a look at the diagram of Obelia’s Life Cycle:

Alteration of Generation

Alternation of generations is a reproductive strategy observed in certain organisms, particularly among some groups of plants and algae, as well as in some animal groups such as cnidarians (e.g., jellyfish and hydroids) and certain fungi.

In Obelia, as in many other hydrozoans and certain other groups of organisms, alternation of generations refers to a reproductive strategy where the life cycle includes distinct multicellular stages, each with its mode of reproduction.

This alternation typically involves two main stages in Obelia: the polyp and medusa. The polyp is responsible for feeding and asexual reproduction by budding. The medusa stage consists of the formation of free-swimming, bell-shaped structures known as medusae. The medusae release gametes (sperm and eggs) into the water, where fertilization occurs externally, forming zygotes.

Structure of Obelia

Obelia have a relatively simple diploblastic body and structure with a gelatinous, transparent body resembling sea anemones that remain attached to the substrate at the bottom, like rock or sea bed. They have only two germ layers: the ectoderm and the endoderm, and a jelly-like substance in between called mesoglia.

They are roughly cylindrical and can grow up to several centimeters in length. They also have tentacles that they use for feeding and reproduction. However, during the life cycle of Obelia, they also have a free-moving stage of life that resembles a bell-shaped medusae form.

Obelia Characteristics

The following characteristics can identify Obelia:

• These are diploblastic organisms with an ectoderm and an endoderm but no mesoderm. There is the presence of mesoglia, a jelly-like substance between the two layers.
• They show radial symmetry.
• They do not have a brain or organized nervous system, but nerve nets are present.
• They have polyps with tentacles that they use for feeding and reproduction.
• Obelia also has cnidocytes of stinging cells that are characteristic of all Cnidarians.
• Obelia occurs in polyp colonies and as part of the plankton on surface water in the medusoid stages.
• The colonies typically consist of interconnected polyps arranged in a branching, tree-like structure.
• Each polyp within the colony serves a specific function, with feeding polyps equipped with tentacles for capturing prey, reproductive polyps responsible for producing medusae (sexual reproductive structures), and defensive polyps for protection against predators.

Conclusion – Life Cycle of Obelia

The Obelia structure and life cycle involve alternating stages between polyp and medusa forms for reproduction and survival. The polyps stage reproduces asexually (budding), while the medusae stage reproduces sexually. Obelia classification shows that it is a hydrozoan of the Phylum Cnidaria and is a simple diploblastic organism with a unique reproduction mode. The life cycle of Obelia includes 4 stages – fertilization, cleavage formation, polyps, and medusae. The life cycle of Obelia diagram is important to get a clear understanding of the Obelia life cycle.

FAQs on Life Cycle of Obelia

Which Stage of the Life Cycle of Obelia is Free Living?

Both stages of the life cycle of Obelia are free living. However, the polyp stage of Obelia is sessile, while the medusa is free-swimming.

Why is Obelia called Trimorphic?

Obelia is called trimorphic because it has three life forms: the polyp, medusa, and planulae larval stage.

What is the Alteration of Generation?

Alteration of generation is the alteration between two life stages with two distinct modes of reproduction, like in Obelia.

What is Polyp and Medusa?

Polyp and Medusa are the two life stages in Hydrozoans like Obelia, where Polyp is the sessile, asexually reproducing stage while Medusa is the free-swimming, sexually reproducing stage.

Which Stage of the Life Cycle of Obelia is Asexual?

The polyp stage of Obelia is asexual in nature. It divides asexually by the process of budding.

What is the Habit of Obelia?

The habit of Obelia is generally marine, sedentary, and colonial. They are often found attached to the rocks and seaweeds in shallow water.