Life Cycle of Obelia


Sea fur is another name for Obelia. Obelia has a delicate hydroid colony that is semi-transparent and pale to light brown in colour. It comprises vertical branching stems known as hydrocauli and root-like branches known as hydrorhiza.

Obelia’s life cycle includes both asexual and sexual generations, which alternate to complete the life cycle. Obelia’s life cycle alternates between hydroid and medusoid phases on a regular occasion.

Medusae lay eggs, which develop into hydroids following fertilisation. Asexual budding is how hydroids produce medusae. This phenomenon was originally known as the “alternation of generations.”

Table of Contents

Definition of Obelia

Obelia is a sedentary, marine colonial life form that attaches to seaweeds, rocks, timber piles, and molluscan shells in shallow water.

Obelia starts out as small, motionless animals with stalks and tentacles that resemble sea anemones. Hydroid polyps are another name for these little invertebrates.

Obelia is commonly referred to as sea fur. Furthermore, this animal’s reproduction process involves two independent stages. In addition, it takes two generations to finish.

Classification of Obelia

Obelia belongs to the hydrozoa class of animals and contains numerous species. The following is a list of biological classifications for obelia:

Kingdom – Animalia

Phylum – Cnidaria

Class – Hydrozoa

Order – Leptothecata

Family – Campanulariidae

Genus – Obelia

Structure of Obelia

Obelia belongs to the phylum Cnidaria. It takes two forms throughout its life cycle — polyp and medusa. The first form is diploblastic, which has two true tissue layers — an epidermis and a dermis (ectodermis).

The second form is gastrodermis (endodermis), which has a jelly-like mesoglia filling the space between the two tissue layers.

These creatures have a neural network but no brain or ganglia. The gastrovascular cavity is also present, which is where digestion begins. This cavity is later converted into an intracellular space.

The mouth is located at the top of the body, encircled by tentacles, during the polyp stage, but the mouth is located near the end of the major body structure during the medusa stage. The manubrium, or primary body structure, contains four gonads. Food enters the manubrium when it is ingested by the mouth.

Life Cycle of Obelia

The medusa’s primary purpose is sexual reproduction. Obelia is dioecious in nature, which means it only has one pair of reproductive organs per medusa. There are no differences between male and female medusae. On the sub-umbrella, the gonads (ovaries or testis) appear as knobs beneath the radial canals. As a result, gonads are radial in nature.

Fertilisation: When the ova and sperm are fully developed, the gonad’s outer wall ruptures, releasing the ova and sperm into the ocean. Water is used for fertilisation. Flagellated sperms occasionally swim around in the water and fertilise the ova of female medusae. Because medusa is a motile form, it serves two key purposes for the colony: reproduction and gamete dispersal.

Cleavage: The fertilised egg goes through an equal and holoblastic cleavage (complete). The blastula is a hollow ball that encloses the blastocoel in a single layer of cells. This hollow is totally filled with cells that have sprung from the blastula’s wall. The new name for the embryo is Stereo gastrula, also known as solid gastrula.

The embryo is released from the egg membrane as a free-swimming larva known as the planula. The larva swims around for a long time, causing the species to spread widely. The enteron is formed when a cavity emerges in the endoderm cell mass.

Hydrula: Gastrula elongates into a free-swimming ciliated planula larva that swims around, looking for an appropriate substratum to attach to and create a sedentary colony. The solid endoderm breaks and develops an enteron cavity. The fixed planula now has an outer ciliated ectoderm and inner endoderm, making it a true two-layered larva.

A dilatation occurs near the open or distal end. Tentacles emerge in a circle as short buds from this section. At the end of the hypostome, an aperture, or mouth, forms quickly. The immature hydranth, known as hydrula, resembles a basic polyp like hydra and undergoes recurrent asexual budding to produce a complicated Obelia structure.

Alternation of Generation: Metagenesis is the term used for the process of alternation of generation. Obelia alternates between asexual and sexual modes of generation. Polyps asexually produce medusae, while medusae sexually produce polyps.

Finally, asexual budding results in the formation of a new generation of polyps.

Further Reading:

Frequently Asked Questions – FAQs

What is Metagenesis with an example?

The transition between sexual and asexual stages is known as metagenesis. Polyps originate sexually from medusae, and medusae develop asexually from polyps. Cnidarians like Obelia, for example, exist in both polyp and medusa forms and undergo metagenesis.

Which stage is found in the life cycle of Obelia?

Obelia’s life cycle alternates between hydroid and medusoid phases on a regular basis. Medusae lay eggs, which hatch into hydroids following fertilisation. Asexual budding is how hydroids produce medusae.

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

Jellyfish with a bell shape and tentacles are known as Medusas. The Obelia medusa may swim freely at this stage and reproduce sexually by releasing eggs or sperm into the water. After fertilisation, the resultant zygote grows into a free-swimming larva (plural: larvae) covered with cilia or microscopic hairs.

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