Introduction
Polyp and Medusa are two different body forms, here one is nonmotile in nature and other is in free swimming. These two structures are found in invertebrates phylum Cnidaria i.e Coelenterates. Some of them exist in polyp form while others are in medusa form as these forms are staged in their life. Some species of Cnidaria have both stages i.e polyp and medusa, example: Hydrozoa. Some other examples of cnidarians are corals, sea anemones, jellyfish, hydroids, etc.
Polyp:
Polyp is a sessile life cycle stage of species who belong to phylum cnidaria. Famous examples of polyp are sea anemones and adult corals. Polyp is formed with a mouth surrounded with tentacles, referred to as a head and head is attached to the bottom with a foot-like disk. Direction of the mouth and tentacles face towards the water. Mode of reproduction in polyp is either sexual or asexual. Corals have separate sexes, some corals have male organs while others have female organs in the same ways some of the corals are hermaphrodites in nature i.e they have both male and female organs on the same body. Here mode of asexual reproduction is budding i.e through formation of buds that form new individuals and sexual reproduction occur by spawning. At a given time following the secretion of pheromones, corals release a big number of sexual cells in the water.
Medusa:
Medusa is a life cycle stage of the species who belongs to cnidaria phylum. One of the most common examples of species having a medusa life cycle is Hydrozoa or jellyfish. External appearance of medusa is bell shaped and has capability of undergoing muscular contractions which enables the medusa to swim. Several types of tentacles like photoreceptors, gravity-sensing osteocytes surround the bell. Members of the Hydrozoa class specially possess manubrium, whose structure is like a tube hanging down from the bell with the mouth at its end. Mode of reproduction in medusa is sexual, but their development varies within the class of the cnidaria phylum as in case of hydrozoa class new members are formed by budding. While in other cnidarian classes such as cubozoa or rhizostomeae medusa are produced by the metamorphosis of the polyp.
Difference between Polyp and Medusa:
There are many difference seen in polyp and medusa based on their external as well as internal features:
1. Movement of Polyp and Medusa:
Polyp is a sessile life stage of the cnidaria phylum whereas medusa is totally opposite i.e they have a mobile life cycle stage of the cnidaria phylum.
2. Morphology of Polyp and Medusa:
Morphological features of polyp are tubular in shape and they are fixed at their base. Their mouth is present at the opposite end and is surrounded with tentacles forming the head part. Their month and tentacles face the direction of water. Whereas medusa have the shape of a muscular bell which enables them to swim. In the case of Hydrozoa, their mouth is present at the end of a tube hanging down from the bell, which is known as manubrium. Several external structures like tentacles, photoreceptors, gravity-sensing osteocytes surround the bell. Among these external receptor photoreceptors and statocytes act as a sense organs present only in medusa and lacking in the polyp.
3. Reproductive Ability of Polyp and Medusa:
Reproductive ability of poly is asexual mode i.e by budding through evagination of a circular are of tissue including the endosperm and ectoderm, or they also reproduce sexually by spawning following the release of pheromones. Some of the polyps exist in hermaphrodite form. Budding in polyp can form either polyp or medusa. Whereas medusa reproduce sexually only and they give rise to medusa live stage cycle only.
4. Evolution Happened in Polyp and Medusa:
Medusa is the evolved form of poly as poly is the primitive form of life stage in cnidaria phylum. There is a large diverse feature between polyp and medusa which are evolved by passage of time, some of these features are medusa sexual reproductive ability, medusa free swimming ability as all these features are complex morphological features than that of polyp, as polyp lacks all these features. Presence of a photoreceptor is also an evolved feature of polyp.
Comparison between Polyp and Medusa:
Many species of the phylum Cnidaria have two separate life cycle stages: polyp and medusa. The phylum Cnidaria contains species that only exist in the polyp stage (Anthozoa), species that only exist in the medusa stage (Cnidaria), and species that occur in both life cycle stages (Cnidaria) (Hydrozoa).
Cnidarians include corals, sea anemones, jellyfish, and hydroids.
Polyp
A polyp is a sessile life cycle stage in the Cnidaria phylum of organisms. Polyps are seen in adult corals and sea anemones.
A polyp is made up of a tube with a mouth surrounded by tentacles. It is called a "head,". It also has a foot-like disc that is attached to the bottom of the tube. The mouth and tentacles are pointing in the direction of the water.
Polyps can reproduce either sexually or asexually. Separate sexes exist in corals, with some corals being males and others females, while other coral species are hermaphrodites, with one individual being both male and female.
Budding happens when a circular area of tissue, comprising the endoderm and ectoderm, is evaginated during asexual reproduction.
Sexual reproduction takes place by Spawning. Corals discharge a large number of sexual cells into the water after secreting pheromones at a specific moment.
Medusa
Medusa is a species' mobile life cycle stage in the Cnidaria phylum.
Hydrozoa species can take the form of a medusa or a jellyfish.
A medusa is formed morphologically by a bell capable of muscular contractions, allowing the medusa to swim.
There are tentacles with a different shape than polyps, photoreceptors, and gravity-detecting osteocytes that surround the bell.
Hydrozoa members also have a manubrium, which is a tube that hangs down from the bell and has a mouth at one end.
The gastric cavity is defined as the space between the manubrium's base and the bell. Medusa procreates sexually.
Medusa development differs among the Cnidaria phylum's classes. Medusa is created through budding Hydrozoa class members. Following the evagination of the endoderm and ectoderm, ectodermal epithelial cells proliferate near the bud's tip, generating an interior cavity.
The cavity then opens, tentacles grow, and the tissue that connects the medusa to the parent polyp constricts, allowing the freshly born medusa to emerge.
Medusa is created by the transformation of the polyp in other cnidarian classes, such as Cubozoa or Rhizostomae.
Polyp and Medusa: Major Differences
FAQs on Difference Between Polyp and Medusa
1. What is the polyp form in the phylum Cnidaria?
The polyp is one of the two primary body forms found in Cnidarians. It is typically sessile (fixed to a substrate), has a cylindrical or stalk-like body, and is oriented with its mouth and tentacles facing upwards. This structure is well-suited for a stationary, predatory lifestyle, capturing food that drifts by. Examples include sea anemones and the individual animals that form coral reefs.
2. What is the medusa form in the phylum Cnidaria?
The medusa form is the other primary body form in Cnidarians, commonly associated with jellyfish. It is typically motile (free-swimming) and has a bell-shaped or umbrella-shaped body. The mouth and tentacles hang downwards from the bell. This form allows the organism to move actively through the water, often by contracting its bell, to hunt or escape predators.
3. What are the main differences between a polyp and a medusa?
The main differences between the polyp and medusa forms are based on their structure, mobility, and reproductive roles:
- Mobility: Polyps are generally sessile and attached to a surface, while medusae are motile and free-swimming.
- Shape: Polyps have a tubular or cylindrical body, whereas medusae have a bell or umbrella-shaped body.
- Orientation: In polyps, the mouth and tentacles face upwards. In medusae, they face downwards.
- Reproduction: Polyps often reproduce asexually through budding to create more polyps or medusae. Medusae typically reproduce sexually to create larvae that develop into polyps.
- Mesoglea: The gelatinous layer called mesoglea is thin in polyps but thick and well-developed in medusae, giving them their jelly-like consistency.
4. What are some examples of organisms that exhibit polyp and medusa forms?
Many cnidarians exhibit either one or both forms. Examples include:
- Primarily Polyp Form: Sea anemones and Corals exist only as polyps. Hydra is another common example.
- Primarily Medusa Form: Most large jellyfish, like Aurelia (moon jelly), have a dominant medusa stage and a very brief, inconspicuous polyp stage.
- Both Forms (Alternation of Generations): Obelia is a classic textbook example that clearly shows an alternation between a colonial polyp stage and a free-swimming medusa stage.
5. How are the polyp and medusa stages related in the life cycle of a single cnidarian?
In many cnidarians, such as Obelia, the polyp and medusa stages are part of a single life cycle in a process called metagenesis, or alternation of generations. In this cycle, the sessile polyp colony reproduces asexually (through budding) to produce free-swimming medusae. These medusae then reproduce sexually, releasing eggs and sperm. The resulting fertilised egg develops into a larva, which settles down and grows into a new polyp, thus completing the cycle.
6. Why do some cnidarians have both a polyp and a medusa stage?
Having both a polyp and a medusa stage provides a significant evolutionary advantage. The sessile polyp stage is excellent for colonising a favourable area and reproducing asexually to increase numbers rapidly. The motile medusa stage allows for sexual reproduction and, more importantly, dispersal. By swimming to new locations, medusae ensure the species can spread over a wide geographical area, reducing competition and increasing the chances of survival if the original location becomes unsuitable.
7. Despite their differences, what fundamental structures do the polyp and medusa share?
Despite their opposite orientations and lifestyles, the polyp and medusa are homologous structures built on the same basic body plan. Both are diploblastic, consisting of an outer epidermis and an inner gastrodermis separated by a gelatinous layer called the mesoglea. Both have a single opening that functions as both a mouth and an anus, which leads into a central digestive cavity called the gastrovascular cavity. A medusa can be thought of as an upside-down, free-swimming polyp with a much thicker mesoglea.
