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Cnidaria: Polymorphism | Zoology Optional Notes for UPSC PDF Download

Polymorphism refers to the presence of diverse physiological and morphological forms within a single species, encompassing a wide range of variations. Another definition of polymorphism is when individuals (zooids) of a particular species exhibit multiple forms and perform different functions.

Causes of Polymorphism

Polymorphism is due to the division of labour, diversifica­tion of forms and specialization. Two general types of interactions, viz., co-operation and disoperation are exhibited by the members of an animal association. In the colonial forms, disoperation ceases gradually and is replaced by co-operation.  Finally the whole colony appears as a single individual, and the zooids function collectively for the inter­est of the colony (Barrington 1979).

Basic Units of Polymorphism

All forms of zooids can be divided into two fundamen­tal forms which can be derived from each other.

Polyp form (L. Polypus = polyp)

  • Sedentary tubular form with one end closed.
  • Free conical end (preoral end) bear­ing hypostome, mouth and tentacles.
  • Aboral end fixed.
  • Mouth situated on hypostome lead­ing to coelenteron.
  • Un-branched elongated tentacles sur­rounding the mouth.
  • The polyp may be encased by a trans­parent covering, the hydrotheca (e.g., Hydra).

Medusoid form (Gk. Medousa = one who rules)

  • Umbrella-shaped with convex exumbrella and ventral concave subumbrellar surface.
  • Subumbrellar surface with mouth and manubrium.
  • Radial and circular canals present.
  • Marginal tentacles are present.
  • Presence of gonads.
  • A velum is often present.
  • Free-swimming forms.

Origin of Polymorphism

Polymorphism in cnidarians is virtually regarded to be the division of labour, where different zooids perform diverse functions. As regards the origin of polymorphism in cnidarians, the following theories have been advanced:

  • Poly-organ theory: The main support­ers of the theory are Huxley, Metschnikoff and Eschscholtz. They regard that each poly­morphic colony is an individual and the polyps or medusae, which are budded off from it, are the organs. 
  • Poly-person theory: The supporters of this theory are Vogt, Leuckart, Gegenbaur, Cuhn and Kukenthal. This theory suggests that cnidarian colony is constituted of inde­pendent and separate individuals which re­main in organic connection with one another. According to this view each zooid is a sepa­rate individual, where some portions may be either lost or obliterated in course of time. 
  • Medusa theory: This theory is for­warded by Haeckel, Balfour and Sedgwick. The theory advocates that the primitive zo­oid of polymorphic colony was, with all probabilities, a medusa which produced other medusae by the process of budding.
    These medusae possess the power of locomotion as well as the power of reproduction. In this view many organs of the colony are nothing more than the parts of such medusoid indi­viduals which have subsequently shifted their attachments from the original medusa.
  • Theory of neoteny (supported by A. C. Hardy): Garstang first postulated the idea of the neotenous retention of larval characters and the members of Siphonophora giving rise to polymorphism.

Significance of Polymorphism

  • Polymorphism is intimately associated with life-history. The life cycle is simple in the monomorphic forms (e.g., Hydra). With the advent of polymorphism reproductive powers are divided. 
  • The polyp is capable only of asexual reproduction while sexual reproduction is confined to the gonophores. Thus arises the alternation of generation or metagenesis.
  • Polymorphism is also concerned with the division of labour. So polyp are mainly associated with the function of feeding, test­ing, protection and also asexual reproduc­tion while medusa is concerned with sexual reproduction.
The document Cnidaria: Polymorphism | Zoology Optional Notes for UPSC is a part of the UPSC Course Zoology Optional Notes for UPSC.
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FAQs on Cnidaria: Polymorphism - Zoology Optional Notes for UPSC

1. What are the causes of polymorphism in Cnidaria?
Ans. Polymorphism in Cnidaria can be caused by various factors such as genetic variation, environmental conditions, and evolutionary pressures. Genetic variation can lead to the expression of different traits within a population, resulting in polymorphism. Environmental conditions, such as temperature or food availability, can also influence the development of different forms or morphs. Additionally, natural selection and evolutionary pressures can favor certain morphs over others, leading to the maintenance of polymorphism in Cnidaria populations.
2. How does polymorphism originate in Cnidaria?
Ans. Polymorphism in Cnidaria originates through a combination of genetic variation and natural selection. Genetic variation arises through mechanisms such as mutation, recombination, and gene flow, leading to the presence of different alleles within a population. These genetic variations can result in the expression of different phenotypic traits, giving rise to polymorphism. Natural selection then acts upon these different traits, favoring those that provide a survival or reproductive advantage in a particular environment. Over time, this process can lead to the establishment and maintenance of polymorphism in Cnidaria populations.
3. What is the significance of polymorphism in Cnidaria?
Ans. Polymorphism in Cnidaria is significant as it allows for increased adaptability and survival in changing environments. Different morphs or forms within a population may possess specialized traits that enable them to exploit different ecological niches or respond to specific environmental conditions. This diversity of forms can enhance the overall resilience and evolutionary potential of Cnidaria populations. Polymorphism also plays a role in various ecological interactions, such as defense mechanisms, prey capture, and reproductive strategies, contributing to the overall biodiversity and functioning of marine ecosystems.
4. Are there any specific examples of polymorphism in Cnidaria?
Ans. Yes, there are several examples of polymorphism in Cnidaria. One well-known example is found in the class Anthozoa, which includes sea anemones and corals. Within this class, there are various morphs with distinct feeding strategies. Some morphs have long, stinging tentacles specialized for capturing small prey, while others have shorter tentacles adapted for filter-feeding on plankton. Another example is the Portuguese man o' war (Physalia physalis), which is not a true jellyfish but a colonial organism composed of specialized polyps. The colony exhibits polymorphism with specialized polyps for feeding, reproduction, and defense.
5. How does polymorphism contribute to the survival and evolution of Cnidaria?
Ans. Polymorphism contributes to the survival and evolution of Cnidaria by increasing their adaptability and resilience in changing environments. The presence of different morphs within a population allows for the exploitation of diverse ecological niches and resources. This increases the chances of survival and reproductive success for the population as a whole. Additionally, polymorphism provides the raw material for natural selection to act upon, allowing for the adaptation and evolution of Cnidaria species over time. The maintenance of polymorphism in Cnidaria populations ensures their ability to respond to environmental changes and persist in dynamic marine ecosystems.
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