Polymorphism: Definition, Causes and Significance | Cnidarians

In this article we will discuss about Polymorphism in Cnidarians:- 1. Definition of Polymorphism 2. Causes of Polymorphism 3. Basic Units 4. Origin 5. Significance.

Definition of Polymorphism:

Polymorphism may be defined as the “phenom­enon of existence of different physiological and morphological forms represented by an extensive range of variation within a single species”.

It may be defined in another way, poly­morphism means “the existence of indi­viduals (zooids) of a single species in more than one forms and 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.

(A) Polyp form (L. Polypus = polyp) (Fig. 12.32A):

(i) Sedentary tubular form with one end closed.

(ii) Free conical end (preoral end) bear­ing hypostome, mouth and tentacles.

(iii) Aboral end fixed.

(iv) Mouth situated on hypostome lead­ing to coelenteron.

(v) Un-branched elongated tentacles sur­rounding the mouth.

(vi) The polyp may be encased by a trans­parent covering, the hydrotheca (e.g., Hydra).

(B) Medusoid form (Gk. Medousa = one who rules) (Fig. 12.32B):

(i) Umbrella-shaped with convex exumbrella and ventral concave subumbrellar surface.

(ii) Subumbrellar surface with mouth and manubrium.

(iii) Radial and circular canals present.

(iv) Marginal tentacles are present.

(v) Presence of gonads.

(vi) A velum is often present.

(vii) Free-swimming forms.

Remarks:

Polyps and medusae are consid­ered as homologous structures and can be theoretically derived from a sac-like body. Possessing of manubrium and mouth points to the basic similarity (Hyman, 1940).

These two forms alternate with each other in the life history of a typical cnidarian—the polyp pro­ducing medusa asexually and the medusa producing polyp sexually.

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:

I. 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.

II. 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.

III. 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.

Remarks:

This concept makes a compro­mise between the two previously de­scribed theories. It agrees with the sec­ond theory in asserting the colonial nature and also admits that asexual re­production and specialisation of certain parts of the colony, as advocated in the first theory.

IV. 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:

1. 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.

2. 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.