Updated by: Arabinda Naik
Polymorphism: Polymorphism is the phenomenon of occurrence of the same species of the organism in more than one form with different functions. Polymorphism is a Greek word, polys meaning many and morphe meaning form. It is more evident in class Hydrozoa and Anthozoa of Cnidaria/Coelenterate. Coelenterates are basically colonial forms. Individuals of a colony of coelenterates called zooids. Class hydrozoa is the best example of polymorphism.
These zooids are of two basic types: polyps and medusae.
Two Basic Forms
In Hydrozoa (or coelenterates), which may be single or colonial there occur two main types of individuals or zooids —polyps and Medusae.
1. Polyps. Polyps are cylindrical, sessile, and fixed forms. These have mouth Surrounded by tentacles and a wide gastrovascular cavity while the other end is usually attached by a pedal disc to the substratum. These are the nutritive zooids of the colony, so, feed the colony. Polyps are also called gastrozooids or trophozooids.
2. Medusae. Medusa has an umbrella-like body with marginal tentacles and a centrally located mouth on a concave side. Medusae are generally motile. They bear gonads and help in sexual reproduction. Therefore, these are called sexual zooids or gonozooids. They are free swimming and derived from polyps.
Hydromedusae. The medusae of class Hydrozoa are called hydromedusae. They are of following types:
l. Trachymedusae. Bell margin of these medusae is not scalloped (decorated with a series of small curves along the edges). These medusas contain long manubrium or pseudo manubrium. In them gonads are present on radial canals, e.g., Aglantha.
2. Narcomedusae. Bell margin of these medusae is scalloped. These medusae contain reduced radial canals and lack manubrium. In them gonads occur on the floor of gastric cavity e.g., Cunina.
3. Anthomedusae. These medusae are taller, retain the tetradiate form, have gonads on the manubrium and usually have ocelli, e.g., Sarsia, Pennaria and other gymnoblasts.
4. Leptomedusae. These medusae are flatter, may have lost their tetraradiate symmetry by branching of the radial canals, have gonads borne on these canals. They have statocysts the bell margin, e.g., Obelia, Tima, Aequora and other leptomedusae.
According to the types of zooids, coelenterates may be of following types:
l. Dimorphic having two types of zooids.
2. Trimorphic having three types of zooids.
3. Polymorphic having more than three types of zooids.
1. Dimorphism The coelenterate colonies bearing two types of individuals or zooids are called dimorphic Such colonies and the phenomenon is termed dimorphism. For example, Obelia, Tubularia. They have only two types of zooids (individuals). Gastrozooids or hydranths are concerned with feeding, while gonazooids or blastostyles with asexual budding forming sexual medusae or gonophores. Such colonies, bearing only two types of individuals are called dimorphic. and the phenomenon is termed dimorphism.
2. Trimorphism The coelenterate colonies bearing three types of zooids are called trimorphic colonies and the phenomenon is termed trimorphism. For example, Obelia colony has
(i) Polyps or gastrozooids
(ii) Blastostyles or blastozooids or gonozooids which bear medusae
(iii) Medusae or sexual zooids.
In Millipora, Plumularia, etc., the zooids are of following three types:
(l) Gastrozooids. These are the polyps or nutritive zooids.
(2)Dactylozooids. These are the protective zooids with long and knobbed tentacles. These are functionally non-feeding and defensive polyps bearing batteries of nematocysts.
(3) Medusae. These are gonads-bearing sexual zooids.
3. Polymorphism In some coelenterates both polyps and medusae are of various types and are modified carry out different functions of the colony. For example, siphonophores are invariably free swimming, colonial and polymorphic. The polyps and medusae are differently modified to present a large variety of zooids.
The polypoid zooids of siphonophores are of three types:
(i) Gastrozooids or feeding polyps are each with a mouth and a long tentacle.
(ii) Dactylozooids or protective polyps are without mouth and usually with a long unbranched basal tentacle. They bear many nematocysts and are also called palpons, tasters or feelers.
(iii) Gonozooids or reproductive polyps lack mouth and tentacles. They reproduce asexually by budding and form medusae. In Physalia, gonozooids are branched stalks, called gonodendra, and bear grape-like clusters of gonophores or medusae.
Similarly, the medusoid zooids are of following four types:
(I) Nectophores. They are also called nectocalyces, nectozooids or swimming bells. These are free-swimming zooids with swimming bells, velum, four radial canals and a circular canal. Nectophores do not possess mouth, manubrium, tentacles, and sense organs. They serve as the locomotory organs of the colony.
(2) Pneumatophores or floats. These are bladder-like, or vesicle-like structures filled with gas. Each pneumatophore represents an inverted medusa bell; it is devoid of mesoglea and consists of an external exumbrellar wall.
(3) Hydrophyllia. These are also known as bracts or phyllozooids. These are shield-like, leaf-like, helmet - shaped or prismatic covers. They are studded with nematocysts, serving for protection.
(4) Gonophores. They may occur singly on separate stalk or in clusters on polypoid gonozooids. They represent buds of immature or sessile medusae and are plate-like or bell-shaped. Gonophores are dioecious but the colonies are hermaphroditic bearing both types of gonophores in the same or separate clusters.
Polymorphism in Hydractina with five types of polyps, each performing a specialized function. These are.
(i) Gastrozooids tor feeding,
(ii) Spiral dactylozooids for protection,
(iii) Long sensory tentaculozooids with sensory cells
(iv) Skeletozooids as spiny projections of chitin, and
(v) Gonozooids or reproductive individuals, bearing male or female gonophores (sporosacs) Or medusae for sexual reproduction.
As in Hydractinia, both polypoid and medusoid individuals, specialized for various vital functions, occur in the same colony. Polymorphism reaches its peak in siphonophora.
Polymorphism in Halistemma Coelenterates of the order Siphonophora represent the most specialized hydrozoa attaining the highest degree of polymorphism. The composition and arrangement of various zooids is very different.
Halistemma. It possesses a long and slender stem. Its upper end is expanded into a bubble-like pneumatophore which help the animal float on the surface of water and the stem is divided into following two parts.
I. Proximal part. It is also called nectosome. Below the float a long stalk is present called coenosarcal stalk. It bears closely set bell-shaped swimming bells, nectocalyces or nectophores. They lack mouth, manubrium, tentacles, and sense organs. They are modified medusae, muscular and rhythmic contraction and relaxation of nectoclyces bring about locomotion for the colony.
2. Distal part. It is also called siphonosome. The coenosarcal stalk below the swimming bells is siphonosome. It carries repeated groups of closely set polyps, called cormidia which are arranged in group at regular interval just like nodes and internodes of a plant. Each cormidium consists of following types of zooids.
(i) Bract. It is the protective zooid and is also called hydrophyllium. It is in the form of flat scale or leaf-like bract.
(ii) Gastrozooid. It is the nutritive zooid. It bears mouth and long branched basal tentacles. They are helpful in the capturing the prey. The tentacles show branches called tentilla.
(iii) Dactylozooid or feeler. It is without mouth but with a single unbranched basal tentacle. It is a protective zooid, defensive in function.
(iv) Gonozooid. It is a reproductive zooid. It has either testes or ovaries.
Polymorphism in Phalysia
It is called Portuguese Man of war. The medusa is modified into a big pneumatophore which floats above the water. Below the pneumatophore stalk is absent. All zooids are arranged below the pneumatophore.
Pneumatophore: It is a big colourful. It is filled with gas. It contains gas glands. They secrete gas. With the help of the pneumatophore the colony floats on the water.
Gastrozooids: It is a modified polyp. It has a mouth at its tip. It has a long tentacle at its base. The tentacle bears nematocysts. They are helpful in capturing prey.
Dactylozooids: it is modified polyp. It has a mouth and tentacles. It bears numbers of nematocysts. Dactylozooids is defensive in function.
Gonozooid: A branched gonozooids with male and female gonophores is present.
Polymorphism in Porpita
This organism will show polymorphic nature. The pneumatophore is large and circular. It contains gas chambers inside. On the lower side of float a big gastrozooid is present. Around the gastrozooid several gonozooids are present. Gastrozooid is a modified polyp. It has a mouth at its tip. It takes up the function of nutrition. The gonozooids are useful for production. On the rim of margin of the disc several tentaculozooids are present. They are called feelers. They are defensive in function. Thus, porpita shows polymorphic tendency. Body is discoidal, enclosing a chambered, chitinous, and porous shell, having air corresponding with pneumatophore of Physalia. Margin of disk contains numerous dactylozooids or tentacles armed with nematocysts. Nectocalyces or swimming bells are absent. Each air chamber opens to outside by a pair of pores. Space between the gastric cavity of the gastrozooid and the pneumatophore is occupied by a large cellular mass traversed by canals, the so-called liver, supposed to have an excretory function. The reproductive zooids are liberated as free medusae.
Polymorphism in Velella
Velella is referred as sail by wind animal. It is most beautiful open-sea form. Deep blue colony consists of a rhomboidal disk pneumatophore or float containing air chambers without marginal indentations. Float differs from Porpita having a clear transparent vertical ridge or crest passing obliquely across the float and called as sail. Both float and sail are filled with gases. On ventral surface hangs a single large gastrozooid and edge of disk is beset with numerous dactylozooids or tentaculozooids. Gonozooids are found around gastrozooid. Gonozooids produce free medusae. Velella is one of the most beautiful open-sea forms.
Forms of Polymorphism
Polymorphism has two main forms: metabolic and reproductive.
I. Metabolic form. Metabolic form of polymorphism is represented by the zooids having vegetative functions such as feeding, swimming and defence. It includes gastrozooids, dactylozooids pneumatophores, nectocalyces, etc. All these individuals are polypoid.
2. Reproductive form. Reproductive form of polymorphism is represented by the zooids having reproductive function. It includes the asexual zooids, such as blastostyles or gonozooids and sexual zooids such as gonophores or medusae. The gonozooid is polypoid while the medusae is medusoid.
Regarding the origin of polymorphism, there are many views. According to one view the original coelenterate was a polyp and through specialization the sexual function was relegated to secondarily developed medusoid form and this led to alternation of generation. According to another view, the ancestral coelenterate was a medusoid form, while the polypoid generation represent a persistent larval form, thus, leading to polymorphism.
Theories of Origin of Polymorphism
Several theories have been put forward by various zoologists; some of them are as follows:
1. Polyorgan theory. This theory was put forward by Huxley, Eschscholtz and Metschnikoff. According to this theory individuals of a colony are organs of a medusoid individual. Certain organs of these zooids such as manubrium, tentacles and umbrella have become multiplied independently and have assumed different forms to perform different functions. Thus, a polymorphic individual is formed. According to this theory the polymorphic individual is a single organism.
2. Polyperson theory. This theory was put forward by Leuckart, Vogt and Gegenbaur. Colony is not a single individual, but various parts of colony are modified individuals. This theory maintains that the part of a polymorphic colony is either polyp or medusae, but the primitive zooids of the colony are of polyp type.
3. Medusome theory. This theory was proposed by Haeckel (1888) as a compromise between the above theories. The theory says that the siphonophore larva formed from gastrula was a medusoid individual, from which zooids appeared by budding from the subumbrella.
Importance of Polymorphism
1. Polymorphism is essentially a phenomenon of division of labour.
2. Different functions are assigned to different individuals, rather than to parts or organs of one individual.
3. Thus, polyps are concerned with feeding, protection, and asexual reproduction. while medusae are concerned with sexual reproduction.