Polymorphism in Cnidaria

 

Polymorphism Cnidaria

 

Polymorphism, derived from Greek poly (many) and morphe (form), refers to the occurrence of different types of individuals or zooids within a single species.

Hydrozoan Polymorphism: 

Coelenterates, including Hydrozoa, exhibit polymorphism in two basic forms: the polyp and the medusa. These forms manifest in different variations but can be classified based on their function.

Polyp:  Polyp zooids have a tubular body with a mouth surrounded by tentacles. The other end is attached to a substrate via a pedal disc.

Medusa:  Medusoid zooids have a bell or umbrella-shaped body with marginal tentacles, and the mouth is centrally located.

Patterns of Polymorphism

 

Polymorphism varies among hydrozoans. It can range from simple to highly complex patterns.

1. Dimorphism: 

   In colonies like Obelia and Tubularia, two types of zooids are present:

   Gastrozooids (Hydranths): Responsible for feeding.

   Gonozooids (Blastostyles): Involved in asexual reproduction, producing medusae or gonophores.

 

2. Trimorphism: 

   In some species, a third type of zooid appears, called:

   Dactylozooids:  These individuals are specialized for defense and protection.

 

3. Polymorphism: 

   In more complex colonies like Hydractinia, five types of zooids can be found:

   Gastrozooids: For feeding.

   Dactylozooids: For protection.

   Tentaculozooids: Sensory function.

   Skeletozooids: Structural support through spiny projections.

   Gonozooids: Responsible for reproduction.

Polypoid Zooids

1. Gastrozooids: 

   These zooids handle feeding for the colony. They have a mouth and tentacles armed with nematocysts for capturing prey. The digested food is distributed through a shared canal system called the coenosarc.

2. Dactylozooids: 

   Also known as palpons, these zooids serve a protective role. Although they resemble gastrozooids, they lack a mouth and possess long, unbranched tentacles.

 

Medusoid Forms

 

1. Pneumatophores: 

   Pneumatophores act as floats. They are inverted medusan bells with a gas gland that secretes gas into an air sac. This structure helps colonies like Physalia (Portuguese man of war) stay afloat.

2.  Nectocalyx (Nectophores): 

   These bell-shaped zooids propel the colony by swimming. They do not have sensory organs or tentacles but are muscular, aiding in locomotion.

 3. Bracts: 

   Also called hydrophyllia, these leaf-like structures protect other zooids in the colony.

 4. Gonophores: 

   These zooids bear gonads responsible for producing sperm or ova.

Examples of Polymorphic Hydrozoans

Physalia (Portuguese Man of War): 

This free-floating colony has a large pneumatophore for buoyancy, with specialized zooids such as dactylozooids armed with stinging tentacles for capturing prey.

Porpita: 

Features a medusoid disc and concentric gas chambers, with zooids like gastrozooids and gonozooids for feeding and reproduction, respectively.

 

Theories on the Origin of Polymorphism

 

1. Poly-organ Theory: 

   This theory suggests that polymorphic colonies originated from a single medusoid individual, with various parts of the medusa evolving into distinct zooids.

 

2. Poly-person Theory: 

   According to this view, colonies are made up of modified individuals that evolved from a primitive polyp due to the division of labor.

 

3. Medusa Theory: 

   Proposed as a compromise, this theory suggests that siphonophores originated from a medusoid individual, with various zooids budding off from it.

 

Significance of Polymorphism

 

Polymorphism in Hydrozoa represents an evolutionary adaptation for survival. It allows colonies to perform specialized functions such as feeding, protection, and reproduction. This division of labor ensures efficiency, giving polymorphic colonies a competitive advantage in their environments.