Polymorphism in Coelenterata

Aristotle Knew the stinging qualities of coelenterates, considered these organisms intermediate between plants and animals, and termed them Acalephe or cnide (Gr., akalephe =nettle; condos = thread). The animal nature of coelenterates was established by Peyssonnel (1723) and Trembley (1744).

1.0Characteristics of Coelenterata  

• Coelenterates are Metazoa or multicellular animals with tissue grade of organization. These are aquatic, mostly marine, except for a few freshwater forms like hydra.  
• These are sedentary or free-swimming and solitary or colonial.  
• Individuals are radially or bi-radially symmetrical, with a central gastrovascular cavity communicating to the exterior by the mouth.  
• Diploblastic animals' body walls consist of an outer layer of cells called ectoderm and an inner layer of cells called the endoderm.  
• These animals exhibit the phenomenon of polymorphism.  
• Acoelomate animals because they do not possess a second body cavity, the coelom. Short and slender tentacles are provided with nematocysts; they encircle the mouth in one or more whorls and help capture food, ingestion, locomotion, and protect it.  
• These are usually carnivorous; digestion is extracellular as well as intracellular. Anus is not found. 
• The nervous system is primitive, consisting of diffuse nerve nets.  
• Respiratory, circulatory and excretory systems are wanted.  
• A ciliated planula larva is usually present in life history.  
• The life history exhibits the phenomenon of alternation of generations or metagenesis, in which the asexual polypoid, sessile generation alternates with the sexual medusoid, free-swimming generations.

2.0Classification of Coelenterata

The phylum includes nearly 11000 known species, half of which are extinct. According to Hyman, the Phylum Coelenterata has been divided into three classes: Hydrozoa, Scyphozoa, and Anthozoa.

1. Class: Hydrozoa (Gr: Hydra=water, zoon=animal)

• Hydrozoa are solitary and freshwater or mostly colonial and marine, sessile and free-swimming forms. 
• They exhibit tetramerous or polymerous radial symmetry. The body wall consists of an outer ectoderm and an inner endoderm separated by non-cellular mesogloea. 
• Gastrovascular cavity without stomatoeum, septa or nematocysts bearing gastric filament. 
• Skeleton or horny structure is horny peri- sarc in some forms, while coenosarc sec-retes a skeleton of calcium carbonate forming massive stony structure or coral in other forms. Ex: Hydra, Obelia, Porita, Physalia (Portuguese man of war), Velella (Little sail).

2. Class: Schyphozoa: Gr: Skyphos= cup, Zoon= animal 

• Scyphozoa comprises giant jellyfish or true medusae found exclusively in marine environments. Medusae are typically large, bell-shaped or umbrella-shaped, lacking a true velum, and they can be free-swimming or attached by an aboral stalk.
• Marginal sense organs are tentaculocyst having endodermal statoliths. 
• Polypoid generation is absent or represented by a small polyp, the scyphistoma, which gives rise to medusae by stabilization or transverse fission. 
• Gonads are endodermal, and the sex cells are discharged. 
• Ex: Aurelia, Cassiopea, Rhizostoma, Lucernaria, Periphylla. 

3. Class: Anthozoa gr: anthos=flowers, zoon = animal or actinozoa: gr: actin = ray, zoon = animal. 

• Solitary or colonial exclusively marine forms and exclusively polyploids. 
• The medusoid stage is altogether absent. The body is usually cylindrical with hexamerous, octamerous or polymerous biradial or radio bilateral symmetry. 
• The stomodaeum is present, often provided with one or more ciliated grooves, the siphonoglyphs. 
• The gastrovascular cavity is segmented into compartments by complete or incomplete septa or mesenteries. These mesenteries have nematocysts along their inner free edges. The exoskeleton, composed of calcium carbonate, frequently forms large coral structures. 
• Ex: Metridium(Sea anemone), Adamsia (sea anemone), Corallium (Precious coral or red coral), Gorgonia (Sea fan), Pennatula (Sea pen). 

3.0Polymorphism in Coelenterata

• In Coelenterata, particularly in Hydrozoa, which can be solitary or colonial, there are two primary types of individuals or zooids: polyps and medusae. Polymorphism (Gr., poly = many; morphe = form) refers to the occurrence of multiple distinct forms or zooids within a single species during its life cycle or as colony members. 
• These different forms perform specific functions, leading to a division of labor among the members. Thus, polymorphism can be defined as the presence of more than one type of individual or zooid within a single organism, each differing in form and function.

Polyp:  It has a tubular body with a mouth surrounded by tentacles at one end. The other end is closed and usually attached to the substratum by a pedal disc.

Medusa: A medusa has a bowl or umbrella-shaped body with marginal tentacles and a centrally located mouth or manubrium.

Patterns of polymorphism

The degree of polymorphism varies significantly among different groups of Hydrozoa. 

Dimorphic -  The simplest and most common polymorphism pattern is seen in many hydrozoan colonies such as “Obelia” and “Tubularia”. These colonies have two types of individuals or zooids:

Gastrozooids (Hydranths): Responsible for feeding.

Gonozooids (Blastostyles):  Involved in asexual budding to form sexual medusae or gonophores. This phenomenon, involving two distinct types of zooids, is termed dimorphism.

Trimorphic- Some hydrozoan colonies exhibit a third type of individual in addition to gastro zooids and gonozooids. The presence of three types of zooids is referred to as trimorphism.

1. Gastrozooids (Hydranths)  and 
2. Gonozooids (Blastostyles), 
3. Dactylozooids:  Specialized for defense are also present.

Polymorphic - In some hydrozoan colonies, more than three types of individuals are known as polymorphism . An example of greater polymorphism can be found in the encrusting colony of Hydractinia , which has five distinct types of polyps, each performing a specialized function.

• Gastrozooids - feeding 
• Dactylozooids - protection. 
• Tentaculozooids - Sensory cells 
• Skeleton Zooids - Spiny projections of chitin 
• Gonozooids - Reproductive individuals.

4.0Origin of Polymorphism

Theories Explaining the Origin of Polymorphism in Coelenterates

Poly-organ Theory:

• This theory, proposed by Huxley (1859), Eschscholtz (1829), E. Metchnikoff (1874), and Muller (1871), suggests that a polymorphic colony originates from a single medusoid zooid. The various components of the colony are considered to be modified organs of this medusoid zooid. According to this theory, parts of the zooid, such as the manubrium, tentacles, and umbrella, multiply independently and have evolved into different forms to perform distinct functions.

Poly-person Theory:

• First proposed by Leuckart (1851), Vogt (1848), Gegenbaur (1854), Kolliker (1853), and Claus (1863), and later strongly supported by E. Haeckel (1888), Balfour (1885), and Sedgewick (1888), this theory posits that a colony is composed of multiple modified individuals rather than a single organism. According to this view, the various parts of the colony are derived from primitive zooids, specifically polyps, which have altered their structure to specialize in different functions due to the division of labor.

Medusa Theory:

• Haeckel (1888) proposed this theory as a synthesis of the viewpoints above. According to this theory, siphonophores originated from a gastrula that developed into a medusoid individual. Zooids or individuals within the colony then emerged through budding from the subumbrella of this medusoid.

5.0Significance of Polymorphism in Coelenterates

• Polymorphism in coelenterates represents a division of labor where specific functions are allocated to different specialized individuals within a colony. For instance, polyps are adapted for tasks such as feeding, protection, and asexual reproduction, whereas medusae primarily focus on sexual reproduction. This distribution of functions and the subsequent adaptations among diversified individuals likely arose from their initial simple organizational structure and lack of specialized organs.
• Polymorphism provides coelenterate colonies with a competitive edge in survival, particularly regarding protection and food gathering. By having morphologically distinct individuals specialized for specific roles—feeding, reproduction, and defense—these colonies can effectively exploit their environment and enhance their overall survival prospects.