POLYMORPHISM IN COELENTERATES

POLYMORPHISM IN COELENTERATES
 The presence of polymorphism in cnidarians is one of their characteristic features. It is defined as the occurrence of structurally and functionally different types of individuals within the same organism during its life cycle.
 A species that exhibits this phenomenon is called polymorphic.
 Polymorphism is predominantly exhibited by the different animals of class- hydrozoa.
 Hydroid colonies which bear two types of zooids are known as dimorphic, while colonies which bear more than two types of zooids are called polymorphic colonies.
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. Polyp: It is sessile (fixed to the substratum) with a hydra like body attached to the main colony by narrower end. Its free end is wider and raised into hypostome that bears a mouth surrounded by tentacles. It faces upwards and carries the function of feeding the colony. Polyps are specialized for feeding and thus known as gastrozooids.






 2. Medusa: It is a free swimming zooid with an umbrella shaped body having exumbrellar and subumbrellar surfaces. It has a mouth facing downwards (present on the tubular growth called manubrium hanging down from subumbrellar surface) in contrast to polyp in which mouth faces upwards.
 Like hydranth, it can feed on its own for its survival until its function is over. It bears four gonads (testis or ovary) which produces either sperms or ova at the time of maturity and is responsible for sexual reproduction therefore also known as gonozooid. They normally die after reproducing the colony. Therefore, they not only help in sexual reproduction but also play an important role in dispersal of the colony.



Importance of Polymorphism
 Polymorphism is essentially a phenomenon of division of labour. Different functions are assigned to different individuals, rather than to parts or organs of one individual.
 Thus, polyps are concerned with feeding, protection and asexual reproduction, while medusae are concerned with sexual reproduction.
Patterns of polymorphism

Degree of polymorphism varies greatly in different groups of Hydrozoa
1. Dimorphic. Simplest and commonest pattern of polymorphism is exhibited by many hydrozoan colonies like Obelia, Tubularia, Campanularia etc.
 They have only two types of zooids. Gastrozoids or hydranths are concerned with feeding. While gonozoids 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. Trimorphic. Some forms like Plumularia, are trimorphic. Besides Gastrozooids and gonozooids, they also possess a third type of individuals, the dactylozooids. These are functionally non-feeding and defensive polyps bearing batteries of nematocysts.
3. Polymorphic. Coelenterates having more than three types of individuals are called polymorphic.
 A somewhat greater degree of polymorphism is found in the encrusting colony of Hydractinia with five types of polyps each performing a specialized function.
These are: 1. Gastrozooids for feeding
2. Spiral dactylozooids for protection
3. long sensory tentaculozooids with sensory cells
4. skeletozooids as spiny projections of chitin
5. gonozooids or reproductive individuals, bearing male or female gonophores or medusae for sexual reproduction




Extreme examples of polymorphism are seen in the pelagic or swimming colonies of the orders
 Siphonophora (Diphes, Halistemmia, Stephalia, physalia) and Chondrophora (porpita, velella).
 Polymorphis reaches its peak in siphonophora.









(a) Modifications of polyps
Polyps structurally get modified into different types of zooids according to the requirement of an individual, which are described below:
i. Gastrozooids: These are feeding zooids and resemble the structure of polyp without usual tentacles. They are tubular, elongated, with a mouth facing towards the bottom of the colony.
ii. Dactylozooids: These are protective zooids. They may be also called as feelers or palpons. Structurally, they look like gastrozooids but are blind structures without any mouth.
iii. Gonozooids: They may resemble gastrozooids having mouth but are without tentacles and bear medusa. In other the gonozooids may form stalked branches bearing grape like structures called gonophores. Sometimes tentacles like dactylozooids are attached to them which are called gonopalpons.
iv.Pneumatophore:
It is a hydrostatic apparatus present in siphonophores. It is gas filled chamber that appear to be a highly modified polyps (although previously considered as derived from medusae).
 It helps in keeping the body in an upright condition while floating.
 It is without mesogloea but the umbrella cavity contains an air chamber called a pneumatocyst (Fig. 8a). Cells lining the pneumatocyst secrete the gases or may expel out of it through one or more small openings called stigmata.
 Thus pneumatophore is a balloon like structure or a hydrostatic chamber containing air.
 When pneumatophore is filled with air, the colony becomes lighter and floats at the surface of the water, but when the gas is expelled out of the pneumatophore, colony sinks down.






b. Modifications of medusae . The medusoid individuals are of the following types
1. Nectophore or nectocalyx or swimming zooid with a muscular bell without manubrium or tentacles.
2. Pneumatophore or float as a bladder-like medusa filled with secreted gas.
3. Phyllozooid or bract, usually leaf-like and studded with nematocysts, serving for protection of other zooids.
4. Gonophore bearing gonads, which may be either male, producing sperms, or female producing ova.









Origin of polymorphism
 As we have seen, colonies of Siphonophora represent the most specialized of Hydrozoa attaining the highest degree of polymorphism and presenting the greatest number of medusoid and polyploid types.
 There are two views regarding which came first, polyp or medusa, during the evolution of polymorphism in Coelenterata.
 According to one view, the ancestral coelenterate was a hydra-like polyp which arose from gastraea. It gave rise to hydroid colony by asexual budding. In the sessile colony some polyps became modified into medusae for sexual reproduction and pelagic life.
 Thus, through division of labour, the hydroid colony became polymorphic.
 According to second view (Brooks, 1886), which seems to be more acceptable, the ancestral coelenterata was a primitive medusa.
 It arose from metagastreae by developing tentacles and becoming free swimming.
 According to Huxley, and Metschnikoff, the manubrium, tentacles and umbrella of this p0rimitive medusoid individual were multiplied and shifted from their original positions to become various zooids of the polymorphic colony.
 According to this view, polyploid stage is considered the persistent larval stage and medusoid the completely evolved coelenterate.

















CORALS AND CORAL REEF FORMATION
 Meaning of corals: Coral animals or corals are marine, mostly colonial, polyploid coelenterates, looking like miniature sea anemones and living in a secreted skeleton of their own.
 Their calcareous or horny skeleton is also commonly known as coral.
 Some corals grow into massive, solid structures others form large, branched colonies.
 Most of the corals belongs to the class Anthozoa and a few to the class Hydrozoa of phylum coelenterata.
Structure of coral polyp:
1. soft structure
 A typical coral polyp from a colony is a small organism about 10mm long and 1to 3 mm in diameter. Solitary coral polyps are much larger reaching up to 25 cm in diameter.
 A basal disc is absent because the basal region of polyp is surrounded by calcareous exoskeleton.
 Oral disc bears numerous tentacles, in several rows around an elongated, oval or circular mouth. Pharynx or stomodaeum is short and without siphonoglyphs.
 Mesenteries are restricted to the upper part of coelenteron and mesenterial filaments contain only one glandular lobe bearing nematocysts.
 A typical coral polyp from a colony is a small organism about 10mm long and 1to 3 mm in diameter. Solitary coral polyps are much larger reaching up to 25 cm in diameter.
 A basal disc is absent because the basal region of polyp is surrounded by calcareous exoskeleton.
 Oral disc bears numerous tentacles, in several rows around an elongated, oval or circular mouth. Pharynx or stomodaeum is short and without siphonoglyphs.
 Mesenteries are restricted to the upper part of coelenteron and mesenterial filaments contain only one glandular lobe bearing nematocysts.







 Living polyps are found only on surface layers of coral masses. They feed at night both by raptorial and suspension feeding. When not feeding they withdraw into cup-like cavities of skeleton.
Structure of coral skeleton:
 Skeleton of a solitary coral is known as corallite. It is a calcareous exoskeleton secreted by epidermis.
 In a colonial coral corallites of individual polyps fuse together to form a skeletal mass, called corallum.
 Each corallite is like a stony cup with a basal part or basal plate, and a cup wall or theca, enclosing the aboral portion of the polyp.
 Cavity of cup contain a number of vertical radiating ridges called sclerosepta, proceeding from theca towards the centre of the cup.








 Inner ends of sclerosepta are fused to form an irregular central skeletal mass or columella.

Formation of coral skeleton:
 In coral polyps sexual reproduction takes place by fusion of gametes. Zygote develops into a free swimming ciliated planula larva which settles down and metamorphosis into a young coral polyp.
 There is no medusa stage. By asexual budding, single become s the parent of all other members of the colony.
 The coral polyp begins to secrete a skeleton rudiment or prototheca. It is secreted by ectoderm, first as a basal plate. Following it, radial folds develop which secrete sclerosepta. At the same time a rim is built up as a thecal wall around the polyp, lying at the top.
 Mean while further skeletol material is added into the gaps between sclerosepta of skeleton which usually alternates with mesenteries of the polyp.
 Coral colony grows in size continuously by budding of new polyps, particularly along the margins and on surface layers of coral masses. Variety in form of compound corals results due to cavities patterns of budding.


Coral reefs
Meaning of coral reefs:
 Coral colonies grow continuously in size by budding of polyps and often forms extensive masses, known as coral reefs.
 A coral reef is a ridge or mound of limestone, the upper surface of which near the surface of sea and which is formed chiefly of CaCo3 secreted by coral polyps.
 Principal builders of coral reefs are stony corals (Madreporaria), but other important contributors are the hydrocorallines and alcyonarians. Coralline algae and Foraminiferan Protozoa also take part in the formation of coral reefs.










 Reef building corals require warm shallow waters (normally above 200C). They are therefore limited to the Indo-Pacific, the Central Western Pacific, and the Caribbean regions north of Bermuda.
Kinds of coral reefs:
There are three types of coral reefs:
• a. Fringing reefs
• b. Barrier reefs
• c. Atoll
 a. Fringing reefs: Fringing reefs are developed in shallow waters on or near the shores of the volcanic islands. These are the simplest kind of reefs. They extend from the sea shore towards the sea as a platform ranging from few meters to half a kilometer and then slopes down towards the bottom of the sea








 Fringing reefs consist of several zones that are characterized by their depth, the structure of the reef, and its plant and animal communities.
Reef edge or reef front
Seaward slope
Reef flat
Boulder zone
Lagoon
b. Barrier reefs.
 A volcanic island with the fringing reefs is surrounded with a big channel of water called lagoon. Its depth may vary from 20 to 100 meters and even ships can pass through it. Lagoon is further surrounded by reefs called barrier reefs.
 As their name suggests, they act as a barrier for ships between sea shore and the main sea.
 Lagoon has fringing reefs towards the volcanic coast and barrier reefs on the other side of it. Sometimes both fringing reefs and barrier reefs may join each other at the bottom of the sea.
 There is a Great Barrier Reefs of Australia. It is not a single structure but is made up of many strings of separate reefs joined to each other at the bottom and thus forms a very big structure which extends along the north eastern coast of Australia for over 2000 kilometers.
 It is separated from the main land by a lagoon which is around 15 to 250 kilometers wide and 70 meters deep. During high tide, big ships can sail over it without realizing that reefs are present nearby and may crash. The Great Barrier Reef is the contribution of all different kinds of coral growth over the years.








 c. Atoll: Atolls are coral reefs which are present within sea water hundreds or thousands of kilometers away from the nearest sea shore.
 There is no volcanic island present. It is more or less circular or horse shoe shaped reef enclosing a central lagoon which may be 40 or 50 miles across and 20 to 90 meters deep.
 It may be a complete or broken into many small reef islands separated from each other by water channels.
 At some places reef is low so sea water simply covers it and reef is not visible. Sometimes a large atoll is formed by many small islets joined together along a line of reef. Thousands of such atolls are found in the South Pacific.
 It must be noted that reefs are not continuous rigid structures but they are broken up into many reefs and islands by water channels. Suvadiva is the largest atoll present in Maldives.
 Bikini Atoll has 2.87square miles land area with a lagoon area of 280 square miles. It was inhabited by the people but people moved to different places as it was selected by the United States for testing hydrogen and atomic bombs. Horse shoe shaped atoll of West Texas is 70 to 90 miles across and 1,000 meters thick.









Theories explaining the formation of coral reefs:
a. Darwin’s subsidence theory: Darwin believed that the reef began as fringing reefs on a sloping shore. Then the island subsides very slowly, so slowly that the reef grows upward at about the same rate, naturally the island becomes small, the channel between the reef and land widens and thus the fringing reef transforms into a barrier reef. Further subsidence of the land till it sinks completely out of site results in the formation of an atoll (Fig. 18). This is substantiated by the fact that all the known coral reefs were in regions where a sinking of the land was known to have taken place or where there were evidences that it had probably occurred.
b. Samper Murray solution theory: This theory states that calcareous skeletons of animals and other sediments form big mounds on the floor of the oceans. Over the time, these deposits grow to certain heights and corals grow on them and reach the water surface. Barrier reef is formed by the luxurious growth of coral at the outer edge while atoll is formed by dissolution of the inner coral rock.
c. Submerged bank theory This theory states that both barrier reefs and atolls grew upon pre-existing flat surfaces. Extensive coral growth occurred on a flat surface which got submerged in the water with the passage of time. Exposed regions formed the barrier reefs, while the shape of the atolls is obtained by the action of prevailing water currents and winds.













Significance of corals:
i. They show a high degree of physiological integration, division of labor, and perfect coordination with other groups of animals staying together, mutually benefiting each other in getting shelter, protection and food. Thus they constitute an ecologically important aquatic ecosystem.
ii. They help in studying the evolution of the animals as fossils of the animals are preserved in the coral reefs over the years.
iii. By studying the lines of growth on the fossils of some mollusk help in knowing about the seasonal fluctuations as the thickness of the growth line varies from season to season.
iv. A few stony corals because of the presence of minute pores are used in surgical procedures as human capillaries can easily pierce through equal sized pores which are helpful in interconnecting two bones with each other. These corals are being used in the surgery for bone grafts and jaw surgery etc.
v. Horseshoe shaped Atoll is the largest limestone reservoir for the oil production in North America.
vi. Many of the corals are precious stones which are used in making jewelry and have aesthetic value.
vii. Coral reefs are very hard structures and are an important source of mortar, cement, lime etc as they contain enough amount of CaCO3, therefore their rocks can be used for making roads and houses etc.
viii. Reefs are also a rich source for medical formulations, used to treat a wide range of diseases like asthma, heart diseases, and viral, fungal and bacterial infections. It has been reported in 2006 that Yellow coral (Isis hippuris) collected off the coast of Okinawa island of Japan has yielded a compound that can slow down and possibly prevent virus replication and also treat cancer.
ix. Lastly, corals act as affective buffers against erosion and storms occurring in the sea thus help in preventing tsunami disaster.