Q.8. What are coral reefs? Describe various types of coral reefs and the process of their formation. (2008, 18)
Ans. The building blocks of coral reefs are the generation of reef-building, and other organisms that are composed of calcium carbonate. For example, as a coral head grows, it lays down a skeletal structure encasing each new polyp. Waves, grazing fish (such as parrotfish), sea urchins, sponges, and other forces and organisms break down the coral skeletons into fragments that settle into spaces in the reef structure. Many other organisms living in the reef community contribute their skeletal calcium carbonate in the same manner. Coralline algae are important contributors to the structure of the reef in those parts of the reef subjected to the greatest forces by waves (such as the reef front facing the open ocean). These algae contribute to reef-building by depositing limestone in sheets over the surface of the reef and thereby contributing also to the structural integrity of the reef.
Reef-building or hermatypic corals are only found in the photic zone (above 50 m depth), the depth to which sufficient sunlight penetrates the water for photosynthesis to occur. The coral polyps do not photosynthesize, but have a symbiotic relationship with single-celled algae called zooxanthellae; these algal cells within the tissues of the coral polyps carry out photosynthesis and produce excess organic nutrients that are then used by the coral polyps. Because of this relationship, coral reefs grow much faster in clear water, which admits more sunlight. Indeed, the relationship is responsible for coral reefs in the sense that without their symbionts, coral growth would be too slow for the corals to form impressive reef structures. Corals can get up to 90% of their nutrients from their zooxanthellae symbionts.
Corals can reproduce both sexually and asexually. An individual polyp may use both reproductive modes within its lifetime. Corals reproduce sexually by either internal or external fertilization. The reproductive cells are found on the mesentery membranes that radiate inward from the layer of tissue that lines the stomach cavity. Some mature adult corals are hermaphroditic; others are exclusively male or female. A few even change sex as they grow.
Internally fertilized eggs are brooded in the polyp for a period ranging from days to weeks. Subsequent development produces a tiny larva, known as a planula. Externally fertilized eggs develop during a synchronized spawning. Polyps release eggs and sperm into the water simultaneously. This spawning method disperses eggs over a larger area. Synchronous spawning depends on four factors: time of year, water temperature, and tidal and lunar cycles. Spawning is most successful when there is little variation between high and low tides. The less water movement there is over the reef, the better the chance that an egg will be fertilized. Ideal timing occurs in the spring, release of eggs or planula larvae usually occurs at night and is sometimes in phase with the lunar cycle (3-6 days after a full moon). The period from release to settlement lasts only a few days, but some planulae can survive afloat for several weeks (7, 14). They are vulnerable at this time to heavy predation and adverse environmental conditions. For the lucky few which survive to attach to substrate, the challenge comes from competition for food and space.
Q.9. Give a detailed account of characters and affinities of ctenophora. (2013)
Related Questions -
Q. Write short note on Ctenophoran affinites. (2014)
Q. Write short note on Affinities of Ctenophora. (2018)
Ans. General Characters: -
1. Exclusively marine and planktonic organism. Body transparent with biradial symmetry and oral aboral surfaces.
2. Body-wall consists of three layers: the outer epidermis without namatocysts, the inner gastrodermis and the middle thick mesogloea with scattered cells and muscle-fibres.
3. Body with 8 vertical rows of comb plates for locomotion.
4. Digestive system consists of a mouth, a stomodaeum and a system of gastrovascular canals.
5. Special adhesive cells, the colloblasts on the tentacles help in food capture.
6. Coelom is absent.
7. There are no special organs for excretion, respiration and circulation.
8. Skeletal system is also absent.
9. Nervous system is in the form of subepidermal plexus. There is an aboral sense organ, the statocyst for equilibrium.
10. Sexes are united, i.e. male and female gonads are found in the same organism. Gonads are endodermal in origin.
11. Development includes a cydippid larva.
Affinities with Coelenterata: -
Ever since the time of Eschscholtz (1829 - 1833), ctenophores have been regarded as a class of phylum Coelenterata. Some writers still continue to group them as subphylum Acnidaria together with the subphylum Cnidaria in the phylum Coelenterata.
1. Resemblance with Coelenterata: -
Ctenophores are believed to be an offshoot from the ancestral medusoid coelenterate. They resemble the coelenterates in several characters, such as:
1. Biradial (tetramerous) symmetry and tentacles.
2. Arrangement of parts along an oral-aboral axis.
3. Two cell layers in bodywall, i.e., epidermis and gastrodermis.
4. Thick gelatinous mesogloea in betwen the two cellular layers.
5. Absence of coelom or other internal spaces except the digestive cavity.
6. Endodermal gastrovascular cavity with branches.
7. A single aperture or mouth leads from the gastrovascular cavity to the outside.
8. Diffused nerve plexus.
9. Presence of statocyst.
10. Endodermal origin of gonads.
11. Absence of nephridia.
12. Presence of lasso cells similar to the nematoblasts of the coelenterates. Exitence of namatoblasts in a ctenophore (Euchlora rubra) is additional evidence of the coelenterate origin of the Ctenophora.
13. No excretory, respiratory and circulatory systems present.
On account of these similarities they are considered by many zoologists to be a class of Coelenterata.
2. Resemblance with Hydrozoa: -
Attempts have been made to demonstrate affinities of Ctenophora with Hydrozoa. An anthomedusan from, Ctenaria, presents remarkable similarities with a cydippid, such as Hormiphora, in the following characters:
1. The general body surface of a ctenophore corresponds with the ex-cumbrellar surface of a medusa.
2. The pharynx or stomodaeum of a ctenophore corresponds with the sub-umbrellar cavity of a medusa.
3. Both possess a thick, gelatinous mesogloea.
4. Presence of a simple gastrovascular cavity.
5. Presence of two opposite tentacles in sheath.
6. Eight radial canals given off from stomach, and corresponding with them are eight bands of nematocysts diverging from the apex of ex-umbrella.
3. Resemblance with Anthozoa: -
The ctenophores also present certain anthozoan characteristics.
1. Ciliated ectoderm of Anthozoa is probably a forerunner of the ciliated band of ctenophore.
2. Presence of well-developed stomodaeum.
3. Gonads develop in connection with gastrodermis and sexual elements passed out through the mouth.
4. The gut in embryos of both is 4-lobed, thus presenting a biradial symmetry.
5. Mesogloea is cellular.
But the aboral sense organ and rows of comb-plates of a ctenophore (1) have no parallel parts in an anthozoan, (2) lasso cells differ structurally from the nematocysts, and (3) the tentacles are hollow in an anthozoan while solid in the ctenophore.
4. Differences from Coelenterata: -
The ctenophores, however, differ greatly from Coelenterates in having:
1. Oppositely placed tentacles which suggest a biradial symmetry.
2. An aboral sensory region.
3. Absence of nematocysts, except in one or two cases.
4. Presence of eight locomotory meridional ciliated bands of comb-plates over the body.
5. Presence of special adhesive cells and the colloblasts over the tentacles.
6. Mesenchymal muscles. With the development of a definite mesoderm, the ctenophotes may be considered triploblastic.
7. More definite organisation of the digestive system with anal pores.
8. Development of determinate type.