In this article we will discuss about the conditions for the growth of coral polyps.
(1) Corals are found mainly in the tropical oceans and seas because they require high mean annual temperature ranging between 68°F and 70°F (20°C- 21°C) for their survival. It may be pointed out that they cannot survive in the waters having either very low temperature or very high temperature.
(2) Corals do not live in deeper waters i.e. not more than 200-250 feet (60-77m) below sea-level because they die in waters deeper than 77m due to lack of sufficient amount of sunlight and oxygen which are very much required for the growth of coral polyps. According to M.S. Land and J.E. Hoffmeister (1936) the maximum depth for ideal growth of corals is 200 to 300 feet (60m to 91m) below sea-level while Gardiner located some corals thriving at the depth of 150 to 170 fathoms (273m to 310m) below sea-level.
(3) There should be clean sediment-free water because muddy water or turbid water clogs the mouths of coral polyps resulting into their death.
(4) It may be pointed out that though coral polyps require sediment-free water but fresh water is also injurious for the growth of corals. This is why corals avoid coastal lands and live away from the areas of river mouths.
(5) Very high proportion of oceanic salinity is injurious for the growth of coral polyps because such waters contain little amount of calcium carbonates whereas lime is important food of coral polyps. The oceanic salinity ranging between 270/00 and 30%, is most ideal for the growth and development of coral polyps.
(6) Ocean currents and waves are favourable for corals because they bring necessary food supply for the polyps. It is obvious that corals grow in open seas and oceans but they die in langoons and small enclosed seas because of lack of supply of food. Currents and waves also determine the shapes of coral reefs.
(7) There should be extensive submarine platforms for the formation of colonies by the coral polyps.
Such platforms should not be more than 50 fathoms (300 feet or 91 m) below sea-level. The polyps start their colonies from a firm base of hard rocks and grow upward until they reach the sea-level. Besides, polyps also grow outward from the submarine platforms.
(8) Human economic activities viz. deforestation, industrialization etc. causing global warming adversely affect corals in their habitats. Corals are more susceptible to long-term climatic change. Corals are generally termed as rainforests of the oceans. These cannot survive in extreme warm environment. The scientists claim that about 10 per cent of the corals have died and become skeletons due to global warming caused by anthropogenic factors mainly industrialization.
According to report published in Down to Earth (August 15, 1999) 30 per cent of corals are in critical condition and a further 30 per cent are under severe environmental stress. According to the report of the United Nations’ Inter-Government Panel on Climate Change (IPCC) ‘If the projected levels of climate change are not stopped, the doom may be just 30 year away’.
The increase in temperature causes bleaching in the corals wherein the corals lose their algae and become white in colour. This process is called coral bleaching, which causes death to corals. According to Clive Wilkinson of the Global Coral Reef Monitoring Network (GCRMN) coral bleaching has occurred at large scale off the coasts of West Asia, East Africa, South, Southeast and East Asia, in the Indian Ocean, East Pacific, the Caribbean Sea and the Atlantic Ocean (1998).
According to him the Indian Ocean is the most adversely affected region wherein ‘more than 70 per cent mortality has been observed off the coasts of Kenya, the Maldives, the Andamans and the Lakshadweep islands. The studies have shown that coral bleaching begins when the temperature rises 1°C above normal temperature.
The year 1998 has been reported to be the warmest year in the last 1200 years. The temperature in the Indian Ocean was recorded 2°C higher than the normal temperature in 1998. El Nino phenomenon has also been associated with coral bleaching (coral death). El Nino was the strongest on record in 1997-98 and hence caused large-scale bleaching of corals.
(9) Besides global warming, human activities at local to regional levels such as pollution of oceanic water through excess flux of sediments and nutrients, industrial effluents, urban wastes, sewage; over fishing; clearance of maritime forest and filling of wetlands; mining of coral rocks; collection of rare species etc. cause fatal diseases to corals. Recent studies have shown that 58 per cent of the world’s coral reefs are threatened by human activities.