The question asks to identify the primary organisms responsible for building coral reefs. This requires knowledge of the biological components of coral reefs.
B) Tiny marine animals called polyps — Coral polyps are small, soft-bodied invertebrates that belong to the phylum Cnidaria. They live in colonies and secrete calcium carbonate (limestone) to form a hard, protective exoskeleton. Over thousands of years, the accumulation of these skeletons forms the massive structures known as coral reefs.
The question asks about the primary chemical composition of the hard skeleton of coral reefs. This is a factual question related to marine biology and geology.
Correct Option: B) Calcium carbonate is the primary material that forms the hard skeleton of coral reefs. Coral polyps extract calcium ions and carbonate ions from seawater to build their exoskeletons, which are chemically represented as \(\text{CaCO}_3\).
The question asks to identify which condition is NOT essential for coral reef formation. We need to evaluate each option based on the known requirements for coral growth and survival.
Correct Option: C) Fresh water inflow from rivers. This is not an essential condition; rather, it is detrimental to coral reef formation and survival because corals require saline water and are sensitive to reduced salinity.
The question asks about the optimal water temperature range for coral polyps to thrive. Coral reefs are sensitive ecosystems, and their survival is highly dependent on specific environmental conditions, including temperature. We need to identify the temperature range that best supports coral growth and health.
Correct Option: C) 20°C to 25°C
This range falls within the optimal temperature conditions for most coral species. Corals are typically found in warm tropical and subtropical oceans, where water temperatures generally stay within this bracket, allowing for efficient photosynthesis by zooxanthellae and healthy coral growth and calcification.
The question asks about the optimal depth for coral reef formation. Coral reefs are complex ecosystems that thrive under specific environmental conditions, including light penetration, temperature, and water clarity. Understanding these requirements is key to determining the correct depth range.
Correct Option: C) Less than 50 metres. Coral reefs thrive in shallow, clear, sunlit waters, typically at depths of less than 50 meters, where their symbiotic algae (zooxanthellae) can perform photosynthesis effectively. Some corals can be found deeper, but the most robust and diverse reef formations occur in this upper zone.
The question asks to identify the largest coral reef system in the world. This requires knowledge of major coral reef systems and their relative sizes.
The Great Barrier Reef, located off the coast of Queensland, Australia, is the world's largest coral reef system. It is composed of over 3,000 individual reefs and 900 islands stretching for over 2,300 kilometers (1,400 miles) and covering an area of approximately 344,400 square kilometers (133,000 square miles). Its immense size and biodiversity make it a globally significant natural wonder.
B) Great Barrier Reef is the correct answer because it is universally recognized as the largest coral reef system in the world, both in terms of area and length.