Coral Reefs under threat
Not all Cnidarians are soft-bodied, and some produce skeletons of limestone in a similar way to the sea sponges and are better known as corals. These animals secrete their skeletons from their base. Each polyp is connected with its neighbours by strands that extend laterally. As the colony develops new polyps form, leaving a limestone skeleton that is riddled with tiny cells were polyps once existed. Live polyp are restricted to a thin surface layer. The size of these colonial polyps are enormous and create entire coral islands called atolls and created the Great Barrier Reef running parallel to the east coast of Australia. This coral reef extends for over a sixteen hundred kilometres and is the greatest animal construction prior to man's artefacts.
Humans continue to represent the single biggest threat to coral reefs. In particular, land-based pollution and over-fishing are the most serious threats to these ecosystems. The live food fish trade has been implicated as one driver of decline due to the use of cyanide in the capture of fish. Rising water temperatures produce toxins in the coral tissue, due to bleaching.
High levels of land development have also been threatening the survival of coral reefs. Within the last 20 years, the once thick mangrove forests, which absorb massive amounts of nutrients from runoff caused by farming and the construction of roads, buildings, ports, channels, and harbors, are being destroyed. Nutrient-rich water causes algae to thrive in coastal areas in suffocating amounts, also known as algal blooms.