A coral is an invertebrate animal that belongs to the phylum Cnidaria. Cnidarians come in a variety of shapes, sizes, and colors. They have a relatively simple structure with a single opening surrounded by stinging tentacles. Corals do not live singly but in massive colonies. A single coral is called a polyp and it multiplies by budding to generate thousands of genetically identical copies of itself that live together as a colony. Corals are classified as hard corals and soft corals with the former also being known as reef-building corals. Hard corals have a rock-like calcareous skeleton which the soft corals lack. About 27% of the world's reef-building corals are threatened.
Formation of Coral Reefs
Over centuries, colonies of hard corals aggregate to from coral reefs. These reefs are thus large carbonate structures formed of millions of tiny polyps. The coral reefs provide the habitat for the survival of hundreds to thousands of other marine species. They are some of the busiest fish spawning grounds in the oceans. The species living here are part of a massive food web that also attracts larger species like whales and sharks to such reefs. Thus, coral reefs are vital to the survival of many marine species.
Over millions of years of their existence, corals have developed a symbiotic relationship with microscopic single-celled plants or algae called zooxanthellae. These algae live within the polyp tissue and receive protection and photosynthesis raw materials from the polyp. In turn, the products of photosynthesis performed by the algae are used by the polyp as nutrients to synthesize proteins, fats, calcium carbonate, and carbohydrates. The oxygen produced during photosynthesis also helps the polyp to detoxicate itself. Thus, both organisms benefit each other from this symbiotic relationship. The coral-algae relationship is thus one of the most productive ones in the living kingdom and helps in nutrient recycling in nutrient-poor tropical waters. The bright colors of the corals are also derived from the algae living inside them.
From the above discussion, it becomes clear that the coral-algae relationship is vital to the survival of both. However, when the corals are stressed, they release the algae sheltered within them. Such corals often appear bleached or white in color. The process is called “coral bleaching.” It indicates the health of the ecosystem is at stake. In the absence of the zooxanthellae, corals start dying and so does the reef formed by the coral. Ultimately, all life in the ecosystem ceases to exist.
Optimal Environmental Conditions
Reef-building corals are extremely sensitive to environmental changes. Water temperatures below 18° Celsius will kill the corals. Different corals have different optimal temperatures for survival with most growing optimally between 23° and 29° C. Most coral species also require high salinity levels in the waters they inhabit with salinity typically ranging from 32 to 42 parts per thousand. Clear waters are also necessary for the survival of reef corals as sunlight passing through such waters allows the algae living in these corals to perform photosynthesis. Thus, most reef-building corals are found in the euphotic zone of the ocean.
Factors Destroying Corals
Climate change is currently the biggest threat to corals. With global warming, ocean temperatures are expected to rise. Warmer oceans will trigger the death of corals in large numbers.
Oceans are regarded as the carbon sink of the planet. The huge volumes of carbon-dioxide released into the atmosphere are absorbed by the oceans keeping the temperatures low. However, as human activities are releasing increasing amounts of carbon dioxide, the oceans are absorbing more of this gas and turning more acidic. The lowered pH of ocean waters adversely affects the ability of corals to form their calcareous shells that in turn destroys the coral colonies.
Pollution of ocean waters also destroys coral reefs. Pollution increases water turbidity that decreases the amount of sunlight reaching the corals. The algae are unable to photosynthesize in the absence of adequate light. Pollution also stresses the corals causing them to expel algae living within them. Coral bleaching then leads to the death of the corals.
Destructive fishing and degradation of coastal habitats have also led to the death of corals.
The Present Status of Reef-Building Corals
Recent studies regarding the status of reef-building corals have yielded shocking results. One-third of these corals have been found to be threatened with extinction. 1% of the 868 reef-building coral species that have been studied is critically endangered. 3% of these species are endangered, 23% are vulnerable, and 20% are near threatened.
According to scientists, Staghorn corals are at the greatest risk of extinction. 52% of species belonging to this group are in a threatened category. Most of the Endangered and Critically Endangered coral species can be found in the Caribbean region which shows that the corals in this region receive little protection. The high population density of humans in the Indo-Malay-Philippine Archipelago region in the western Pacific Ocean also threatens the coral biodiversity in the area.
Species from the genera Porites and Favia appear to be the least threatened as they can withstand bleaching to a greater extent than other types of corals. They are also less susceptible to coral diseases. 34% of the coral species studied were placed in the Least Concern category while 19% lacked sufficient information for any classification.
The Reef-Building Corals Must Be Saved
The reef-building corals of the world are essential to both humans and many species of marine flora and fauna. Humans depend on the coral reefs as a source of fish. Fishing in the coral reefs supports the livelihood of thousands of people across the world. Coral reefs also protect coasts from damage by waves and winds. They are important to the tourist industry as snorkeling, scuba diving, boating, etc., are popular tourist activities based on coral reefs. These reefs are home to great marine biodiversity and many larger marine species depend on these reefs for their food source. Thus, it is vital to save the reef-building corals.