OASES Corals Education

Mission
Vision
Goals

by Bridget Zachary
Coral Education

Coral reefs are one of the most diverse and productive communities on earth. Found only in areas that have shallow, clear, warm waters, the reefs provide food and shelter for a variety of plant life, fish and invertebrates while protecting the shoreline from erosion caused by strong wave action. The building of the reefs is performed by the stony (hermatypic) corals that have a special relationship (called symbiosis) with zooxanthellae, (unicellular algae) making them the primary source of production in the reef community. Reefs are formed largely from calcium carbonate that is extracted from surrounding waters by coral polyps. However, building of reefs is not accomplished by the corals alone, coraline algae cements various corals, mollusks shells, and tube worm structures together.

A single coral colony many consist of thousands of polyps where an entire reef having thousands of corals, might consist of many millions of polyps. Due to the polyps symbiosis with the zooxanthellae (contained within the polyps), they can only tolerate a narrow range of environmental conditions. Those conditions are defined by both physical and chemical parameters such as temperature fluctuation, light intensity, wavelengths of light, salinity changes, optical water clarity, and changes in the pH to name a few. Environmental changes can make the colonies susceptible to diseases and a phenomena called bleaching. However, since the colonies are made up of many individual polyps, the disease may only affect a portion of the colony.

Threats to Reefs

There are both natural and human caused threats to coral reefs. Natural threats to the reefs include hurricanes, diseases, natural outflows of freshwater runoff, non-indigenous invasive species, changes in the water current flows, and other weather related circumstances. Some invasive species such as starfish or Crown-of-thorns can devour entire tracks of coral reefs leaving nothing but the exoskeleton (calcium carbonate).

Though natural occurrences may cause dramatic changes in coral populations and health, most of the decrease is actually caused by humankind. One of the largest threats is the over development of coastal regions that are in proximity to coral reefs. Developments that do not work with the natural flow of the land can create outflows of large amounts of freshwater runoff. This runoff can increase the amount of sediment that reaches the reefs and covers the corals, which decrease the needed light for the zooxanthellae to produce food for the corals. Additional effects are changes in nutrient levels, salinity, temperature, and oxygen content. Increases in nutrient loading cause other organisms, such as sponges and algae, to grow rapidly and out compete the corals, effectively choking them out of existence.

Another significant human threat to the coral reef is overfishing. Through the selective reduction of large predatory fish species, an imbalance occurs allowing more competitive species to take over reefs that are normally kept in check by the those key fish species. Additionally, fishing with explosives and poisons is common in many parts of the world and causes severe damage to the reefs. Additionally, coral reefs are also damaged directly by groundings of both commercial and private ships. Boat anchors may damage or break entire colonies. A decrease in coral cover is becoming more noticeable not because of any one factor but because of a combination of many.

Coral Reproduction

Corals reproduce both sexually and asexually. The coral colony expands by budding. Budding can occur either intratentacular or extratentacular. Intratentacular budding occurs when the new bud forms on the oral discs of the old polyp. Extratentacular budding occurs when the new polyp forms from the base of the old polyp.

A common type of asexual reproduction in corals is fragmentation. Fragmentation is successful when a broken piece of coral lands on a suitable ground surface and grows to produce a new colony.

Many corals reproduce sexually in a mass spawning event. Within a 24 hour period, all the corals from one species in a certain area will release their eggs and sperm at the same time. Intraspecies fertilization does occur, but mass spawning raises the possibility of hybridization. Some coral species will brood their larvae. Brooding means the egg is fertilized and remains attached to the colony until the larvae is able to survive on its own in the water column.