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Although they are often associated with ample light conditions, relatively shallow depths, and as part of reef structures, both solitary and colonial hard corals can also be found in the low-light conditions of abyssal depths and do not form the reef structures with which they are commonly associated. The majority of stony corals, however, are colonial and are responsible for creating the basic framework for most coral reefs.
A close relative of sea anemones, hard corals secure nutrients through the use of cnitocytes, which are stinging cells that either kill its prey on contact or render it immobile prior to ingestion. Colonies of hard corals are influenced by the surrounding environmental conditions, with the light conditions, water movement, and depth all affecting the shape, color, and overall structure of the individual colony. Zooxanthellae, symbiotic unicellular organisms living within the tissues of the hard corals, also influence the color of the coral.
Reproduction occurs both asexually and sexually, with the former most commonly occurring through fragmentation, a process in which part of a colony of hard corals detaches and subsequently reattaches to the seabed in a new location. The latter typically occurs through the release and fertilization of gametes, with the planula larvae drifting along as part of the plankton until settling on the seabed where it can found a new colony and protect itself by secreting the calcium carbonate needed to build its hard skeleton.
Due to the inherent limitations of classification efforts made preceding the development of scuba diving -- along with the accepted rules of nomenclature asserting that precedence is granted to the first description regardless of the accuracy of that description -- the hard coral’s taxonomy is somewhat difficult to parse. In many cases, hard coral species lack a defining distinction from hard coral subspecies, and the morphological and geographic boundaries merge with the boundaries of other species.
Although the taxonomy is undeniably challenging, recent methodologies have assisted in providing some clarification. Using mitochondrial and ribosomal RNA, zoologists used the resulting molecular data to support the following scientific classification:
• Kingdom - Animalia
• Phylum - Cnidaria
• Class - Anthozoa
• Subclass - Hexacorallia
• Order - Scleractinia
According to the World Register of Marine Species, hard corals can claim each of the following families within its order:
Although hard coral classification has been marked by a great deal of complexity, most zoologists agree that future classification efforts should be guided by molecular systems as well as morphological systems.
Inhabiting all of the world’s oceans, stony corals can be divided between two ecological groups: hermatypic corals and ahermatypic corals. Reef building is the principal difference between the two groups, with hermatypic corals engaging in reef building and ahermatypic corals existing as a non-reef building counterpart. Another key difference -- which is not quite a universal difference that clearly divides the two groups -- is the presence of the symbiotic unicellular organisms called zooaxanthellae, a dinoflagellate found in the tissues of hard corals that also plays a role in the color of the corals.
Ahermatypic corals, the non-reef building type of coral, tend to lack the presence of zooaxanthellae and, although some can be found in tropical waters, the majority are located in polar waters, temperate seas, or at abyssal depths of up to 6,000 meters. Ahermatypic corals begin to appear in abundance at depths in excess of 500 meters, with the low-light conditions and lack of zooaxanthellae resulting in a growth rate far slower than its hermatypic counterpart.
While ahermatypic stony corals prefer the colder and deeper waters that tend to be lacking in light penetration, hermatypic corals thrive in the warmth of tropical waters and in the light conditions most common at relatively shallow depths, particularly when the hard corals have a firm seabed on which to settle and build a reef. Hermatypic hard corals grow up to three times faster than ahermatypic corals, due to not just the well-lit conditions and warm-water environs, but also due to the presence of zooaxanthellae, which produce as much as half of the nutrients the polyps consume.
Most hard corals are equipped with tentacles that can be extended for predation, as the tentacles benefit from the stinging cells that either immobilize or kill its intended target. Although most hard corals feed on plankton, those that are equipped with larger tentacles are able to prey on other invertebrates as well as smaller species of fish. Other hard corals are capable of producing mucus films that can be positioned in different parts of its body through the use of its cilia, thereby allowing it to capture small organic particles on which they are then able to feed.
Like most corals, hard corals are able to reproduce through a wide range of sexual and asexual methods. While hard corals benefit from these varied reproductive strategies, hermatypic hard corals are mostly hermaphroditic and utilize synchronous spawning events in which eggs and sperm are simultaneously released throughout the year, often in accordance with the moon cycle.
Hard corals also benefit from the ability to reproduce asexually, most commonly through a process known as fragmentation. Environmental factors such as storms or even the strong movement of water can cause fragments of hard corals to detach and then reattach elsewhere, often forming new -- and frequently massive -- colonies right from its new location on the seabed. In other cases, the polyps of a hard coral colony may detach themselves when the parent colony dies, allowing it to protect itself until it settles in a suitable location for forming a new colony from the seabed.
Scleractinian corals are more commonly referred to as hard corals or stony corals for a reason. Although hard corals possess a hard skeleton known as coralite, they also possess the soft parts commonly found on other types of coral. The individual polyps, which can retract within the coralite, are marked by a cylindrical body and possess an oral disc surrounded by tentacles, forming a ring. The tentacles extend to secure food, which can then be moved toward the mouth that is located at the center of the polyp’s oral disc.
At the base of the polyp, calcium carbonate is secreted in order to form the hard skeleton that creates a protective layer for the polyp’s soft parts. The skeletons of the hard corals are composed of calcium carbonate that takes on the form of aragonite, but the fossil record indicates that a number of hard coral ancestors had skeletons that instead took on the form of calcite. This difference is why contemporary corals tend to be light and porous while some of those found in the fossil record were much more solid.
Colonial corals grow through a process in which new polyps bud from existing polyps. There are two different types of budding processes utilized by hard colonial corals, including extratentacular and intratentacular. Separate polyps result from the extratentacular budding process, with each new polyp possessing its own wall of coralite.
The second growth process common to colonial corals is intratentacular budding, a process in which the new polyps form within the ring of tentacles and on the oral disc. Sometimes this process results in the formation of separate polyps, but it is also common for a row of polyps -- which are only partially separated from one another -- to form on an elongated oral disc.
Although colonial corals can grow through these budding processes, solitary corals cannot. Instead, they simply increase in size by secreting additional calcium carbonate to create the whorls of septa that make up the hard skeletal exterior. Some species of solitary coral can reach up to 10 inches in length while producing over 1,000 septa.
The rate of growth stemming from the aforementioned budding processes varies from species to species and is influenced by a number of external factors, with some species growing at approximately the same rate of human hair (up to four inches within a single year) while others grow as slowly as one-tenth of an inch each year.
It has become increasingly apparent that a number of environmental issues are threatening hard corals and coral reefs all over the world, with the overwhelming majority of these issues caused by human activity. In addition to sea level rise, ocean acidification, increased water temperature, and pollution, overfishing -- including through the use of destructive practices like blast fishing -- and canal-digging have likewise endangered coral reef ecosystems and the hard corals that serve as the basic structure for these reefs.
Hard corals have been both directly and indirectly affected by a number of these issues. Overfishing of the giant triton sea snail -- valued for its aesthetically pleasing shell -- has allowed crown-of-thorns starfish populations to go relatively unchecked due to the sudden absence of its principal predator. Hard corals, which are generally protected from predation due to the fact that the overwhelming majority of predators are unable to digest the wax found in their tissues, are at increased risk due to an overabundance of crown-of-thorns starfish, one of the few predators capable of digesting the wax cetyl palmitate.
An excess of nutrients -- especially nitrogen and phosphorous -- is also problematic for hard corals. With nutrient-rich conditions, certain types of algae and phytoplankton thrive and subsequently absorb all of the available oxygen, creating hypoxic conditions while also blocking the light from reaching the seabed. Without light and oxygen, hard corals and coral reefs are unable to survive. The excess of nitrogen is toxic to the hard corals, while the excess of phosphorous reduces the rate of growth by a significant degree.
Climate change has also been a major contributor to the increased threat posed to hard corals and coral reefs in general. Rising ocean temperatures are responsible for bleached corals and have killed a large percentage of the coral reefs in oceans all over the world. This is because the increased ocean temperatures have adversely affected the zooaxanthellae, the symbiotic dinoflagellates that are so critical to the growth and survival of most hermatypic hard corals and contribute to the color of the hard corals they inhabit. The increased temperatures also create conditions more conducive to diseases capable of wiping out the hard corals and coral reefs, and the rate at which the temperature continues to rise will play a major role in whether or not the coral is able to adapt quickly enough to ensure its continued survival in increasingly extreme conditions.
A host of other climate-related issues exist and continue to affect coral reefs and indeed threaten entire ecosystems all over the world, and there is the potential for a domino effect that may not yet be entirely understood or even clearly evident at the present time. It has only recently been demonstrated, for example, that both air pollution and the destruction of mangrove forests have had a devastating impact on the coastal water quality inshore coral reefs inhabit.
While overfishing and blast fishing are certainly practices that pose a threat to coral reefs all over the world, marine aquaria hobbyists can take comfort in the fact that the hard corals made available through retailers do not require the use of any adverse or extensive harvesting practices. Since the overwhelming majority of hard corals are capable of reproducing asexually through the fragmentation process, there is no need to harvest an overabundance of hard corals from the ocean. Instead, a detached fragment of a branching coral can be utilized to increase retail stock through the fragmentation process rather than through repeated coral harvesting.