Coral reefs consist of many diverse species of corals. These corals in turn are made up of tiny organisms called polyps. The structure of the polyps and the skeleton of the coral is a rather simple combination. A polyp is made up or two cell layers: the epidermis and the gastrodermis. The non-tissue layer between the gastrodermis and the epidermis is called the mesoglea. The corallite is the part of the skeleton deposited by one polyp. The skeletal wall around each polyp is called the theca. The coral anatomy also includes calcareous plate-like structure known as septa. The septa radiate from the wall to the center of the corallite. 12-5 Coral Anatomy
Zooxanthellae are unicellular yellow-brown (dinoflagellate) algae which live symbiotically in the gastrodermis of reef-building corals (Goreau et al., 1979). It is the nutrients supplied by the zooxanthellae that make it possible for the corals to grow and reproduce quickly enough to create reefs. Zooxanthellae provide the corals with food in the form of photosynthetic products. In turn, the coral provides protection and access to light for the zooxanthellae. It was once believed that all zooxanthellae were the same species, Symbiodinium microadriaticum (Rowan and Powers, 1991). However, recently, zooxanthellae of various corals have been found to belong to at least 10 different algal taxa. 12-5 Coral Reefs
Corals exhibit sexual and asexual reproduction. The coral colony expands in size by budding. Budding may be intratentacular, in which the new bud forms from the oral discs of the old polyp, as in Diploria, or extratentacular in which the new polyp forms from the base of the old polyp, as in Montastraea cavernosa. A common type of asexual reproduction in corals is by fragmentation. Broken pieces of corals that land on a suitable substrate may begin growing and produce a new colony. This type of reproduction is common in branching corals like Acropora cervicornis in which a positive correlation was found between fragment size and survival. Coral Reproduction
Many coral species mass spawn. Within a 24 hour period, all the corals from one species and often within a genus release their eggs and sperm at the same time. Some species of coral brood their larvae. The sperm fertilizes the egg before both are released from the coral. The larvae float to the top, settle, and become another colony. Species of Acropora release brooded larvae. Coral Reproduction
Corals can obtain food in a variety of ways. Reef-building corals rely on the photosynthetic products of zooxanthellae for the majority of their nutrients. However, corals also capture zooplankton for food. Corals are suspension feeders. They utilize two main methods of prey capture: nematocyst adhesion and mucus entrapment (Sebens and Johnson, 1991). Nematocysts on the tentacles and mesentarial filaments can be used to sting prey and move it into the mouth. Some corals will trap prey in sticky mucus on their tentacles and move the prey into the mouth using the mucus and cilia. Prey size is correlated to polyp size (Sebens and Johnson, 1991). Most corals feed at night. This may be because night is when the zooplankton travel into the water column and become available for capture. Keeping the tentacles retracted during the day may also help corals avoid predation, protect themselves from UV light, and avoid shading their zooxanthellae. 12-5 Coral Feeding
Four coral conditions have been identified as diseases: white band disease (WBD), black band disease (BBD), bacterial infection, and shut down reaction (Richmond 1993). They are also susceptible to tumors and parasitic worms. These maladies are all stress related, and anthropogenic stresses can increase a coral's susceptibility to these diseases. Diseases such as BBD and WBD actually kill coral tissue while advancing in a band around the coral and leaving the white coral skeleton behind. Edmunds (1991) stated that BBD, caused by cyanophyte Phormidium corallyticum, may have a role in maintaining coral diversity because it is most prevalent in coral species that form large colonies and provide a structural framework for the reef. WBD, which is believed to be caused by a bacteria pathogen yet unknown, has much of the same effect on corals, leaving behind a white, lifeless coral skeleton. Gladfelter (1982)
The crown-of-thorns starfish (Acanthaster planci) The crown-of-thorns sea star feeds on polyps. It spreads it's stomach out through the mouth over a lump of living coral, secreting digestive juices, kills the coral and then sucks in the resulting "soup". After feeding it moves on leaving a patch of white, or a coral skeleton. The Crown of Thorns seastar usually feed twice a day for several hours. Small and juvenile Crown of Thorns starfish feed at night, so they can avoid predators which are more active during daylight. Depending on its size the Crown of Thorns can eat from 2 to 6 sq. meters of coral a year. That can be up to 180 times as much coral destroyed compared to its own size per year When times are bad, like many other starfish, it may survive without feeding for up to 9 months.
Preferred Coral The Crown-of-thorns starfish will eat most types of coral but prefer the branching tubular and staghorn type corals called Acropora (which is fast growing) They do not tend to eat Brain corals or large, hemispherical (termed massive) corals (such as the coral Porites)
Coral bleaching is the whitening of coral colonies due to the loss of symbiotic zooxanthellae from the tissues of polyps. This loss exposes the white calcium carbonate skeletons of the coral colony. Corals naturally lose less than 0.1% of their zooxanthellae during processes of regulation and replacement (Brown and Ogden, 1993). However, adverse changes in a coral's environment can cause an increase in the number of zooxanthellae lost. There are a number of stresses or environmental changes that may cause bleaching including disease, excess shade, increased levels of ultraviolet radiation, sedimentation, pollution, salinity changes, and increased temperatures.