Coral Reefs Shipley Marine Bio Summit High School Chapter 14 Coral Reefs Shipley Marine Bio Summit High School

Corals Corals are cnidarians, a phylum of animals characterized by the presence of stinging cells on their tentacles These stinging cells are used to capture prey primarily, but can also be defensive The soft body of the coral (polyp) is enclosed in a hard shell of calcium carbonate created by the coral (whitish color in diagram on right).

Basic Types of Corals Hermatypic corals- Reef builders Normally contain zooxanthellae, symbiotic photosynthetic dinoflagellates (shown at right) Restricted in distribution by water temperature and water quality Ahermatypic corals- Do not build reefs May not contain zooxanthellae Not restricted by water temperature and water quality

Anatomy of a Coral Reef Coral reefs are groups of coral polyps interconnected by thin layers of tissue These grouped polyps share a nervous system and digestive connection – this is due to the fact that the entire colony came from one original polyp that reproduced asexually to produce the colony

Anatomy of a Coral Reef Each polyp continues to lay down new CaCO3 beneath the polyp body so the entire coral colony continues to grow upward Each polyp is normally 1 to 3 mm in width However, the collection of polyps together can collectively grow to extremely large sizes and weigh several tons

Coral Nutrition The zooxanthellae housed inside the coral produce organic matter through photosynthesis Some of this organic matter is passed to the coral This “sharing” aids the coral and helps the entire reef grow faster In fact, if zooxanthellae are supplied with enough light, they can feed the coral completely

Coral Nutrition Of course, coral do also possess nematocysts to sting potential prey that come within reach of the tentacles These cells allow the coral to feed on small organisms in the water, mostly zooplankton Some corals also produce sheets of mucous to capture zooxanthellae or detritus

Coral Nutrition Coral can also feed using extensions of the gut wall called mesenterial filaments These filaments secrete digestive enzymes The coral can extend the filaments with its enzymes allowing the coral to feed and digest food outside the body

Conditions Required for Reef Growth Hard Substrate Light Optimal Narrow temperature range Narrow salinity range Low sediment load in water Low pollution Narrow range of pH

Reproduction and Growth of New Coral Corals reproduce both sexually and asexually Asexual reproduction allows for the growth of buds from a single polyp Sexual reproduction results in the creation of a planula larvae that is able to ride on the water currents to a new environment These larvae will settle out of the currents to begin growth on existing reefs or other hard substrate

Light Requirements Light must be able to penetrate the water to the depth of a coral to allow the symbiotic zooxanthellae to photosynthesize Due to this, corals are normally found in shallow waters Some may be found to depths over 150 feet if water clarity permits light penetration to that depth Corals are mostly found on continental shelves, near islands or on seamounts due to this light/depth dependency

Temperature Requirements Reef building (hermatypic) corals tolerate a low temperature range compared with most marine organisms Corals can only reproduce and grow if water temperatures are over 68 degrees F Higher water temperatures are preferred for optimal growth However, water temperatures above 86 degrees (or higher in some species) causes coral stress Coral stress can lead to “bleaching,” an expulsion of zooxanthellae from the coral polyp

Coral Stress and Bleaching The expulsion of zooxanthellae is termed bleaching because the normally colored coral is bleach white after this expulsion The zooxanthellae give the corals their distinctive color

Coral Stress and Bleaching Other events besides high temperatures can cause coral bleaching Poor water quality, increased sediment in water column, wave stress or disease can also cause bleaching If corals remain bleached for too long, it can result in death of the coral Widespread bleaching often occurs as a result of events such as El Nino (increased water temps) and hurricanes

Salinity Requirements Normal sea salinity is around 35 parts per thousand Corals maintain good health in an environment around 35 ppt Corals normally do not exist near areas where rivers deliver large amounts of freshwater to the marine environment because of this salinity requirement Low salinity can result in coral bleaching as well

Sediments and Wave Action Corals due not normally grow in areas where wave action is high This is due to the fact that heavy wave action results in the suspension of sediments in the water column Suspended sediments can settle on corals causing damage as well as reduce water clarity Reduced water clarity means that zooxanthellae photosynthesis will be reduced or stopped

Coral Sensitivity to Pollution Even low levels of pollution can kill coral polyps High nutrient levels can also allow algae levels to rise Increased algal growth can shade light-sensitive corals and their zooxanthellae As an example, read the case study in chapter 14 in Kane’ Ohe Bay in Hawaii on eutrophication and the decline of coral reefs

Types of Coral Growth Forms Coral colonies can come in a variety of shapes and sizes Taller, more branching more tend to be more common in shallower areas of the reef due to intense competition for space and light Flatter forms tend to be more common in deeper areas of the reef; this probably assists with capturing all possible light available.

Types of Coral Growth Forms

Other Organisms that Contribute to Reef Structure Besides Stony Corals Coralline algae (also produce calcium carbonate) Soft corals such as sea whips and sea fans Other cnidarians such as hydrozoans or anemones Sponges (help cement coral rumble into a substrate) Bryozoans On the following page, there is a table that outlines important cnidarians on coral reefs

Types of Coral Reefs Fringing Reefs Barrier reefs Atolls

Fringing Reefs Simplest and most common form of reefs These reefs develop near the shore in tropical waters These reefs develop as narrow strips along the shore The fringing reef shown on the right completely encircles the island it has formed near

Fringing Reefs Fringing reefs consist of an inner reef flat and an outer reef slope (shown at right) The reef flat is wide, gently sloping and may be exposed in places at low tide The reef slope is much more steep and is not exposed to air The reef crest is the shallow, upper edge of the reef slope Growth is normally high in this area

Fringing Reef The reef slope contains living and dead corals, seagrasses, soft corals, algaes and invertebrates Growth of these organisms is normally fairly high on the reef slope and especially at the reef crest The number of species present is also higher on the slope and the crest Growth is slower on the reef flat due to runoff from land which includes sediment, freshwater and pollution Wave action is normally higher in this area as well

Barrier Reefs Barrier reefs also grow along the shore or may be farther from shore They are different from fringing reefs by the presence of a deep lagoon between the shore and the reef At right, you can see an aerial photo of the barrier reef in Belize, Central America – this is the second largest barrier reef in the world

Barrier Reefs The illustration at right shows the typical construction of a barrier reef: a back-reef slope, a reef flat and crest and a fore-reef slope The slope of the fore-reef slope is normally much more pronounced than the back-reef slope The depth of the water drops quickly in this area! The slop of the back-reef slope may also be steep, but not to the degree of the fore-reef slope

Barrier Reefs Waves often wash sediment onto the back reef slope or reef flat which causes reduced coral growth in this area Enough sediment may accrue to form small islands on the reef These islands are called keys in the US or cayes elsewhere

Barrier Reefs Coral growth is normally the highest at the reef crest or just below the crest on the fore-reef slope As with fringing reefs, species diversity is greater in this area as well

Great Barrier Reef, Australia The Great Barrier Reef is considered the largest reef structure in the world with regards to total coral area The length of the reef systems exceeds 1200 miles The width varies from 10 – 200 miles Many cayes exist along the reef system

Atolls An atoll is a circular reef structure surrounding a central lagoon Sand cayes may be a part of the atoll structure Width varies from less than one mile to over 20 miles

Atoll Formation Atolls form when a volcanic island is formed The coral begins as a fringing reef around the newly formed island Over time, weathering and geologic activity lowers the level of the island Eventually, the island disappears below the water’s surface, leaving only the outer band of coral surrounding a lagoon The diagram at right shows the progression of these steps

Atolls Atolls possess a reef flat and inner and outer reef slopes The inner slope has a more gradual slope than the outer slope and the lagoon normally has a depth of 200 feet or less The two largest atolls are in the Maldives (Indian Ocean) and the Marshall Islands (Pacific)

Coral Reef Ecology Like any marine environment, the coral reef community is plagued by limiting resources, competition and predation The water column surrounding the reef is normally very low in nutrients This is due to the position of reefs away from rivers and other sources of nutrients (remember corals form best away from possible sedimentation sources)

Coral Reef Ecology Due to this low nutrient content, the water surrounding the reef does not support the degree of primary production by phytoplankton seen in other marine communities The food chain here is based around the primary production of zooxanthellae located in the body of the corals and other organisms (in some cases) The coral benefits the zooxanthellae by providing carbon dioxide, nitrogen and phosphorus (by-products) The zooxanthellae provide organic molecules to the corals (and any organism that feeds on the corals or their parts

Coral Reef Ecology Cyanobacteria present on the reef are able to fix nitrogen which is then available to other reef inhabitants who feed on the cyanobacteria Seagrasses and algae can also feed reed inhabitants Water currents may bring some phyto- and zooplankton to the reef community

Typical Coral Reef Food Web

Many Organisms Feed Directly on Corals or their Products (mucous, eggs, larvae)

Deterring Predation Many coral reef inhabitants can deter predation by producing toxic or foul tasting chemicals – this is common in soft corals and sponges These chemicals can be released into the water to kill organisms or limit growth surrounding these organisms Other reef dwellers may produce hard structures to deter predation – sponges produce spicules of calcium carbonate or silica while soft corals produce calcium carbonate needles known as sclerites Coralline algaes as well as other organisms grow in an encrusting form that is difficult to feed upon (tissue layer is VERY thin)

Competition on the Reef There is tough competition for space and light on the reef To limit competition, some corals grow fast and upright to maximize their ability to reproduce quickly Others grow slow but massive to out-compete other organisms in the long run Corals may feed directly or sting one another if they contact each other Shown at right is direct coral on coral competition (and some coral death in the pink area) Remember that corals must also compete with algae, seagrasses and invertebrates for space and light

Competition on the Reef Other organisms such as fish and crustaceans must also compete for limited resources They normally do this by feeding in slightly different areas or in slightly different ways Each has its own ecological niche