Chapter 14 Coral Reefs Copyright © 2016 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.

Corals Corals are cnidarians, which are characterized by the presence of stinging cells (nematocysts) on their tentacles The coral polyp secretes a calcium carbonate skeleton Most reef-building corals are colonial: interconnected polyps that develop asexually from a single polyp

Basic Types of Corals Hermatypic corals Reef builders Contain zooxanthellae, symbiotic photosynthetic dinoflagellates Tropical in distribution: warm, shallow water Ahermatypic corals Do not build reefs May not contain zooxanthellae Can be found in cold, deep water (see Box 14.2, “Deep-Water Coral Communities”)

Coral Nutrition The coral zooxanthellae produce organic matter through photosynthesis Some of this organic matter is passed to the coral Zooxanthellae help in the deposition of the calcium carbonate skeleton Coral polyps also ingest plankton and other small prey aided by the nematocysts in their tentacles

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

Conditions for Growth of Reef-Building Corals Hard substrate for settlement of first polyp of a colony Optimal light for zooxanthellae Narrow temperature range Narrow salinity range Low sediment load in water Low pollution levels

Reproduction of Corals Corals reproduce both sexually and asexually Asexual reproduction allows for the growth of a colony of polyps from a single polyp Sexual reproduction results in the release of planktonic planula larvae for dispersal into new locations The mass spawning of planulae often occurs at the same time in some coral reefs (see Box 14.1, “Coral Reproduction”)

Temperature Requirements Reef-building (hermatypic) corals tolerate a low temperature range compared with most marine organisms: limited by cold currents Corals can only reproduce and grow if water temperatures are over 20˚C (68˚F) Higher water temperatures are preferred for optimal growth Temperatures above 30-35˚C (86-95˚F) causes coral stress Coral stress can lead to bleaching, an expulsion of zooxanthellae from the coral polyp

Coral Stress and Bleaching [insert Fig. 18.4 (Ch. 18) in 10th ed.] - The expulsion of zooxanthellae is termed bleaching because the normally colored coral is bleached white after expulsion - 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 Niño (see Chapter 18)

Other Requirements Corals normally do not occur near areas where rivers deliver large amounts of freshwater because they have a narrow salinity tolerance Low salinity can result in coral bleaching as well Suspended sediments can settle on corals causing damage as well as reducing water clarity, which reduces the sunlight needed for photosynthesis by zooxanthellae High wave action also increases the amount of suspended sediment

Coral Sensitivity to Pollution Even low levels of pollution can kill coral polyps High nutrient levels due to runoff from land can also allow seaweeds to take over (see “The Kane’ohe Bay Story,” Chapter 13) Increased seaweed growth shade light-sensitive corals and their zooxanthellae

Types of Coral Growth Forms Coral colonies can come in a variety of shapes and sizes Taller, more branching colonies 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, probably to help with capturing all possible light available.

Types of Coral Growth Forms

Other Organisms that Contribute to Reef Building Calcareous green algae and coralline algae (also produce calcium carbonate) Soft corals such as sea whips and sea fans Other cnidarians such as hydrozoans that also produce a calcium carbonate skeleton (fire coral [Millepora]) Sponges (help cement coral rubble into a substrate) Bryozoans

Types of Coral Reefs Fringing Reefs Barrier reefs Atolls

Fringing Reefs Simplest and most common form of reefs Develop as narrow strips along the shore Consist of an inner reef flat of varying width and an outer reef slope Growth of corals is typically more intense on the outer reef slope Growth is slower on the reef flat due to runoff from land, which includes sediment, freshwater, and pollution

Barrier Reefs Barrier reefs also develop along coasts, often farther from shore A deep lagoon is found between the shore and reef Waves often wash sediment onto the back reef slope or reef flat, which causes reduced coral growth Enough sediment may accrue to form small islands on the reef: keys or cays Ex.: Great Barrier Reef, Australia; Belize off Central America

[Insert Fig. 14.18 of 10th ed. here]

Atolls An atoll is a circular reef surrounding a central lagoon Sand cays and islands may be part of the atoll Width varies from less than one mile to over 20 miles The inner slope has a more gradual slope than the outer slope; 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)

[insert Fig. 14.23 of 10th ed.

Atoll Formation Atolls begin developing when a volcanic island is formed: a hypothesis first presented by Charles Darwin, now supported by a wide body of evidence The coral begins as a fringing reef around the newly formed island Over time, weathering erodes the island The island eventually disappears below the water surface, leaving only the outer band of coral surrounding a lagoon: the round fringing reef becomes an atoll

Coral Reef Ecology Coral reefs are among the most biodiverse and productive ecosystems on Earth The coral reef ecosystem is very complex and limited by many physical (light, depth, nutrients, wave action) and biological (competition, predation, grazing) factors The warm, tropical water where reefs develop is low in nutrients (phosphates, nitrates) due to stratification of the water column and distance from rivers and other sources of nutrients

Coral Reef Ecology Due to low nutrient content, the water surrounding reefs has a low primary production (poor in phytoplankton) The food web is basically based around the primary production of zooxanthellae in corals and in a few other invertebrates The coral benefits the zooxanthellae by providing carbon dioxide and nitrogen and phosphorus in excretory products The zooxanthellae provide organic molecules to the corals (and to all animals that feed on the corals)

Coral Reef Ecology Cyanobacteria (photosynthetic bacteria) present on the reef fix nitrogen into nitrogen compounds (including nitrates) Seagrasses, mangroves, and seaweeds can also contribute to primary production in the form of detritus Nitrogen and phosphorus waste products are efficiently recycled, not only be corals and zooxanthellae but by other reef dwellers

Typical Coral Reef Food Web

Many Organisms Feed Directly on Coral (mucus, eggs, planulae larvae)

Competition on the Reef There is tough competition for space and light on the reef Some corals grow fast and upright, others grow slow but massive to out-compete other organisms Corals may feed directly or sting one another if they contact each other Many coral reef inhabitants compete for space and/or deter predation by producing toxic or foul-tasting chemicals, common in soft corals and sponges

Competition on the Reef Other animals must also compete for limited resources The many species of fishes, molluscs, echinoderms, crustaceans, and members of other groups normally do this by feeding in slightly different foods or areas or in slightly different ways and/or times of the day, sometimes by reproducing at different times of the year Each species has its own, well-defined ecological niche

Coral Predation, Grazers Both are important in structuring of coral reef communities Examples of coral predators: fishes (butterflyfishes), sea stars (crown-of-thorns [Acanthaster]) [see “The Crown-of-Thorns Sea Star”, Ch. 14) Grazers (fishes, sea urchins) keep seaweeds from taking over coral surfaces

[Insert Fig. 14.32 in 10th ed.]