Marine ecosystems 5: Coral Reefs Unique features The foundation of the ecosystem is produced by living things Reef-building corals Similarities with tropical rain forests Richness and complexity 3-dimensional skeleton where thousands of species live What creates a coral reef? Calcium carbonate (CaCO 3 ) deposited by corals Are cnidarians belonging to the group Anthozoa Only the polyp stage is present Each coral is like a tiny sea anemone with stinging cells Why are corals able to form huge reefs? (1) Asexual reproduction (cloning) A big area covered by corals in reef may have aroused from only one individual (2) Mutualism with Zooxanthellae Dinoflagellates living within the polyp s tissues Most of the food corals need is taken from the algae Corals only need to get about 2% of their food feeding on zooplankton with their tentacles Corals reproduce both sexually (most are hermaphrodites) and asexually coral Coral eggs Polyps provide raw materials algae need in order to make photosynthesis Polyp s tentacles with symbionts algae Polyp body Asexual reproduction allows to rapidly colonize a suitable place Sexual reproduction allows to produce new genotypes that may adapt better to other environments
Why does a colony behave as one individual? (1) They share a common nervous system Brainless animals, extremely simple nervous system (2) Digestive systems of polyps are connected Each individual has only a mouth and a digestive cavity Incomplete digestive system (only one opening, no anus) How does a colony grow? A planula larva settles on a hard bottom Changes to a polyp form And divides asexually over and over to form a colony The polyps continually place new layers of CaCO 3 Building up the skeleton beneath them Growing upward and outward The living coral is a thin layer on the surface
How does a coral feed? (1) Zooxanthellae gives about 98% of the food a polyp needs (2) But they use their nematocysts to eat zooplankton whenever they have a chance! Nematocysts = stinging cells (4) Mesenterial filaments secrete digestive enzymes Polyps can extrude them to digest and absorb food particles (5) They can also absorb DOM (dissolved organic matter) Organic matter released by organisms as metabolic waste (3) They also catch food secreting mucus Gathering it with hair-like cilia, and passing it along the mouth Other reef builders Corals alone cannot build up a reef! (1) Algae are the most important organisms that help making a coral reef Zooxanthellae (dinoflagellates) are essential to corals growth Coralline (red) algae growth encrust the corals And secrete, sometimes, even more CaCO 3 than corals Porolithon onkodes Porolithon gardineri Lithothamnion glaciale They grow over the sediment as it deposits Cementing it in place, holding the reef together Halimeda (green) algae are composed of 95% CaCO 3 (2) Calcareous sponges (3) Bryozoans (Moss animals)
Conditions for reef to growth Shallow waters Where there is enough light for the Zooxanthellae to make photosynthesis Reefs only develop in coastal waters of continental shelves, around islands, or on top of seamounts Temperature Reef building corals can only grow and reproduce with temperatures above 20ºC (68ºF) Depending on the species, if temperature is above 30-35ºC (86-95ºF) Bleaching occurs Because of heat stress Corals expel its zooxanthellae and slough off big amounts of slimy mucus Salinity, sediments Low salinity and sediments do not favor coral reef growth Coral reefs are always located in warm and relatively salty waters Pollution Corals are very sensitive to pollution Even to very low concentrations of chemicals discharged to the water or eutrophication pollution by an excess of nutrient added to the water Larvae are particularly sensitive
Types of coral reefs (1) Fringing reefs The simplest and most common type of reef Are near-shore reefs that develop where rocky shores or at least a patch of hard bottom exist The longest fringing reef in the world 1,200 mi Is located on the shores of the Red Sea Which part will have the densest cover, the flat or the slope? The slope, is away from the coast and the effects of sediments and freshwater (2) Barrier reefs Also lie along the coast But farther from the shore Sometimes as far as 60 miles Sand cays (Keys in the US) Originated by sand accumulated by waves and currents Protected from waves and currents Usually with a soft sediment bottom and seagrass beds The largest barrier reef Is the Great Barrier Reef in Australia 1,200 mi long 10-200 mi wide
(3) Atolls Are rings of reefs, islands or sand cays Surrounding a central lagoon India Australia Most atolls occur in the Indo-West Pacific region Why do atolls always form rings? What provides the foundation for the reefs? Atolls form on subsiding islands Beginning with fringing reefs all around the island (Darwin!) Coral reefs food webs Organisms interactions Grazing, competition (space, food, mates), and predation Maintain the structure of a healthy coral reef Keep in check the seaweeds Preventing them to take over the reef Keep in check the coral grazers and other organisms
Why are coral reefs so productive? (1) The mutualistic relationship Between corals and the zooxanthellae If the surrounding ocean waters lacks nutrients? (2) Coral reefs communities use nutrients Very efficiently as a result of recycling Rates of nitrogen fixation by cyanobacteria are between the highest in natural communities Bacteria live as symbionts in sponges and also corals Ocean currents bring in phosphorus and other nutrients Water is nutrient poor But nutrients add up if enough water washes over the reef