Particle feeding

Many of the organisms that live on coral reefs feed on particles such as plankton or small pieces of debris suspended in the water column, or deposited on submerged

Water Colum Trophic

Figure 7.6 Trophic interactions on coral reefs. A, Trophic pyramid showing the flow of energy from primary producers such as the benthic algae to apex predators such as the White-tip reef shark shown. B, Herbivores such as parrotfish leave characteristic scrape marks. C, Other invertebrates such as chitons graze the surfaces in between coral colonies and may be significant reef grazers. D, Another important input of energy to the reef system occurs through the large numbers of particle feeders that line the seaward edges of coral reefs forming a 'wall of mouths'. (Photos: O. Hoegh-Guldberg; Figure: D. Kleine and O. Hoegh-Guldberg.)

Figure 7.6 Trophic interactions on coral reefs. A, Trophic pyramid showing the flow of energy from primary producers such as the benthic algae to apex predators such as the White-tip reef shark shown. B, Herbivores such as parrotfish leave characteristic scrape marks. C, Other invertebrates such as chitons graze the surfaces in between coral colonies and may be significant reef grazers. D, Another important input of energy to the reef system occurs through the large numbers of particle feeders that line the seaward edges of coral reefs forming a 'wall of mouths'. (Photos: O. Hoegh-Guldberg; Figure: D. Kleine and O. Hoegh-Guldberg.)

surfaces. Particle feeders are part of an important energy and nutrient loop (see next section). Particle feeders range from the largest (whale sharks) to the smallest organisms (copepods) of reef associated species. They include sponges, which use special cells called choanocytes to drive water through small channels and trap particles. Many simple (hydroids, anemones) and advanced invertebrates (polychaetes, bivalve molluscs, ascidians) use cilia to drive water past their feeding structures and use sticky mucous to trap them prior to ingesting them. Other organisms such as cri-noids use tube feet to pick particles out of the water while orienting feeding structures into the currents. Polychaete worms and sea cucumbers have specialised feeding structures that allow them to feed on particles deposited on sediment surfaces. Some fish actively pick particles out of the water column, while others sift sediments using their gill rakers to find small pieces of detritus for ingesting.

The assemblage of particle feeders along the reef crest has been termed the 'wall of mouths'. The 'wall' plays a crucial role in the accumulation of energy and nutrients on coral reefs. Few large zooplankton from the open ocean survive contact when they float by the cloud of particle feeding fishes at the edges of coral reefs (Fig. 7.6D). The amount eaten (and hence the energy acquired by the reef ecosystem) is highly significant, as shown when feeding rates are calculated per unit time per metre of reef-ocean interface (in one study, approximately 0.5 kg m_l d_l wet weight zooplankton, mostly larvaceans and copepods, entered the reef economy). It appears that the 'wall of mouths' represents

BOX 7.1 THE MUTUALISTIC ENDOSYMBIOSIS OF CORALS AND DINOFLAGELLATES

Reef-building corals and invertebrates from at least five invertebrate phyla form close associations with dinoflagellates from the genus Symbiodinium. Often referred to as zooxanthellae (a loose, non-taxonomic term), these single-celled plant-like organisms live within the endodermal cells (gastroderm) of reef-building corals. Here, they photo-synthesise like other phototrophs, but instead of retaining the organic carbon that they make, Symbiodinium releases up to 95% to the host. This energy is used by the coral to grow, reproduce and produce copious amounts of calcium carbonate, which forms the framework of coral reefs. Only corals that have a symbiosis with Symbiodinium are able to calcify at the high rates that are typical of reef-building corals.

In return for this copious energy, Symbiodinium receives inorganic nutrients from the waste metabolism of the animal host (Fig. 7.7). Given the shortage of inorganic nutrients such as ammonium and phosphate ions in tropical and subtropical water columns, the provision of these nutrients is critical to the high rates of photosynthesis and energy production of Symbiodinium. Because there are benefits for both partners in this symbiosis and one cell lives inside the cells of another, this symbiosis is referred to as a 'mutu-alistic endosymbiosis'. The tight recycling of energy and nutrients between the primary producer and consumer avoids the problem of the low concentrations of these materials in typical tropical seas. This is thought to be one of the key reasons why coral reefs are able to prosper in the otherwise nutrient 'deserts' of tropical seas.

yet another important part of the puzzle for how coral reefs maintain themselves in the nutrient-poor (oligo-trophic) conditions of tropical seas.

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