Food Chains

Coral reefs form in the warm, clear, shallow waters of tropical oceans worldwide. They provide food and shelter for fish and invertebrates. The organisms in a coral reef co-exist in complex interconnected food chains. These organisms can share the same area as they have different behaviour patterns although they all share the common aims of feeding, reproduction and occupying and protecting their own space. Any organism that is not successful at any of these aims will vanish from the reef.

All these organisms eat different kinds of food and are also, themselves, a source of food for other organisms. Some animals and plants have a symbiotic relationship with other species so that they cannot survive without them.

Members of a food chain are classed as either producers, organisms that produce energy or consumers, organisms that consume their energy by eating other organisms. Producers usually generate energy from photosynthesis; these are plants and plant-like organisms. Consumers are animals and fungi. Producers are the basis of the food chain. They are very important as all energy comes form them. The three main producers in a coral reef are blue-green, algae, seaweed, and reef-building corals.

Coral

Coral Polyps have two different ways of feeding. Firstly, they have tiny algae called zooxanthellae living in them that use sunlight to make sugar for energy and secondly, polyps are predatory, using tentacles they catch tiny floating animals called zooplankton.

Zooxanthellae is a plant-like algae that lives inside the coral polyps and performs photosynthesis. They share the energy produced by photosynthesis with the coral. The coral, in turn, provides shelter, protection, carbon dioxide and access to light. It is the zooxanthellae that give the colour to the coral. It is because the zooxanthellae need light for photosynthesis that coral reefs are found in clear, shallow water where light can filter down to the polyps. The polyp’s wastes are processed by the zooxanthellae to keep the nutrients and give the polyp oxygen. Up to 98 per cent of the corals nutritional needs are supplied by the zooxanthellae.

Corals also feed on tiny floating animals called zooplankton. The polyps come out of their skeletons at night to feed on the zooplankton. At night, they stretch out their long tentacles to capture their prey as it floats by, so that the reef looks a “wall of mouths”. The tentacles sting the plankton with their toxic nematocyst cells then transfer it to the polyp’s mouth and it is digested in its stomach cavity. Solid wastes are excreted through the mouth.

Many corals collect fine particles in mucous film or strands as well as zooplankton, which are taken to the polyp’s mouth by cilia. Some planktons feed only on mucous suspension whilst others can take small fish.

Consumers

Consumers eat the producers. There are two kinds of primary consumers on coral reefs, herbivores and carnivores. There are also some carnivores that eat coral called corallivores. Herbivores are animals that eat plants, such as the sea urchin. Sea urchins are the main reef herbivores, they control the amount of seaweed on rocky surfaces by constant grazing.

Coral Reef Inhabitants

There are several species of sponges inhabiting coral reefs. These porous animals provide shelter for fish, shrimps, crabs and other small animals. The close relative of the coral, the sea anemones have a symbiotic relationship with clownfish and anemonefish. The fish find refuge in the tentacles of the anemone and lay their eggs among them. Anemonefish remove parasites from their host anemones and keep away predators such as butterflyfish.

A reef may be encrusted with microscopic invertebrates called bryozoans, which form branching colonies over coral skeletons and reef debris which cements the reef structure. Several varieties of worms, including flatworms, bristleworms and polychaetes, live on a reef. Sea stars, sea urchins and sea cucumbers live on coral reefs. The crown-of-thorns sea star eats coral polyps. These can cause devastation to a reef if there are too many of them, stripping away the polyps and leaving only the calcium carbonate skeletons.

Crevices and the gaps between coral branches are great hiding places for shrimps, crabs, lobsters and other crustaceans away from predators. Crustaceans can also be predators, such as the coral crab which crushes sea urchins and clams with its claws. A cleaner shrimp such as the banded coral shrimp removes parasites and dead skin from reef fishes.

Many mollusc species live on or near reefs. These include octopuses, squid, clams, scallops, marine snails and nudibranchs. Some of these are filter feeders, as they filter food particles from the water and some are predators. Carnivorous snails drill holes in the shells of the prey.

Schooling and solitary fish are both predators and prey in the food chain of a reef. Some eat other fish; some eat the coral and the algae within it; others are cleaner fish that remove parasites form larger fish; and they also provide nutrients for other reef inhabitants with their leftover food scraps and wastes.

 A coral reef, not only a living organism itself, provides the base for a whole world of other organism to live on. This beautiful and seemingly delicate structure is the foundation for a balanced ecosystem where many species are dependant on each other for survival.