Most coral reefs are large complex ecosystems found in the tropics comprised of billions of tiny polyps, minute animals, and algae. The reefs are home to thousands of other creatures. Those surrounding creatures are similar to those found in any other ecosystem: herbivores that eat plants such as “goat fish”, carnivores such as barracudas and sharks, and omnivores that eat whatever is available. There are some other, more unusual animals found feeding around coral reefs such as filter feeders that have sieves built into their jaws which filter sea water and catch the microscopic plankton to use for sustenance. There are also Parrot fish with beaks that eat the coral and produce sand!
One question that often arises is how do the actual coral reefs survive in those tropical waters that are crystal clear, but also nutrient poor? The polyps are animals and animals must feed, what and how do they feed? What do the actual coral polyps feed on and how? They are symbiotic with some algae, but what does the symbiosis involve and does the algae supply enough to sustain the coral polyps of do they actually eat other things?
For starters the coral polyps are in the family cnideria, same as anemones and jellyfish. They have small sack-like bodies with minute tendrils and a mouth, but no true head. The tendrils radiate out evenly from the body. They have associated algae which gives them color and nutrients while removing waste products that could damage the coral. In the meantime the coral protects the algae from being eaten and gives it a fixed place to live. Thus clear water actually helps magnify the sunlight and provide energy to the polyps, but the polyps do not eat the algae, they live in a mutually beneficial fashion with it, this is symbiosis.
During the day, the coral polyps retreat into their calcium carbonate exoskeleton and rest. The algae stays active processing the sunlight and building up energy stores, in the evening the polyp becomes active. It protrudes from its “shell” and puts forth the tentacles, each arm has small, microscopic stinging cells referred to as nematocysts. A nematocyst is a tiny, poisonous barb packed with a spring. These nemacysts are released as soon as anything touches the polyp!
The tiny barb shoots out and injects the intruder, usually a type of zoo-plankton and the poison kills it. The tendrils then latch onto the tiny morsel and places it into the mouth, the digestive juices of the polyp takes over and the rest is food history! The plankton is broken down into its constituent parts and adsorbed by the polyps. The waste products are actually secreted and utilized by the algae.
So, coral polyps which form the coral reefs rely on algae for energy and plankton for nutrients and feed using poisonous bards at night! The other creatures feed around, and sometimes on, the coral reefs, but are not the coral reef feeding!
