Part 2: Partners, Competitors, and Enemies on the Great Barrier Reef Q2. Stretching over 1,200 miles, Australia's Great Barrier Reef is the largest structure on Earth built by living organisms. Even for its size, it houses an incredible biodiversity -- literally thousands of species, many yet to be identified. How has such variety evolved? Simply put, individual organisms must change and then survive in their new forms. They can be pressured to change -- sometimes into entirely new species-- both by the physical environment and by other organisms with which they interact. Below are descriptions of Great Barrier Reef species that have evolved as partners, competitors, and enemies in the never-ending challenge to survive and reproduce. Use arrows and signs (i.e., +, -, or 0) to show the type and direction of each interaction between the species in boldface. a. Some multicellular algae on the reef produce calcium carbonate (limestone) skeletons very similar to those made by hard corals. These calcareous algae play a major role in barrier reef construction, acting as a sort of living mortar that holds together individual coral colonies. Growing between corals and wrapping around the bases of branching corals, calcareous algae protect the corals from erosion, especially in high-energy areas. Individual coral colonies, especially branching corals, can easily be toppled in high-energy reef zones, such as the reef front and rock rim. Waves can easily scour away sediments from a colony's base, uprooting it and pushing it along like tumbleweed. So how do branching corals ever get a solid foothold in such zones? Calcareous algae grow between corals and around their bases, preventing erosion and stabilizing the reef structure. Calcareous algae Corals mutualism, C. algae needs a hard substrate aka home (the coral) + b. It takes a lot of energy to secrete the calcium carbonate exoskeletons (hard outer structures) that make up coral reefs. Reef waters are typically very low in nutrients, so most coral animals can't filter out enough food to provide the extra energy they need. To make up for this deficiency, hermatypic corals shelter microscopic algae (zooxanthellae) within their tissues; in exchange, the algae supply the corals with carbohydrates so the corals have enough energy to build reefs. Zooxanthellae (pronounced "zoe-zan-thelly") are microscopic algae that live within the tissues of host animals, including hermatypic coral animals. Like all plants, zooxanthellae make their own food by a process called photosynthesis. Using solar energy absorbed by special pigments, they transform carbon dioxide into carbohydrates and oxygen. What they don't need themselves passes directly into the coral's gut cavity, providing the extra energy the coral needs to produce a calcium carbonate exoskeleton. C. + + Zooxanthellae → Corals + + Hard corals have evolved to have large amounts of a wax (cetyl palmitate) in their tissues. Very few predators can digest the wax, which has allowed corals to flourish and produce massive reefs. Recently, epidemic populations of one predator -- the crown-of-thorns starfish -- have done extensive damage to many reef regions. Covered with long, venomous spikes, the crown-of-thorns starfish (Acanthaster planci) is a voracious feeder that can eat living corals because of a unique adaptation: a wax-digesting enzyme system. Mutualism Corals Crown-of-thorns starfish Antagonism d. One of the few predators of the crown-of-thorns starfish, the giant triton (Charonia tritonis) has evolved a tolerance to the starfish's toxins. Unfortunately, tritons can no longer keep starfish populations in