Rising water temperatures due to global warming are causing coralPolyps to expel their Zooxanthellae as part of Coral Bleaching. When coral polyps die, their exoskeletons are left for new polyps to build upon.
Over long periods of time this process creates reefs. Most coral reefs are large complex ecosystems found in the tropics comprised of billions of tiny polyps, minute animals, and algae.
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? 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. It protrudes from its “shell” and puts forth the tentacles, each arm has small, microscopic stinging cells referred to as nematocysts.
So, coral polyps which form the coral reefs rely on algae for energy and plankton for nutrients and feed using poisonous bards at night! When a coral reef dies without a lot of plant-eating fish around to save it, there is usually one direction it tends to move toward: total annihilation.
When coral polyps are ailing, they tend to expel the symbiotic single-celled plants called zooxanthellae that give corals their color and the nutrients they need to survive. A dead, bleached reef can come back to life as the skeleton of its remains becomes a substrate for new baby corals to grow on.
But the reef's new enemy will be the growth of blanketing algae that prevents new, living coral from growing. Overfishing by humans tends to reduce the number of big, herbivorous fish that would otherwise eat the algae and give the reef a chance at rebirth.
Altair and Caitlin D. Temple, a PhD fellow from the University of Queensland in Australia, set out to find out why the bleached coral hadn't been covered in algae. Sea urchins the size of ping-pong balls (Echinoderm irides) and a diminutive species of parrot fish (Scars Isère) were grazing algae on the reef.
The ability of these tiny grazers to step in and fill the role with their larger relatives is somewhat analogous to taking bison off of a prairie, only to learn that groundhogs were able to maintain it. But some existing research suggests that small species of sea urchins may have become ecologically important in the past, taking on the key role of algae clean-up in the absence of larger grazers.
Altair and other scientists at the Smithsonian Tropical Research Institute are waiting for the corals to re-colonize the reef naturally. The research gives some new hope to the well-known plight of The Great Barrier Reef, which has recently entered the bleaching phase of a die-off.
While the exact mix of species on the Great Barrier Reef is different, the ecological principle of tiny herbivores eventually expanding their population enough to do the job of their larger cousins could potentially be repeated. Like their relatives, the jellyfish and the sea anemone in the Cnidaria phylum, each individual coral, or polyp, has barbed, stinging cells called nematocysts it can extend to capture prey like zooplankton or small fish.
Coral almost could be considered half-plant because of the zooxanthellae (pronounced zoo-zan-thelly) algae that live just inside each polyp's cell walls. In turn, the polyp shelters the zooxanthellae and provides the carbon, nitrates and phosphates the algae need for photosynthesis.
Up to 90 percent of the energy produced by zooxanthellae's photosynthesis is transferred to the coral host . They secrete the limestone from their base, creating a protective skeleton and a hollow chamber called a cup.
They simply secrete more calcium carbonate under and around their current cup, creating the framework of the reef and causing it to grow both upwards and outwards. Corals may reproduce either asexually, by dividing and producing identical clones, or sexually, by sending out eggs or sperm.
Either way, new coral polyps settle towards the ocean bottom until they find a hard substrate to call home, either combining with a pre-existing coral colony or starting one of their own. In addition to being connected at their bases, coral polyps link to one another laterally by a thin tissue called the coenosarc.
Relatives of anemones and jellyfish, corals help to create reefs that house some of the greatest biodiversity on earth. Corals were first believed to be plant-animals but later were established to be solely marine invertebrate animals because they reacted to touch.
The subclass, Hexacorallia has the mantids, sea anemones, and stony corals. The stony corals secrete a tough structure made of calcium carbonate around them.
Since most corals are attached to the substrate, they feed by taking advantage of the water current that brings them an abundance of food. Small fish and tiny zooplankton form the primary diet of corals.
At the right moment, the nematocysts of the coral's tentacles are utilized to immobilize the prey. Some species of corals subsist on photosynthetic dinoflagellate algae that grow within its polyp structure.
The photosynthetic process performed by the algae also helps to toughen the coral's outer calcium carbonate structure. The majority of corals colonize shallow-water reefs with the help of energy obtained from photosynthetic algae harbored within them.
Canal digging near inland bays and coves pose a threat to fringing reefs. Underwater springs that go directly to the ocean and transform the water salinity also threaten the sensitive reefs that cause coral and fish die-offs.
Today, countries working with conservation groups have created many opportunities for coral reef protection. The creation of marine reserves, underwater parks, and heritage sites have all contributed to the saving and protection of coral reefs and its inhabitants.
Additional efforts such as habitat protection and fishery management have also made coral conservation more effective. This method entails the cultivation of coral fragments that are put back in the depleted reefs.
Corals can multiply both asexually and sexually, but the latter is the predominant form of reproduction. Reproduction is influenced by the cycle of the moon, length of daytime, and natural chemical signals.
About 75% of the stony corals release their eggs and sperms into the water which after fertilization develops into tiny larvae called plane. These larvae grow in the water until they are ready to settle on the substrate to form new colonies.
Another type of coral, the brooders inhabit waters with strong currents and waves. The asexual reproduction methods endow regeneration abilities to the coral species, creating more individuals in a given population.
Coral larvae are either fertilized within the body of a polyp or in the water, through a process called spawning. In some areas, mass coral spawning events occur one specific night per year and scientists can predict when this will happen.
Once attached, they metamorphose into a coral polyp and begin to grow, dividing in half. As more and more polyps are added, a coral colony develops and eventually begins to reproduce.
But tropical reef-building corals have tiny plant-like organisms living in their tissue. In return, the algae provide the coral with food.
Learn more in the Ocean Portal's coral reef section and in this zooxanthellae slideshow. The coral polyps, which usually live colonially, remove calcium carbonate from the seawater and use it to build the skeletons that protect and support their tiny, cup-shaped bodies.
For these organisms, the coral reef structure provides an ideal habitat in which to find food and shelter. Algae on the reef remove excess nutrients and add needed oxygen as they photosynthesize.
Some species, known as coralline algae, are calcareous and act like cement to help hold the reef structure together. Algae, in turn, is food for animals who are grazers or herbivores, such as conchs, sea urchins, and parrot fish.
Most coral reefs do not experience tides, and those that do not have the extreme tidal fluctuations seen in temperate ecosystems. There is only a slight seasonal change in the average water temperature and amount and intensity of daylight.
This extremely stable environment is one of the reasons tropical coral reefs are able to support such a tremendous diversity of flora and fauna. Dense blades serve as protective nurseries for many reef species, while roots and underground stems stabilize the soft bottom.
Seagrasses and the algae that grow on their blades are important producers of food and oxygen for the entire reef ecosystem.