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Corals have fascinating colors and interesting shapes. Many types are reminiscent of flowers or filigree plants. However, corals are animals. They live in groups of single individuals, the polyps. There are often thousands united in a colony. Once anchored in the soil substrate, they can no longer move the polyps, which are sometimes only a few millimeters in size. Their tentacles, tiny arms surrounding their only orifice, move passively with the ocean currents. The animals have capsules with nettle poison in special cells. Thus, taxonomically, corals belong to the sessile, colony-forming cnidarians (Cnidaria).
There are various species-rich groups (taxa) that are not closely related to each other, for example hard coralswhich are significantly involved in the formation of reefs, Octocolrallen, to which also soft corals or sea turtles and black corals. All of these groups resemble flowers in appearance and belong to the Anthozoa.
Fire corals and filigree corals, on the other hand, belong to the class Hydrozoa. Over 6000 species of exclusively marine animals are known. They are found in all oceans on earth, often adapted to warm shallow water areas. However, certain cold-water or deep-sea corals can also be found in northern latitudes, for example off the coast of Norway, sometimes hundreds of meters below the sea surface. However, most species live in the tropical belt, in the warm waters of the earth. Corals are filter feeders. They filter microplankton (tiniest crabs and algae), nutrients and trace elements from the water around them. Corals that live in shallow sea water or closer to the surface also feed themselves with the help of unicellular algae, so-called zooxanthellae, which live as endosymbionts in their cells. The metabolic cycle of both organisms is so closely interlinked that the photosynthetic products of the algae serve as the main food source for many coral species.
Why are corals so important
Coral reefs, large structures in seas formed by certain species of coral over thousands of years, are the most biodiverse marine habitats. they offer
countless marine animals, such as snails, crabs, mussels, reef sharks, manta rays or turtles. Coral reefs are also an important biotope for a large number of different fish species. They offer them protection, are suitable places for laying eggs and serve as a nursery for the offspring.
About a quarter of all marine fish live between corals or in the area of coral reefs, are adapted to this habitat and depend on it.
Corals in the seawater aquarium – for beginners and advanced
For Beginner offers the attitude of soft corals (Alcyonacea). Compared to other coral groups, they have fewer demands on their environment and are more tolerant of environmental changes. Their tissue is soft and fleshy, small calcareous needles in their body give it support. They are usually yellow, brown or green in colour, but some species also shine in red, orange or violet. Because their polyps have eight-rayed symmetry, they often resemble small, colored flowers.
Around hard corals (Scleractinia) to be successful, the technology of a seawater tank must work properly and the biology in the tank must be stable. Therefore hard coral species are suitable for experienced marine aquarists. In nature, hard corals are mainly found in tropical waters. They secrete lime at their base. The plant-like, branched single skeleton that forms over time contributes to the support of the colony in which they live.
The colorful individual polyps sit on top. If they die, their calcareous skeleton remains and new polyps settle at this point. In the natural environment, the gradual growth of the animals can form coral banks and finally huge limestone structures, coral reefs. Entire islands were formed as a result of the growth processes of the smallest hard coral polyps, such as the Maldives in the Indian Ocean, the Bahamas in the Atlantic or Kiribati in the Pacific Ocean. A seawater tank that is stocked with stone corals must not be dominated by soft corals and should have a small number of fish. The latter prevents excessive entry of nitrate and phosphate into the pool. Stony coral species with large polyps (LPS) have high nutritional needs.
Jenny (JennyHuang) from Taipei, Nephtheidae 2, CC BY 2.0
Many of these species do not form symbiosis with algae, instead feeding mainly on zooplankton or phytoplankton. They do not tolerate too strong or constantly changing water currents. hard corals with smaller polyps (PLC) need high light intensity and strong and changing water currents to feel comfortable. They feed almost exclusively on the photosynthetic products of the algae, with which they form a symbiosis. They place high demands on the water quality in the aquarium.
Only professionals should dealing with anemones to be reserved. These produce a very strong nettle toxin. Caring for them and cleaning the aquarium requires a lot of experience and special precautions, such as wearing goggles, gloves and a mask.
Toby Hudson, Coral Outcrop Flynn Reef, CC BY-SA 3.0
Saltwater aquarium – important tank parameters
size of the aquarium
The larger a seawater aquarium is, the easier it is to maintain, the more stable the biological system that has established itself in the tank works and the more comfortable the animals feel in it. A mixed aquarium, in which other sea creatures live in addition to corals, should hold 300 liters if possible. Corals can also be kept in a nano tank. In smaller basins, fluctuations in water values have comparatively greater effects on the biology in the system. A nano basin should therefore be compared with more robust species from the group of soft corals be occupied, more sensitive stone corals are not really suitable for a small aquarium.
Light intensity and light spectrum
Many corals enter into a symbiosis with algae. Their photosynthetic products, sugar compounds and amino acids serve as food for the corals. Their light requirement (light intensity) is therefore related to the algae that form a community with them. Too high light intensity can cause excessive algae growth in the tank, too low intensity can cause deficiency symptoms. In the worst case, the corals starve. Species that do not feed on algae, but only on plankton, often prefer darker locations. Also regarding light spectrum coral species have different demands. For example, some require a combination of blue and white light.
In order to find out at which light intensity and with which spectrum a certain species feels comfortable and grows well, it is important to obtain specific information in advance. A conversation with the seller or a person who already successfully keeps the species in question often helps.
Beginners who initially care for corals that require less light, such as soft corals, should not set the light intensity to less than 0.5 to 0.6 watts per liter when lighting with LEDs.
illuminants
Very beautiful and individual lighting moods can be conjured up in coral tanks with LEDs. The more options these lighting devices offer in terms of dimming and control, the better. LED systems can be expanded and offer the possibility of specifically influencing the biological system in the aquarium, such as a selective reduction in lighting when algae growth is too strong.
Fluorescent tubes (T5) illuminate a large area of a coral landscape. The tubes should be changed every six months as they lose their luminosity or
change their light spectrum. Fluorescent tubes heat the aquarium water
must then be cooled again.
temperature
A seawater tank in which corals live must not exceed the critical temperature mark of 28.5° – 29° Celsius.
Many coral species feel comfortable at a temperature of 24 – 26 degrees Celsius.
water flow
Corals are filter feeders. They filter microplankton (tiniest crabs and algae) as well as nutrients and trace elements from the water around them. As sessile animals, corals are dependent on currents. Flow pumps in the pool imitate the movement of the sea. They ensure a constant flow of fresh water, which supplies the tank with oxygen and nutrients and washes away dirt and suspended matter from the animals. Reversible pumps also change the direction of flow, for example every six hours.
nutrients and care
For a balanced supply of nutrients, it is advisable to connect a dosing pump at. The smallest amounts of microorganisms and trace elements are thus continuously released into the water. Caring for corals means caring for the aquarium first and foremost. In addition to regular cleaning, important water parameters must be checked, for example with regard to salt content, silicate, pH value, ammonium, nitrite, nitrate, phosphate and other elements. Commercially available measuring kits, test strips, can be used for this. Other methods, such as ICP analyses, are also suitable for a more precise analysis.