Horn Coral

Thorny Coral, Velvet Horn Coral, Knob Coral

Horn Coral Hydnophora rigida, also known as Thorny Coral, Knob Coral, Velvet Horn Coral and Green Horn CoralHydnophora rigidaPhoto © Animal-World: Courtesy David Brough

   With a bright green color and chiseled branches, the Horn Coral is an striking attraction in the reef aquarium!

   The eye catching Horn Coral Hydnophora rigida is a real beauty. This coral is naturally a green and brown, or when it doesn't get enough light it turns yellow. It can also be a beautiful fluorescent green. It forms an arborescent, branching, colony without an encrusting base, though its reported some specimens do encrust in the reef tank. There are a number of descriptive common names it is known by including Thorny Coral, Velvet Horn Coral, Knob Coral, Green Horn Coral, Branch Coral, and Deer-Horn Coral.

   The Hydnophora genus have several different growth forms, that can be either encrusting or arborescent. But they all have an almost "furry" appearance due to having Hydnophora. This is a unique skeletal characteristic that looks like little mounds, or bumps, on the surface of these corals. They form where the common wall between corallites intersect.

   The Horn Coral is moderate to care for and easy to frag, with the biggest requirement being moderate to bright light and good water flow. In the wild, their polyps only extend at night, though in captivity they are eager feeders and extend their polyps during the day. They will eat a lot of food if offered to them, and being a fast grower they do need it. The Horn Coral is very aggressive. It should not be placed close to other corals as it can extend sweeper polyps and sting them.

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Scientific name    Family: Merulinidae
   Species: Hydnophora rigida

Distribution / Background    Hydnophora Coral Information: The Hydnophora genus was described by Fischer de Waldheim in 1807. The Horn Coral Hydnophora rigida was described by Dana in 1846. Some of the common names they are known for are Thorny Coral, Velvet Horn Coral, Knob Coral, Green Horn Coral, Branch Coral, and Deer-Horn Coral. They have been successfully propagated in captivity. There are around 14 species of Hydnophora, yet the most common in the trade are H. rigida and H. exesa.

   Where Hydnophora Corals Are Found: The Hydnophora rigida are found around Australia in the Great Barrier Reef, Coral Sea, and North West Cape and also from Nicobar Islands to Fiji. Hydnophora genus are found from the east coast of Africa, up to the Mediterranean and Red Sea. To the east they encompass the west, north, and east coasts of Australia and all tropical waters north of Australia to Kyushu Islands and Marianas. They are found from the Lord Howe Islands, east, all the way to the Tubuai and Tuamotu Islands. They then extend northward to the Line, and Marshall Islands, which lead back to water north of Australia.

   Hydnophora Coral Habitat: H. rigida inhabit lagoons and protected reef slopes.

Status    The Hydnophora rigida is not on the IUCN Red List for Endangered Species.

Description    What do Hydnophora Corals look like: The Hydnophora genus have several different growth forms, that can be either encrusting or arborescent. But they all have an almost "furry" appearance due to having Hydnophora. This is a unique skeletal characteristic that looks like little mounds, or bumps, on the surface of these corals. They form where the common wall between corallites intersect. The corallites (holes) are where the tentacles are located. In the wild the tentacles come out at night, yet in captivity, they come out during the day and are not active at night.

  • The Horn Coral H. rigida has thin branches without encrusting bases and can be cream or green, and can have a green fluorescence. They can have splashes of brown or burgundy and the polyps may contrast slightly. They can form a bush like stick structure that can be 3.3 feet (1 m) across.
  • H. exesa can have huge colonies that are encrusting or flat, but can form thick irregular projections or branches which can be cream or green.
  • H. microconos has very uniform Hydnophora and has a rounded growth structure, making for a very neat and tidy appearance. They can be cream, brown or green.

Difficulty of Care    Hydnophora Coral Care: The H. rigida is moderate to care for with the biggest requirement being moderate to bright light and good water flow. They are eager feeders, but are are susceptible to bleaching, recession, and other ailments. Some aquarists have noted that compact fluorescent lighting is generally not sufficient for these corals to live past a year.

Foods / Feeding    Hydnophora Coral Feeding: In the wild, Hydnophora corals have developed several feeding strategies. Through a symbiotic relationship with a marine algae, known as zooxanthellae, they receive the majority of their nutrients. They also capture planktonic organisms, food particles from the water column, and can absorb dissolved organic matter.

   In captivity, like other SPS, these corals use their zooxanthellae for nutrition, but they can be fed larger foods that they digest on the surface. Feed finely shredded shrimp and ocean fish along with copepods and other small crustaceans that naturally occur in established tanks. They should be fed several times a week, and they do grow fast.

Aquarium Care    Pristine tank conditions are typically needed to keep all SPS corals. Keep the nitrate levels low, and maintaining calcium and alkalinity levels. Typically you can do water changes of 20% to 30% a month, 15% every 2 weeks, or 5% a week for SPS corals. The 5% a week also seems to really make a big difference in other SPS corals health.

   The following water supplements are suggested for Hydnophora species:

  • Calcium:  385 to 425 ppm. If a small poly stony (SPS) coral does not have enough calcium, it will not grow.
  • Alkalinity:  3.2 TO 4.8 MEQ/L (8 to 12 dKh)
  • Phosphates:  0, zero. Phosphates are the worst of all and all corals hate them.
  • Magnesium:  1350-1500. Magnesium makes calcium available, so if your calcium is low, check your magnesium levels before adding any more calcium.
  • Strontium:  Strontium (10 for most SPS Corals), and trace Elements are also suggested.

Aquarium Parameters

   A well-feed live rock/reef environment is what is needed for your Horn Coral. A mature tank is recommended. Provide a refugium to produce natural foods. Most SPS corals do well with some fish for organic matter production.

Quick Reference Chart
Lighting: Prefers Low Lighting Levels Prefers Medium Lighting Levels Prefers High Lighting Levels
Water Flow: Prefers Low Water Flow Levels Prefers Medium Water Flow Levels Prefers High Water Flow Levels
Temperament: Peaceful Temperament Semi-Aggressive Temperament Aggressive Temperament

   For tanks with metal halides, you would position SPS corals in the mid levels. With other lighting, position SPS corals at the upper to mid levels depending on the watts used. SPS corals will show whether they are happy or not by the coloring. Make sure that no other corals can come in contact with your Hydnophora. Hydnophora spp. are very aggressive and can extend sweeper polyps and sting other corals.

  • Minimum Tank Size / Length: 50 gallons (190 L) or larger
  • Marine Lighting: Moderate to high
  • Temperature: 74° - 83° F (23° - 28° C)
  • Salinity / Specific Gravity: 1.023 - 1.025
  • Water Movement: Moderate
  • Water Region: Bottom of the aquarium

Compatibility and Social Behaviors   Hydnophora are very aggressive and can extend sweeper polyps and sting or basically eat other corals it touches. The Hydnophora genus will basically win any fight with other corals. Keep well away from other corals in the tank. They sometimes are found with a little commensal shrimp called Metapontonia fungiacola. They are fine with reef safe fish

Sex - Sexual differences    No sexual difference in appearance is known.

Breeding and Reproduction   The small polyp stony (SPS) corals are male and female and can reproduce both sexually and asexually. In the wild they reproduce sexually by releasing eggs and sperm at the same time, resulting in a fertilized egg which then forms into a free-swimming planula larva. Eventually the planula larvae settles onto the substrate, becoming plankters. This then forms a tiny polyp which begins to excrete calcium carbonate and develops into a coral. Planula larvae are extremely vulnerable to predation, and very few survive. Hydnophoras reproduce asexually as well. In the wild Hydnophoras spread from breakage due to storms and fragmentation.

   Propagation is very easy for Hydnophora corals. First you need to choose a healthy coral that is not showing any signs of distress. Then, simply cut a branch at least 2" long. The best choice for Hydnophora with dense structures is to use an electric saw. Although they produce a lot of slime, they actually resist infection once they are cut. The frag should be left on its side or upside-down. keep the cut or wound completely open to water flow so it can heal. If not, the cut area can get infected. They quickly start to grow into beautiful forms. Hydnophora heal very quickly and the mother colony regrows quickly. Allow the frags to heal before moving them to a new tank. Again, give the frag ample water flow.

Potential Problems   The Hydnophora spp. are generally disease resistant, but can still get the same illnesses that any other small polyp stony (SPS) coral can get under poor conditions. They are sensitive to "brown jelly" bacterial infections and that situation needs to be handled quickly. Recession can happen under unfavorable lighting and water conditions, and usually starts from the basal area. If you have problem with cyanobacteria or algae, keeping moderate water movement will prevent them from attaching to your Hydnophora.

Availability    Hydnophora Corals for Sale: The Horn Coral H. rigida is easy to find at pet shops and on line. Online they can run about $38.00 USD or more depending on size and/or color.

References

Author: Clarice Brough, CRS, Carrie McBirney
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