You place a green hydnophora frag two inches from a favia because they look good together under blue light. A week later the side of the favia facing it is bare white skeleton, and the hydnophora looks better than ever. Nothing in the tank changed. The hydnophora reached over at night and took the space.
Looking for one now? See the hydnophora corals in stock: the Green Hydnophora and the Eye of Horus Hydnophora. Quarantined: held at our Key West facility for at least 4 weeks and dipped multiple times before it’s offered for sale.
Hydnophora, often called horn coral or velvet horn, is a stony coral in the family Merulinidae from the Indo-Pacific. It is not hard to keep alive. The hard part is that it is one of the most aggressive corals you can put in a mixed reef, and most of the problems people have with it are problems their other corals have with it.
What a hydnophora is
The genus is named for its hydnophores: small cone-shaped mounds that form where the walls of neighboring polyps fuse. Instead of the separate round cups you see on most stony corals, the surface is covered in these little peaks, with the polyps arranged around the base of each one. That is the easiest field mark. If the surface looks like velvet pulled up into tiny points, it is a hydnophora.
It grows in more than one shape. Hydnophora rigida forms bushy colonies of thin branches with no encrusting base. Hydnophora exesa grows as encrusting or lumpy, submassive colonies. Trade names like “Green” and “Eye of Horus” are color selections, and our listings say so: the name alone does not tell you the species or the growth form, so we do not assign one we cannot confirm. Expect a frag to encrust its plug first and then either thicken or branch.
Sweeper tentacles: how far they reach
Hydnophora fights in two ways. It extends long sweeper tentacles, mostly at night, that sting whatever they touch. And like many stony corals it can push digestive filaments out of its body onto a neighbor in direct contact. Top Shelf Aquatics puts sweeper reach at 2 to 4 inches, more in strong flow. Our listings start at 6 to 8 inches of clearance, and more space on the down-current side, because flow carries the tentacles in that direction.
- Give it its own section of rock with open space on every side.
- Do not put it up-current of soft-tissue corals like favias, acans, chalices or montipora edges.
- Check it with a flashlight an hour or two after lights out. That is when you see how far it actually reaches in your tank.
- If a neighbor starts losing tissue on the side facing it, move one of them that week. Do not wait for it to stop.
Plan for growth too. A frag that is safely spaced today closes the gap as it spreads, so leave more room than it needs now.
Light and flow numbers
Our listings run hydnophora at moderate light, about 100 to 200 PAR, with moderate, variable, indirect flow. Branching forms are often kept brighter. Top Shelf grows its Hydnophora rigida at 200 to 300 PAR with strong random flow. Start a new piece toward the low end and move it up over a few weeks. Sudden bright light can bleach the tips.
Flow matters more than people expect. Detritus settles in the valleys between the cones, and tissue under it dies. Aim for flow that keeps the surface clean without blasting it from one direction. Shaded undersides of a branching colony are the first place tissue goes, so keep light and flow reaching the base. Our reef tank lighting guide covers PAR and placement in more detail.
Why the green glows
The bright green on a good hydnophora is not the algae in its tissue. It is fluorescent protein the coral makes itself, partly as sunscreen. A 2018 study by Quick, D’Angelo and Wiedenmann in Frontiers in Marine Science compared high- and low-fluorescence color morphs of Hydnophora grandis. Under strong blue light the bright morph lost fewer algal cells and grew faster. Under low light it grew more slowly, and both morphs turned their fluorescence down to roughly 30 to 60 percent of maximum. In plain terms: a green hydnophora under too little blue light fades toward brown, and the same coral under steady moderate light holds its color.
Feeding
Hydnophora gets most of its energy from light. It will also catch small food at night. Fine foods such as rotifers, copepods or reef powders one or two times a week help growth, but they are optional in a tank with fish being fed. Do not overfeed to chase growth. Extra nutrients feed algae in the valleys before they feed the coral.
Common problems
| What you see | Likely cause | What to do |
|---|---|---|
| Neighbors losing tissue on one side | Sweeper tentacles or contact | Move one coral now, add space down-current |
| Dead patches on the underside or base | Low light and flow, detritus | Raise it, open up the flow, trim shaded overhangs |
| Pale or white tips | Too much light too fast | Lower it or dim the light, then ramp up slowly |
| Tissue peeling from the base up | Alkalinity or temperature swings | Stabilize alkalinity first. See bleaching vs STN vs RTN |
| Dull brown color | Low light or high nutrients | More light, nitrate and phosphate low but not zero |
Fragging
Hydnophora frags well. Cut between the ridges with a saw or sharp bone cutters rather than crushing the skeleton, glue only the base, and give the cut pieces clean, strong flow for the first week. Wear gloves, as you would cutting any coral.
Acclimation
Corals do not need a drip. Turn the lights off and float the sealed bag for 15 to 20 minutes. Open it into a clean container and add about half a cup of tank water every few minutes for 15 to 30 minutes. Lift the coral out by its plug and throw the bag water away. Dip it before it goes in the display, then place it low in the tank with the light off or low for the first day. Our coral dipping guide explains which dip to use.
FAQ
Is hydnophora aggressive?
Very. It uses sweeper tentacles, mostly at night, and can damage neighbors on contact. Start with 6 to 8 inches of space and more down-current.
Is hydnophora LPS or SPS?
It has small polyps, so many reefers group it with SPS, but it is sold under both labels. We list it with our LPS corals.
How much light does hydnophora need?
Moderate to high, about 100 to 200 PAR to start, and higher for branching forms once settled.
Is hydnophora good for beginners?
Our listings rate it intermediate. Keeping it alive is not the hard part. Spacing it so it does not kill its neighbors is.
Why is my hydnophora turning brown?
Usually too little light or too many nutrients. Fluorescent color needs steady blue light, and extra nitrogen favors algae in the tissue.
How fast does hydnophora grow?
Our listings describe it as an aggressive grower once settled. It encrusts its plug first, then thickens or branches, which is why spacing has to allow for the size it will be next year.
Buy hydnophora coral
Shop the hydnophora category: Green Hydnophora and Eye of Horus Hydnophora. For other corals that spread across rock, see encrusting corals, and use the coral identification guide to compare it with look-alikes.
The Underwater Gardener is a reef aquarium store in Key West, Florida. Quarantined: held at our Key West facility for at least 4 weeks and dipped multiple times before it’s offered for sale. Livestock ships UPS Next Day Air on Tuesdays and Wednesdays.
Corals in stock right now
Photographed in Key West, in stock today. Fill your box: up to 4 spots ship for one shipping charge. A frag takes 1 spot and a colony takes 2. Browse corals under $100 or beginner corals to fill one.
Comments (0)