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Did you know there are over 6,000 species of coral in the world? The types of coral available in the aquarium trade is amazing and with the introduction of designer species, there are new colors and variants every day. Let’s talk about what is a coral, where, they come from, and the 4 major types of corals available in our hobby. Let’s dive in!
What Exactly Is Coral?
Live corals are the crown jewels of the natural marine world, but what exactly are they?
This marine life comes in all shapes and sizes and can be found in almost all saltwater ecosystems. Some corals live in the most tropical and shallow waters getting direct sunlight while others can withstand freezing temperatures and complete darkness. Through all their differences, they are connected.
Corals are animals. They are not plants and they are not parts of the rock. Instead, they are marine species that lack a vertebral column, making them a type of invertebrate. More specifically, corals belong to the phylum Cnidaria, which is a large taxonomic group that contains over 11,000 species of coral, jellyfish, gorgonian, and anemone.
The most outstanding feature of this phylum is their inclusion of stinging cells called cnidocytes; these are what give jellyfish their stinging tentacles. There are different types of cnidocysts including nematocysts which contain venom, spirocysts which are very sticky, and ptychocysts which help the animal build a protective tube. All corals have cnidocytes though not all are dangerous to humans.
From Cnidaria, corals can be broken into different categories.
Corals belong to the Anthozoa class of Cnidaria. Of the 6,000 different known species of marine life in this group, corals make up over a third of its members. Even further, the Anthozoa class is broken into two main subclasses1 which then contain different species:
- Octocorallia subclass
- Hexacorallia subclass
The differences between these two groups are mainly in appearance and growth pattern.
As its name suggests, corals in the Octocorallia subclass have eight tentacles and form colonies. These tentacles grow in a pinnate fashion, meaning opposite from one another. This group includes species of gorgonian, organ pipe coral, as well as the Alcyonacea order of soft corals.
For the most part, the majority of soft corals belong to the Alcyonacea order. These corals lack a complete hard calcium carbonate skeleton but contain small sclerites that give them some structure. Instead, they have very fleshy individual polyps that are easy to divide and reproduce.
Some of the most recognizable species in the Octocorallia subclass are:
The Hexacorallia subclass features tentacles in multiples of six that do not grow in a pinnate fashion. These corals may grow alone or in colonial forms and are known as reef-building corals due to their internal calcium carbonate skeleton. The Hexacorallia subclass contains some of the most recognizable species of hard and stony corals.
Though this subclass is known for having hard structures, zoanthids belong to the Zoantharia order under this categorization as well; zoanthids are colloquially known as soft corals in the aquarium hobby, though they may incorporate sand and other stronger materials into their flesh for some support.
The other order under the Hexacorallia subclass is Scleractinia. The Scleractinia order comprises most of the available coral species in the hobby today, including both large polyp stony coral and small polyp stony coral varieties. As a result, most coral reef ecosystems are built from the hard calcium carbonate skeletons of members from the Hexacorallia subclass.
Many marine mushrooms also fall under the Hexacorallia subclass in the Corallimorpharia order. These corals lack any sense of hard skeleton structure, which does not allow for a fossil record.
Here are some of the most recognizable corals within the Hexacorallia subclass:
Why Is Coral So Important?
Now that we understand corals on a scientific level, we need to understand how each one of these species plays into their larger ecosystem.
You might have heard that coral reefs are dying. But what does this mean exactly and why does it matter?
Coral reef ecosystems are homes to hundreds of animals, plants, and bacteria. These are some of the most unique and biodiverse ecosystems on this planet and extend past their aquatic borders, into estuaries, lagoons, rivers, and beaches.
Corals reefs provide food and shelter for numerous fish and invertebrates. They also provide food, protection, medicine, and recreational value to humans; corals reefs are natural structures that help preserve shorelines and prevent erosion from oncoming storms and strong tides.
Not only would entire ecosystems collapse should coral reefs die, but human life for millions would also be severely impacted by dwindling numbers and extinction.
Why Are Coral Reefs Dying?
Coral reefs are in trouble and it’s estimated that 70-90% of coral reef ecosystems will die in the next 20 years. These mass die-offs have largely been attributed to rising ocean temperatures, pollution, ocean acidification, overharvesting, and poor fishing practices.
Coral death is sudden and usually irreversible. Corals are animals that have a symbiotic relationship, or beneficial exchange, with a type of algae called zooxanthellae; the coral provides shelter for the zooxanthellae while the algae photosynthesize to make food to share.
When environmental conditions change and the coral becomes stressed, it will release these zooxanthellae, leaving the coral an empty white calcium carbonate skeleton; this is known as coral bleaching and can only hope to be reversed when the original stressor is corrected.
Unfortunately, many reefs are already dead or are on their way to becoming fully bleached. Important reef-building corals, like the staghorn coral (Acropora cervicornis) and the elkhorn coral (Acropora palmata), are critically endangered and quickly disappearing.
Where Can You Find Coral Reefs?
The most famous coral reef is the Great Barrier Reef in the tropical waters of the Indo-Pacific off the coast of Australia. This coral reef is so massive that it can be seen all the way from outer space!
Being so large, the Great Barrier Reef is made up of at least 3,000 individual reef systems. It is home to many of the aquarium hobby’s favorite fish and invertebrates, including many types of clownfish, angelfish, and butterflyfish as well as hundreds of different soft and hard corals.
Other famous tropical reefs include the Tubbataha Reef off of the Philippines, the Raja Ampat Reef off of Indonesia, and the Mesoamerican Barrier Reef off of Mexico. The coral reefs we are used to seeing are typically along the equator of the Atlantic, Indian, and Pacific Oceans.
But what about the coral reefs at the bottom of the ocean?
That’s right! There are coral reefs made of deep-sea species that have evolved to withstand high pressure and low temperatures down to about 6,000 feet towards the ocean floor. These corals do not need light and do not photosynthesize in these deeper waters. Instead, they depend on catching the many small organisms that happen to float past.
Most of these deep-sea corals can be found in the North Atlantic Ocean, though deep-sea coral reefs thrive at great depths throughout many of the world’s oceans.
4 Types of Coral in the Reef Hobby
Obviously, not all types of coral can be kept in the home aquarium, though the list of species and varieties available changes every year. For the most part, new corals are always being added but sometimes trade laws and environmental concerns limit what becomes available when. This can cause big shifts in price and availability, though the focus on aquaculture and sustainable harvesting has grown greatly over the past few years.
As a result of these sustainable efforts, the many types coral that aquarists love, are inexpensive, and easy to come by. Let’s look at the four main types:
- SPS Corals
- LPS Corals
- Soft Corals
- NPS Corals
Hard corals, also known as SPS corals, are some of the most desirable in the hobby. These reef-building corals are big, colorful, and an indicator of a high-tech and happy system.
Most hard corals belong to the Scleractinia order. There are two main types of hard corals: branching SPS and encrusting SPS.
Some of the most popular shallow water species of branching SPS belong to the Acropora, Montipora, Pocillopora, and Seriatopora genera. Some of the most popular species of encrusting SPS belong to the Favia and Echinophyllia genera.
Regardless of whether you pick a branching or encrusting type of coral, hard coral are very similar in their needs. They are some of the most challenging corals to keep, demanding high lighting, high water flow, and consistent water parameters.
SPS are regarded as slow-growing as they need to build hard calcium carbonate skeletons. There are a few species, like Montipora and Seriatopora, that seem to grow faster than the others, though those growth rates are relatively slow in comparison to LPS and soft coral species.
LPS corals, or large polyp stony corals, comprise of a large number of species that greatly vary in appearance. Some have long tentacles and aggressive tendencies, like torch corals (Euphyllia glabrescens), while others are compact and nearly harmless, like Blastomussa species.
These types of coral actually belong to the Scleractinia order along with other hard corals. This discrepancy is largely due to the fleshy large polyps of LPS corals that cover most of their underlying hard skeleton, making them difficult to see. Their needs are also much less demanding than those of related reef-building hard corals.
Most species of LPS need moderate lighting and moderate water flow. They grow quickly, but not as fast as slow corals as they need to build a calcium carbonate skeleton.
When talking about soft coral species, LPS and true soft corals are usually lumped together even though they are in completely different taxonomic subclasses; LPS are part of the Hexacorallia subclass while true soft corals belong to the Octocorallia subclass.
True soft corals are regarded as some of the easiest corals to keep. They don’t require high lighting, can tolerate low and moderate water flows, and are pretty forgiving about beginner’s mistakes. In addition, they’re very easy to propagate and grow quickly, making them especially inexpensive and readily available.
Some of the most popular soft coral species may belong to the Sinularia, Capnella, and Sarcophyton genera.
So far we know soft corals, LPS corals, and SPS corals, but there’s actually a fourth kind of coral that can be seen in the aquarium hobby: nonphotosynthetic (NPS) corals.
In terms of difficulty, soft corals are considered the easiest followed by LPS corals. The most advanced hobbyists will keep SPS-dominant systems or mixed reefs that contain all three. NPS corals are much more niche, though, and can certainly prove to be some of the most difficult.
NPS corals do not have zooxanthellae (azooxanthellate) and need to obtain food through other means, mostly by catching available microorganisms in the water column. This means that a lot of food needs to be available a lot of the time in stable water parameters, making them almost as challenging as some of the more difficult hard coral species.
There is no denying that NPS corals are some of the most unique types of coral available. They come in bright, almost fluorescent, colors and have interesting polyp shapes that allow for optimal feeding. They may sometimes be mistaken as a type of soft coral to the untrained eye.
Hobbyists have come up with some solutions for keeping their NPS corals fed, but many fail and lose their corals within a matter of months. Daily broadcast feeding is a must. It is even better if food intake can be controlled by using a plastic or glass container to cover the coral for more direct feedings.
In general, NPS corals are rare to come by in the average aquarium store because of their dietary needs. However, here are some of the species you’re likely to come across if you do:
- Sun coral (Tubastraea spp.)
- Fat head dendro (Dendrophyllia spp.)
- Carnation coral (Dendronephthya spp.)
- Chili coral (Nephthyigorgia spp.)
- Some gorgonians
Each species of NPS will come with its own set of difficulties, though sun corals are usually regarded as one of the easier types of coral in this category.
Wild-Caught vs. Maricultured vs Aquacultured Corals
When shopping for corals, where they come from matters. Most types of coral that are available in the aquarium hobby once originated from the Indo-Pacific. Some are still wild-caught directly from these tropical coral reefs, while other species have been maricultured or aquacultured.
In reality, there are few benefits to buying wild-caught corals.
Some of the reasons for doing so include rarity and variety. Different species may be rare due to being newly introduced into the hobby or being difficult to propagate in the aquarium setting. Many corals also differ in appearance depending on the area of collection, which can be appealing for some hobbyists that are looking for something uncommon.
Collecting corals from the wild has a significant impact on ecosystems, though. Remember, these animals are already suffering from global warming and overharvesting and removing them more is not helping.
In addition, wild-caught coral species come with a lot of problems: parasites, difficulty acclimating, and expense. It is safe to assume that any type of wild-caught coral bought will arrive with parasites or hitchhikers, requiring quarantine. The quarantine will also help ready your new coral for the display tank as they are extremely sensitive to changes in light, water flow, and parameters.
Lastly, wild-caught corals are more expensive than maricultured or aquacultured ones. This is due to vendors having to take the risk of importation and acclimation. Not to mention that coral importation laws can change at any given moment.
Maricultured types of coral are a decent alternative to wild-caught corals, but they still come with a lot of problems. In theory, mariculture is the best of both worlds, growing corals in their natural shallow water ecosystems with the intent to collect.
These coral farms are out in the wild, providing food and shelter for surrounding fish and invertebrates while also supporting local commerce when sold to hobbyists. Even better than that, these corals can easily be placed back onto the reef to start rebuilding damaged and dead ecosystems.
However, the transfer from the wild coral reef to the home aquarium is still difficult and pests and acclimation are still problematic. Though maricultured are much more sustainable than wild-caught ones, they still carry inflated prices and increased risk.
Aquacultured species are arguably the most sustainable, hardy, and attainable types of coral available to the average hobbyist. These corals are fully grown in captivity, far away from the sanctuary of the coral reef.
Over the past few years, more and more coral species have been successfully aquacultured. This allows vendors to quickly and infinitely harvest corals in safe and controlled conditions, lowering costs and nearly eliminating the need for prolonged acclimation. What you see is what you get with these types of coral, with little worry of changing colors, pests, or sudden death.
Though corals get all the spotlight in the coral reef aquarium, there are many other invertebrates that make these fragile systems work as they should. Some of these helpers include clams, sponges, anemones, and marine worms.
It should be noted that most of these invertebrates are filter feeders to some extent, making their care even more challenging than that of some types of coral.
In recent years, clams have made their way into many home reefs. The majority of marine clams available are members of the Tridacna genus, namely the maxima clam (Tridacna maxima) and the crocea clam (Tridacna crocea).
Marine clams are arguably even more difficult than all types of coral, making them an unrealistic addition for most average hobbyists. These invertebrates require high lighting, moderate water flow, and constant feedings.
Interestingly, these clams also share a symbiotic relationship with zooxanthellae similar to photosynthetic corals: the zooxanthellae are protected while the clam is fed. However, like other clams, Tridacna species are filter feeders that will quickly strip the water column of any and all food.
With the addition of their potentially large sizes, marine clams are limited to only the most expert hobbyists.
Sponges are definitely one of the most underrated invertebrates in the aquarium hobby, but surprisingly difficult to keep. That being said, they are a very common hitchhiker, though most don’t survive due to limited food availability or natural predators that are already in the tank; many fish and other invertebrates won’t hesitate to munch away on small pieces of sponge.
Sponges also require moderate water flow to ensure that planktonic foods are always being passed through and that wastes are being carried away. Though some sponges are photosynthetic, they need to be able to get sufficient food from the surrounding aquarium water alone.
Everyone wants an anemone until it becomes too much–which can happen rather quickly.
But what is a clownfish without an anemone? Probably better off, and here’s why.
Anemones are members of the Hexacorallia subclass in the Cnidaria phylum meaning that they’re related to many of the other types of coral found in the aquarium; this is also why they have similar stinging cells and are photosynthetic from symbiotic algae.
The main problem with sea anemones is that they are mobile. Most corals are sessile invertebrates that will stay in one given location, moving or simply dying if conditions become unfavorable. Instead, anemones have the ability to detach their foot and float with the current until landing in a more desirable location. In the aquarium, this could mean landing on top of and stinging other corals.
In addition, anemones can be pretty demanding when it comes to lighting, water flow, and water parameters. They are definitely more appropriate for experienced hobbyists, which limits many beginner hobbyists who want to try the anemone and clownfish pairing.
The other major problem is that they are quick to spread. Though propagation is a sign of good health, it can quickly become unmanageable. Many hobbyists need to constantly find new homes for their sea anemones, which can be difficult to remove from the rock and handle afterward.
If you’re up to the challenge, then some of the most popular species of anemone are:
- Rose/green bubble tip anemone (Entacmaea quadricolor)
- Rock flower anemone (Epicystis crucifer)
- Sebae anemone (Heteractis crispa)
- Long tentacle anemone (Macrodactyla doreensis)
There are thousands of species of marine worm with some being incredibly helpful, like the bristle worm, and others being slightly terrifying, like the bobbit worm (Eunice aphroditois). Hobbyists have managed to find the most beautiful of worms and incorporated them into the home coral reef aquarium.
Some of these marine worms are feather dusters (Sabellastarte spp.), coco worms (Protula magnifica), and Christmas tree worms (Spirobranchus giganteus).
Just like the other invertebrates on this list, marine worms are filter feeders, pulling planktonic foods out of the water column. However, marine worms are not corals and are in a different taxonomic phylum altogether called Annelida. They are not photosynthetic and will need to be fed a vast array of supplemented foods instead.
When stressed, some marine worms have the ability to drop their crown. It is unlikely for the animal to recover after this, so remove the crown and tube as soon as possible to prevent an ammonia spike.
Accurate replication of the coral reef ecosystem has long been the goal of many saltwater enthusiasts. From soft corals to hard corals, these marine animals bring a dimension to the display tank that can’t be understated. With so many types to choose from, there truly is a coral species for everyone at every level.
Before choosing a coral to bring home, make sure to research how these corals are being collected. We can all do our part to save the remaining coral reef ecosystems of our world’s oceans.