The moniker “coral” refers to a large array of different species — and it is necessary to realize how different these animals can be from one another when we aquarists try to provide the best care for them. As a comparison, consider the care of a pet bird versus a wild bird. The care of a budgie would be quite a bit different if that pet is a California condor with a 9-foot wingspan. Almost every aspect of their care would be different, yet they are both birds. A similar point may be made about corals.
The range of diversity among corals is far greater than the diversity found among birds. All birds arose from ancestors that were found within a small group of feathered dinosaurs, and frankly, no modern vertebrate taxonomist has any trouble grouping dinosaurs and birds together. But corals contain stony corals of about a dozen distinct, separate and different lineages; in some ways, a montiporid and a mussid are about as distinctive from one another as condors and budgies. But the immense diversity — and differences — among the stony corals is only a small part of the total diversity of corals.
As examples, there are corals without any skeleton whatsoever (mushroom polyps); sea anemones called zoanthids are sometimes lumped in with corals, though they belong in a different group, and finally there are the soft corals. They are corals, yes, and some of them are even “soft.” However, the last common ancestor between soft and stony corals appears to have lived well before an ancestor for the dinosaurs became distinct from other reptiles, probably well before 250 million years ago.
Hard and soft corals belong to the same general animal group: the class Anthozoa of the phylum Cnidaria. This means their body form is a polyp: a simple cylindrical animal wholly closed on one end, with a single opening on the other end. That opening, generally referred to as the stoma or mouth, acts as the mouth when it closes behind food being eaten and as the anus when it opens to allow indigestible food to be expelled. The thin tissue lining the inside of the polyp is glandular and capable both of secreting digestive enzymes and absorbing digested food. Typically, a ring of thin tissue projections called tentacles surrounds the mouth.
There are obvious structural differences between soft corals and the rest of the anthozoans. However, there is a fundamental problem with evaluating those distinctions: there just aren’t very many of them. All polyps are very simple animals. Compared to complicated animals, such as humans, crabs or snails, corals have almost no structural complexity at all. This makes for serious problems in assessing the importance of the distinctions between the types of corals.
Because there are only a few (and apparently minor) visible differences between soft corals and stony corals, aquarists often assume that that their biological properties are not very different. That is not the case. Any given representative stony and soft coral are about as different as two animals can be. Those few visible differences are fundamental and reflect other invisible differences, such as disparities in metabolism and behavior.
Although the visible distinctions appear minor, they are encoded so deeply in the animals’ genetic structures that they are fundamental to the class. The differences split the Anthozoa into two groups: those that are soft corals and those that aren’t. That there is such a deep-seated division should indicate that the care of these two animal groups should be different. Unfortunately, the animals’ genomes are not visible, and even if they were, the average person, as well as most biologists, would be hard-pressed to find the differences. Soft corals appear to be similar enough to some stony corals that aquarists make the assumption that their care is the same.
Food and Feeding
The major distinctions between soft and stony corals likely to result in different feeding requirements are found in the relationship between the soft parts and the skeleton. Such differences may be seen by comparing two individual corals: a large leather coral (Sinularia), which is a soft coral, and a large Acropora, which is a stony coral. If those two corals are about the same size and fingerlike shape, it would be a natural deduction that they may need about the same types and amounts of food. After all, each colony’s small polyps could be about the same size, and there might even be about the same number of them. Both colonies contain zooxanthellae, and one might reasonably expect that the zooxanthellae would provide each colony with about the same amount of photosynthate — and that may be true. But the similarity stops there.
Stony and Soft Coral Differences
|Common Name||Stony Corals||Soft Corals|
|Polyp Size||Small to Large||Small|
|Body Form||Solitary (many) to colony||Colony|
|Number of tentacles||Many (in sixes)||Eight|
|Tentacle shape||Simple to highly modified||Pinnate|
|*Skeleton’s mineral composition||Calcium carbonate||Calcium carbonate|
|*Organic composition||Little protein||A lot of protein|
|*Properties reflecting different metabolic needs.|
These animals need different foods offered in different manners. An Acropora coral may appear quite large but has very little living tissue on it. The nonliving, largely calcium carbonate skeleton under and external to the thin surface tissue layer comprises most of the colony’s mass. In contrast, a Sinularia that appears to be the same size will have many times the amount of living tissue in it. The internal skeleton is comprised of a myriad of small calcium carbonate spicules and a large amount of proteinaceous material. Additionally, the body is riddled with a large number of water canals, all of them lined with living tissue. For two colonies of the same size, there is a lot more flesh in the Sinularia than the Acropora.
Requirements And Care
Novice aquarists sometimes wrongly assume that corals — no matter what type — get the majority of their nutrition from their zooxanthellae and that relatively little additional food needs to be added as long as the corals are well-illuminated. Zooxanthellae provide all the sugars most reef corals will ever need, however, those sugars are about all the algae provide.
All corals need a source of protein, so all of the living part of coral in an aquarium has to be grown by adding a source of nitrogen for the coral to eat. The so-called SPS (small polyped stony) corals, having relatively little protein for the apparent size of the animal, need relatively small amounts of any foods. The distantly related, larger and fleshier LPS (large polyped stony) corals need more nitrogenous food than SPS corals.
Soft corals, however, need a lot more food than any comparably sized stony corals. Among the many types of soft corals are leather corals, such as Sinularia and Sarcophyton; horn corals or gorgonians; sea pens; and “true” soft corals, such as Alcyonium and Dendronephthya. All of these animals require a lot of particulate food to meet their colonies’ protein requirements. Corals without zooxanthellae, such as Dendronephthya, need a large amount of supplementary food to make up for the sugar that zooxanthellae produce.
There are two main aquarium food sources that may be used to meet these needs. One is small planktonic material added by the aquarist. The other is bacterioplankton, or “bacterial particulate material,” formed in the aquarium. Bacterioplankton grow in the aquarium’s water and feed on either dissolved or very small particulate organic debris. The bacteria often originate in a sandbed or in the debris from other feedings. When they become planktonic, they are often eaten by small coral polyps. In a large aquarium that is well-fed, there is often enough bacterioplankton to provide the nitrogen requirements for a zooxanthellate soft coral.
Unfortunately, there generally will not be enough plankton of any sort in aquariums to support any azooxanthellate (without zooxanthellae) soft coral. Feeding these latter animals is difficult. Providing them with acceptable food in large amounts can foul the aquarium. With some small azooxanthellate gorgonians, each and every polyp needs to consume as many as five to 10 baby brine shrimp or the equivalent per day. Additionally, to be able to feed at all, they need to live within specific flow regimes with very specific current velocities, as their polyp shapes are adapted for particular conditions. They simply cannot capture food if the current is not suitable. Their high volumetric requirements and specific necessary flow regimes make it very difficult to provide sufficient food for them.
Presently, feeding most varieties of zooxanthellate soft corals is pretty easy. Because their sugar requirements are met by their symbiotic algae, sufficient small amounts of proteinaceous food will be generated by a well-maintained sandbed coupled with reasonable supplemental feeding of the aquarium. This situation contrasts with that found with all types of azooxanthellate soft corals.
Azooxanthellate soft corals are often the most beautiful corals, yet they are also the hardest to maintain. All of their food must be obtained from plankton in the aquarium. At the same time, they are difficult to feed because of their need for particular water flow and currents. Care of these corals should probably not be attempted by any but the most experienced aquarists, and even then success may be difficult.