Husbandry: Coral Selection and Companionability
Last week, we discussed fish selection and companionbility. This week, we will focus on corals as part of a balanced aquarium.
More likely than not a saltwater aquarium will house not only fish but also a wide variety of coral. Some saltwater aquariums are referred to as “fish only”, but even in these tanks it is quite common to see one or two species of hard or soft coral, sometimes even an anemone. If you plan to house fish along with coral then the following information should help you in your consideration of species. The term “Reef Safe” is great key term to use when asking about a particular specimen and means that a particular species of fish or other marine life form is relatively harmless to any coral or other live inhabitant of your tank. This term may pop up in your internet research on most websites selling marine livestock so it is a perfect phrase to scan the page for.
The main objective of keeping saltwater coral, along with your fish, is to keep them alive and thriving. This infers that they will show visible signs of healthiness via moving, growing, consuming the nutrients you introduce as food, reproducing, and, in some cases, mating. It is important to note that the level of interaction they have with other inhabitants in your tank is relatively high.
There is a constant and in some cases aggressive competition for resources between corals and other inhabitants of you tank. The morphological (structural and pattern based) competition between corals is the number one concern hobbyist consider when placing their newly purchased piece. Light is one of the most finite of properties utilized by calcifying species and the usable area of light is simply your tanks dimension, so as you place corals throughout the rockwork you are dedicating portions of your light coverage to its specific growth rate and pattern. When considering the growth rates of coral species like Acropora, which was mentioned in last week’s article, it is important to learn that the rates of growth and patterns with which they grow can vary dramatically. Corals like Montipora capricornis (a plating coral) can actually create large shaded areas within the tank and deprive colonies placed lower in the water column of much needed light.
Most hard and soft corals have the potential to cover large percentages of the substrate on a reef and they are always competing for light and space on the rockwork. Some corals are encrusting in that their growth patterns are that of coverage, and the texture or form of the rock it is placed upon will dictate its region or territory. These corals will encrust upon the rockwork, creating new layers of the existing shape defending their conquered territory with formidable weapons.
The sweeper tentacles possessed by bubble corals. Photo by James W. Fatherree, M.Sc
I used the word territory to infer that these corals will also compete utilizing differing forms of defense and utilize weapons at their disposal. The types of behavioral competition can vary, so I will discuss to two most common and tangible forms. First is the use of nematocysts contained within their stinging “sweeper” tentacles. These tentacles are much longer than other "normal" tentacles and are used to search the neighboring water for encroaching corals. They will increase in numbers and length when chemical contacts from other coral colonies are made. Tentacles are capable of use for feeding, but they are primarily used as both offensive and defensive weapons. After sweepers contact a neighboring coral, they continue to discharge nematocysts, damaging the tissues and causing necrosis. The Frogspawn coral (Euphyllia divisa) and Galaxea fascicularis species are known for forming very long, potent sweeper tentacles. The discharge of sweeper tentacles is an external form of chemical defense; All coral also contain an internal form of chemical defense. Extracoelenteric Digestion is when the digestive filaments of coral, which contain cnidocytes (a venomous mucus like fluid), are expelled upon a neighboring coral ultimately consuming it. This form of defense requires close contact to be truly effective and usually does not mean the loss of an entire colony.
In extreme cases certain species of coral can actually consume entire fish. Take for example the remarkably beautiful and hard to find Rhizotrochus typus. This formidable predator gets large enough to devour entire fish with ease. These types of coral are kept with extreme caution and are usually purchased by expert hobbyist, but plenty of coral that remain comparably beautiful can be kept with many different species of fish. Species like Acropora millepora can be kept easily with a wide variety of great looking fish, and they can be an asset in more ways than one. The presence of this coral in a marine tank can provide shoaling fish with a place of protection and refuge from assumed predators. Most mushrooms, zoanthids, and leather corals are also extremely compatible with a huge range of stunning marine fish as they receive nutritional benefit from the fish waste. Moreover, some species of invertebrate like (see James W. Fatherree article and photo [right]) actually live within the coral structure of many SPS (small polyp stony corals) in a symbiotic relationship, warding off predators like Exallias brevis (leopard Blenny) who’s diet consist mainly of small polyps living in stony coral.
The combination of fish and differing species of coral in an aquarium is a great display of nature’s brilliance. Unfortunately, there is no coral and fish compatibility chart. However, merely engaging in a few extra hours of research and asking questions on the forums can achieve the benefits received by the proper husbandry of the two.
Next week we will expand on this concept of symbiosis between coral, fish, and invertebrate to learn more about how choosing the right combination can also help sustain a high level of health in your aquarium.