By general definition, invertebrates are animals without backbones (vertebrae) or internal skeletons. Therefore most any living thing you keep in your aquarium that is not a fish or plant, is an invertebrate. As a class they do best in clean environments with low nitrate levels, and are not tolerant of copper and other metals.
Popular invertebrate groups kept in saltwater aquaria include:
This group broadly consists of “flower-like” animals (coelenterates) that are generally sessile (non-mobile), occurring sometimes as single animals or polyps, but often as colonies of individual animals. Most corals, kept in the aquaria, house symbiotic algae (plant) cells within their tissues, which provide food for the corals via the process of photosynthesis. Photosynthesis is process by which plants produce food (carbohydrates, sugars) in response to light. Therefore, these corals have dependence on intense and specific wavelength lighting. Generally, some combination of intense broad spectrum “white” light is combined with specific “blue” UV (usually called an actinic 03) light, to approximate the effect of natural sunlight.
It is important to note that corals like very clean aquarium conditions. It is often said they want a “nutrient poor” environment. What that means is they don’t tolerate a buildup of waste byproducts (esp. nitrates and phosphates) from fish. The implication of this is that if you keep corals (even “a few”), you’ll be limited not only on the type of fish (no coral eaters!), but also the quantity. And you’ll do better if you favor veggie eating fish over meat-eaters (less waste). As a good rule of thumb, plan to stock no more than 25 – 30% of a normal fish load if you keep corals. You will also fare better with water changes (to remove wastes and restore chemical nutrients) of 20% every two weeks.
Generally we subgroup the corals as:
These animals take in calcium from the seawater and manufacture a “coral skeleton” which is the rock-like structure most people are familiar with as decorative coral. The calcium deposits of these corals, over thousands of years, have formed the great coral reefs. A hard coral specimen often consists of a colony of many individuals. It’s important to note that most of these corals have stinging cells for food collection and defense. As much of their food comes via the algae cell photosynthesis, direct feeding of these corals is usually infrequently needed, but they do require regular additions of certain chemicals and nutrients. Critical elements to add are: calcium, strontium/molybdenum, and iodine. Additions of nutritional (mineral and vitamin) supplements may also be beneficial, as well as occasional feeding of phytoplankton or other “micro” food.
These are sometimes referred to as “anemone corals,” or not classified as true corals at all. However, we are dealing with the reality of the hobby here, and these are usually sold as “corals”. Typically, a soft coral is a single animal (e.g. a Leather coral) or group of animals (e.g. mushroom corals, or soft polyps), that reside on rock or substrate that is not of their making. In other words, unlike the hard corals, they do not produce “rock.” For all practical purposes they are not mobile, unlike true anemones (q.v.), although mushroom polyps can “migrate” over a long period of time (so slowly as to be almost imperceptible!). Chemical additions important to these animals include iodine, as well as nutritional supplements, and phytoplankton feeding.
These are soft-bodied, jelly-like, animals that possess tentacles containing stinging cells (nematocysts). They are quite mobile, though slow (at top speed they can traverse the length of your aquarium overnight). Their bodies are better than 95% water so they have the ability to expand and contract quite quickly and can squeeze through the narrowest of spaces. What most people know about anemones, if anything, is that certain varieties of anemones act as a host to certain types of small fish (usually various “clownfish”) which are immune to the stinging cells in the anemones’ tentacles! The exact dynamics of this fish/invertebrate relationship has been the subject of countless studies and theories, but most agree that mutual protection and feeding are the driving forces for this “odd couple.” The anemone affords hiding and protection for the clownfish, and the clownfish will feed his anemone and attempt to chase off potential “anemone eaters.”
Like many of the corals, most “fish host” anemones possess symbiotic algae cells and require proper type an intensity lighting. They also do best in a clean, low-nitrate, environment.
One popular variety of anemone that is fairly hardy, inexpensive and does not require intense lighting is the condylactis anemone from the Caribbean. As clownfish do not occur in the Caribbean, this anemone will normally not be chosen by clownfish as a host. It is often a good choice for an invertebrate tank that lacks special lighting.
If one attempts to keep anemones in a reef tank with corals, the possibility exists of an anemone bumping into a coral with an ensuing “stinging battle” taking place. The coral will often lose.
Shrimps, lobsters & crabs
These crustaceans share the common characteristic of a shell (or exoskeleton) which must be periodically shed as the animal grows. They are usually hardy, inexpensive, active, and interesting animals which can serve a useful function as scavengers of excess food or unwanted algae.
Their aquarium requirements are not very demanding, although they don’t tolerate high nitrate conditions. It is important to avoid keeping these with predatory fish, such as triggers, lionfish, groupers, or certain wrasses. In some cases even cardinal fish will prey on shrimps. It is also important to ascertain compatibilities with other crustaceans of corals.
Starfish, urchins & sea cucumbers
These are members of a group called echinoderms. They travel very slowly, attached to surfaces (including the glass walls of your tank).
Starfish are active predators, and some are a menace to corals (usually the starfish with knobby surfaces – brittle and serpent stars are usually OK). The brittle and serpent star moves rather quickly, especially in pursuit of food. Their movement is similar to that of a slow moving octopus.
Urchins are usually vegetarian, which often makes them useful in keeping algae under control. One does have to be careful of their spines which may injure coral specimens (or your fingers!).
Sea cucumbers feed mostly on detritus, either swept from the sand surface or filtered from the water, via their tentacles. Some cucumbers (e.g. large sea apples) pose a danger of releasing a toxin when injured or dying, that can kill all fish in your tank.
Snails & slugs
Certain snails (e.g. astrea, turbo, pinnatus) can be useful in controlling algae. Generally, we discourage keeping sea slugs as most are very difficult to keep and some are coral eaters.
Clams, oysters & scallops
These are the bivalve (two shell) animals.
Certain clams of the Tridacna (“Giant Clam“) species are the most popular, possessing a fluted shell and colorful mantle, which is often fluorescent blue or green. They are filter feeders but derive most of their nutrition in the same manner as many corals – food production by symbiotic algae cells. As you might expect, keeping these requires intense light as one would need with corals.
Flame scallops are popular, colorful, and inexpensive bivalves with no special lighting needs. They are filter feeders and are not very long-lived in the aquarium, usually less than a year .
The only oyster of any regular aquarium interest is the thorny oyster whose spiked shell makes an interesting addition to the tank.
Tube worms (feather dusters)
These animals are worms that live in a soft or hard tube, of their own making, and filter feed via featherlike tentacles at the end of the tube.
Most popular are the large Hawaiian feather dusters. More dramatic, but also more costly, is the Cocoworm, which possesses a hard (calcified) tube and colorful (pink to red) “feathers.”