Summary of Marine Invertebrate Families

An overview of invertebrates that do well in captivity.

Most beginning marine enthusiasts start out with fishes. Many ignore, or even avoid, invertebrates — perhaps thinking that such unfamiliar creatures may be very difficult to keep. On the contrary, if one understands their special needs, invertebrates are in many ways hardier and less demanding than marine fishes. In addition, there are many invertebrate species that make good candidates for small aquariums.

With reef tanks the current rage, beginners often get the impression that all invertebrates require complicated, expensive filtration and lighting systems. While it is certainly true that high-tech reef tanks provide ideal aquarium conditions for many invertebrates, such systems are hardly necessary for a very wide variety of invertebrate species.

Invertebrates come in great variety of forms. For example, there are about 20,000 species of fishes in the world, marine and freshwater. There are more than a million species of invertebrates, and we are still counting. Granted, most of these are insects, but there are probably more species of marine arthropods alone than the combined total of all vertebrate species. With more than 600 million years of evolutionary history behind them, the invertebrates are a diverse assemblage, indeed.

Biologists sort the bewildering array of invertebrate forms into some 30 large groups, called phyla (singular: phylum). Of these, representatives of six are of interest to aquarists. The remaining phyla include several that are well represented on aquarium specimens of live rock — for example — which are otherwise seldom seen or discussed by aquarium hobbyists. The six major phyla of interest to aquarists are sponges (Phylum Porifera); corals, sea anemones and their relatives (Phylum Coelenterata); certain worms (Phylum Annelida); mollusks (Phylum Mollusca); shrimps, crabs and their relatives (Phylum Arthropoda); and starfish and their relatives (Phylum Echinodermata).

An understanding of the varied lifestyles of invertebrates is essential to their successful husbandry in the aquarium. Aquarists, however, need not concern themselves with the details of the natural history of every species. Rather, learning to recognize the broad categories of feeding habits and behavior, which often cut across lines of biological classification, is of more utility to the aquarist. Like fishes, marine invertebrates require that water conditions in the tank be maintained correctly. These parameters have been discussed previously in this series, but I repeat them here as a review (see Table I).

If you maintain these parameters in your tank, you will be able to keep any invertebrates that you wish, provided you understand their special needs. For the beginner with an interest in invertebrates, the first major distinction that should be recognized is between invertebrates with a requirement for bright light, and those that do not require special tank lighting.

Light-requiring invertebrates need a light intensity of at least 10,000 lux in order to survive and grow. Without getting into a long discussion of the merits of various lighting methods, suffice it to say that this level of lighting can be achieved only with fixtures that allow for multiple fluorescent lamps or with metal halide lighting systems. Give careful consideration, therefore, to the lighting system if you intend to keep any of the light-requiring species discussed below.

The majority of light-requiring invertebrates are coelenterates. This group includes anemones, corals, soft corals, gorgonians, sea mats, mushroom corals and all other types of polyp animals. There are a few exceptions, such as the orange polyp coral (Tubastrea) that live in darkness, but in the main, tropical coelenterates all need bright light to survive more than a few weeks.

This is because these organisms harbor symbiotic algae, zooxanthellae, in their tissues. The algae are absolutely required for the health of the coelenterate, and the algae, in turn, require light to carry out photosynthesis. In a properly illuminated aquarium, coelenterates with zooxanthellae need no feeding by the aquarist. These animals derive all of their nourishment from their symbionts. (Exceptional coelenterates, such as the Tubastrea — mentioned above — do need to be fed regularly, however.)

Apart from coelenterates, the only other light-requiring invertebrates that you are likely to encounter are the giant clams. There are seven species, five of which are available to aquarists from a hatchery in the South Pacific. The most commonly seen of these is Tridacna derasa. The zooxanthellae of Tridacna impart beautiful colors and patterns to the animal, and no doubt account for the popularity of the giant clams as aquarium residents.

No two clams are exactly alike in color and pattern. Giant clams subsist entirely on the photosynthesis performed by their zooxanthellae, absorbing only oxygen and inorganic nutrients from the water. They grow large and can live to be very old.

With regard to invertebrates that have no special lighting requirements — which we will discuss in the remainder of this article — the mode of feeding is probably the most important factor to consider when choosing specimens. There are many feeding modes among invertebrates. Here are the most common ones, with a few examples of specimens that you may see in your local aquarium shop.

Filter Feeding
Known by a variety of other names — “suspension feeding,” “particle feeding,” etc. — filter feeding consists of straining the surrounding water for small, planktonic food organisms. Filter feeding invertebrates have a variety of nets, strainers and other fishing gear that they use for this purpose. Some are passive, simply extending their capture equipment into the water and waiting for currents to bring food by. Others pump water through their bodies and possess internal organs that do the filtering. Among the latter groups are sponges and bivalve mollusks, such as clams. In the former category are worms, such as the feather duster and Christmas tree worms, as well as a few unusual arthropods — for example, anemone crabs.

Herbivorous organisms are those that ingest plant matter as the bulk of their diet. Generally, algae is scraped or nibbled from the surfaces of rocks and other objects. Sea urchins are all herbivorous grazers, as are many kinds of snails. Herbivorous organisms are among the easiest to care for, as they very often subsist entirely upon algae growth that occurs naturally in the tank.

Omnivores eat both plant and animal matter, and many of the invertebrates that fall into this group are considered “scavengers.” Most shrimps, crabs and hermit crabs are omnivorous, as well as some mollusks, such as cowries. Large, omnivorous mollusks should be avoided in an aquarium that features a variety of encrusting invertebrates.

Only a few aquarium invertebrates are active predators, and most of these are either mollusks or arthropods. Large arthropods, crabs and lobsters especially, cannot be trusted and will often prey upon smaller tankmates. Among mollusks, it is important to make sure you can identify the species in question, because this may be an important key to its feeding habits. One mollusk in particular, the octopus, is an active predator that will hunt down and eat any tankmates.

This has been a thumbnail summary of invertebrate requirements and lifestyles. While it is not possible to cover the gamut of invertebrate types in a single article such as this, it is relatively easy to select a few specific examples from among the most commonly available and popular invertebrate species.

Brightly colored sponges often appear in shops that stock a variety of invertebrates. Most sponges do well if their few simple requirements are met. Above all, they should never be removed from the water. Air trapped inside the sponge will lead to a slow death from within. Otherwise, protect them from overgrowths of algae or accumulations of debris and sponges are long-lived, undemanding aquarium inhabitants.

This group is so diverse that only one representative from each of the major subdivisions can be included here.

Anemones. Most people want an anemone as a host for clownfish. Among the most widely available of the clownfish anemones is the long tentacle anemone, Macrodactyla doreensis. This species has a salmon-colored column, with tentacles that range in color from brown to bright purple depending upon the species. It is a good host for many species of clowns, such as the tomato clown, Amphiprion frenatus.

This anemone likes to bury its column in the substrate, and at least a small amount of gravel or sand should be provided. Given proper lighting and good water quality, it will thrive easily. Never buy an anemone that shows the slightest outward sign of injury, such as a tear or puncture, because this may lead to a fatal bacterial infection.

Corals. True corals, those that produce a skeleton of calcium carbonate, should not be attempted by the beginning aquarist. However, after you have gained experience and are ready for a true reef tank with corals, you might consider elegance coral, Catalaphyllia jardineri, as a first specimen. This species is hardy, grows well in the aquarium (can double in size in six months) and comes in various attractive shades of green, greenish-brown and, occasionally, tinges of pink. It tends to be among the more expensive coral, but is well worth the price paid.

Soft corals. Soft corals do not produce a hard, rigid skeleton. Leather mushroom soft coral, Sarcophyton trocheliophorum, is the most commonly seen species. The fleshy body is shaped like a large toadstool and is usually light tan or brown in color. Whitish or yellowish polyps dot the upper surface.

It requires very bright light and a high, stable pH. It is generally easy to keep, with specimens regularly producing offspring in the tank. From time to time, this and many other soft coral species may appear shrunken and unhappy. This behavior is apparently normal and should not be cause for alarm unless it persists for more than a week. In that case, a large water change may be in order.

Gorgonians. These are soft corals in which the skeleton is rigid but is not made of calcium carbonate. The texture of the skeleton in gorgonians is similar to that of fingernails or horn — rigid but flexible. All gorgonians require good water movement and abundant food. They are not for beginners.

Sea mats. If you see what appears to be a group of little anemones all connected together at the base, you may be looking at a sea mat. These are hardy, colonial coelenterates that need only bright light to do well. I have seen the green sea mat, Zoanthus sociatus, exposed at low tide and baking in the tropical sun. No special care is needed apart from light and good water quality.

Mushroom corals. These coelenterates are sometimes called “false” corals. Most are disk-like polyps about the size of a half-dollar and are usually sold as a cluster of individuals attached to a rock. They are hardy, undemanding species that do not require as much light as their cousins. There are many species of mushroom corals in the genera Actinodiscus, Rhodactis, Amplexidiscus, Discosoma, Paradiscosoma and Ricordea. The last one mentioned, which comes from Florida, was once quite popular and commonplace. It is far less frequently seen now owing to collecting restrictions in Florida.

Annelids. Only a few of the thousands of species of segmented marine worms are imported for the aquarium trade. Without exception, these are filter feeders. The primary attraction is the fan-like crown of tentacles that are employed in both feeding and respiration. In the giant Hawaiian feather duster worm, Sabellastarte sanctijosephi, the appearance of the crown of tentacles gives the animal its name. This species lives in harbors, on docks and pilings and even embedded in mud. It is very hardy in the aquarium. To thrive, it needs weekly feedings of very small foods, such as newly hatched brine shrimp, or liquified preparations made for filter feeders. From time to tome, the worm may cast off its crown of tentacles. They will grow back in a few weeks if tank conditions are good.

Mollusks. Mollusks can cause great confusion simply because there are so many of them. Bivalves, snails and cephalopods are all available from aquarium dealers. Bivalves are all filter feeders and are typified by the flame scallop, Lima scabra, from Florida and the Caribbean. Cephalopods, such as the octopus, are all predatory. With snails, make sure you can correctly identify the species in which you are interested. This information will be crucial for determining feeding preferences.

Arthropods. Most arthropods are omnivores and are easy to keep. The banded coral shrimp, Stenopus hispidus, for example, will thrive on bits of food missed by the fishes, as will the scarlet cleaner shrimp, Lysmata grabhami. Either of these shrimps may be seen in almost any dealer’s tanks. In nature, both these shrimps are cleaners, removing parasites, dead tissue and so on from fishes. Only the scarlet cleaner is prone to exhibit this behavior in the aquarium, however.

Echinoderms. This phylum includes starfish, brittle stars, sea urchins and sea cucumbers. Sea cucumbers are filter feeders, urchins are grazing herbivores and brittle stars are omnivorous scavengers. Starfish may be herbivores or predators, and it is important to know the difference, for obvious reasons. Among the commonly seen starfish, the blue star, Linckia laevigata, is vegetarian and can safely be housed with other invertebrates. Predatory starfish, like the Bahama star, Oreaster reticulata, will eventually eat anything that is slow-moving enough for the star to catch.

Invertebrates are so varied that some type of reference book is essential to the invertebrate aquarium enthusiast. Of the many titles on the market, one of the best is The Manual of Marine Invertebrates by Martyn Haywood and Sue Wells, published by Tetra Press. Along with informative text, this book provides excellent color photographs of correctly identified invertebrates that are widely seen in aquarium shops.

There are literally dozens of other titles that are useful to the marine hobbyist. These most often deal with the species found in a particular region of the world or with the biology of a particular phylum of invertebrate. A good introduction to invertebrate biology is the college textbook Invertebrate Zoology by Dr. Robert Barnes, publishes by the W.B. Saunders Publishing Company. You can find it in college bookstores or at the library.

With such variety to choose from, invertebrates are gaining in popularity among aquarists. Sooner or later, you will want to try them too. Just make sure you read up on the species in which you are interested before you buy.

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Fish · Saltwater Fish