User's Guide to Invertebrate Species Profiles

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Invertebrates are not like fishes, especially when it comes to identifying them from photographic guides. Please read this before trying to use the invertebrate accounts on this site.

Problems with Identifications

Many invertebrate animals are difficult to identify without either specialized training, scientific references, or specialized equipment. This is unfortunate, but it is a fact of life and we have to live with it. In my book A PocketExpert Guide to Marine Invertebrates and in the excerpted profiles found here, I discuss the animals as precisely as I can.

In many cases, the animals are sufficiently distinctive as to allow a discussion of particular species. This is particularly true in such well-known groups as the mollusks, arthropods, and echinoderms.

However, in many other common groups, such as sponges, hydroids, and annelid worms, the characteristics necessary for specific identification are generally microscopic or internal, and thus beyond the reach of the average hobbyist.

For example, sponge identification is generally done by dissolving sections of the animal and microscopically examining the remaining internal hard structures. Few aquarists would want to destroy their animals to identify them. Consequently, for such animals, I will discuss only the general groups.

The “Scientific Name” and Nomenclature

Hobbyists, particularly advanced hobbyists, often want to know the so-called “scientific names” for the animals they keep. There are numerous reasons to want to do this, but primarily this is done to allow the hobbyist to find additional information on the care of the particular organism. This is an admirable goal.

Unfortunately, it is also one fraught with problems and errors. Most of these errors have to do with the use of the “scientific name.”

A little background about these scientific names is necessary. The scientific name of an animal is in two parts, and because of this duplex nature, it is termed a binomial, literally a word that means “two names.”

The first part, always capitalized, is the name of the genus. The second part, never capitalized, is the name of the species. These names are written in a derivative form of Latin, which is, of course, a foreign language. Because of this, by convention, the names are often italicized when printed or underlined when written.

However, other names also go with the specific and generic names. Biologists group similar organisms together, and these are considered to be many levels of similarity. Consequently, a hierarchy of names exists. Ranging from the least similar to the most similar, the basic levels in the hierarchy are:


  • Phylum (plural = phyla)
    • Class
      • Order
        • Family
          • Genus (plural = genera)
            • Species

The fundamental biological unit, the species, is the smallest level in this hierarchy and is considered to be a group of all animals that can, in nature, sexually reproduce with one another and give rise to fertile offspring. Similar species are grouped into a genus, and similar genera are grouped into a family. Likewise, similar families are grouped into orders, orders into classes, and classes into phyla. All animal phyla are in the animal kingdom, the most inclusive group in the hierarchy.

A point worth remembering is that except for the “species,” all of these groups are arbitrary and may be changed or challenged by various specialists, depending on their interpretation of, or new knowledge about, relationships.

This description of binomial nomenclature is of its ideal use in scientific literature and research. The reality of its use within the aquarium trade and hobby is something else again. Before a specific name can be accurately applied to a given animal, it has to be carefully examined by someone who knows what they are looking for and at. In most cases in the aquarium hobby, where scientific names are applied, this has never been done. Generally, a photograph is compared to the organism and the presumptive “scientific name” is determined. For a few distinctive invertebrate animals and some fishes, this method will work well. For most invertebrates, this process is, at best, deeply flawed. At its worst, it provides a false sense of security about the validity of the identification. For many animals, this is no big deal. For animals that are potentially dangerous to either the aquarist or other denizens of the aquarium, misidentification may be a very important point, indeed.

Some sea urchins, snails, and cnidarians, for example, have the capability of killing humans. I have seen misidentified examples of some of these animals for sale in dealers’ tanks. Fortunately, to my knowledge, no hobbyist has yet died of cardiac arrest from handling a misidentified sea urchin or snail, but it really is only a matter of time before this occurs.

Obviously, the scientific names applied to an animal in a dealer’s tank or on a mail-order list should always be considered to be suspect. Given such a situation, what can the hobbyist do to get accurate identifications?

First, compare with a number of different references. Don’t neglect using scientific descriptions as well; there is nothing magical or devious about these. You really can take a bit of effort to look up the creature in the scientific literature, and compare descriptions of it to your organism.

Look at pictures in hobbyist publications, but remember they may be misidentified as well. Many of the identifications given in hobbyist publications are doubtful. Examine the authors’ credentials for identification of the animals they are selling or discussing. See if they say that the identification of the animals they are describing or picturing have been verified by some real taxonomic authority. I have taken a conservative approach with regard to names, and many hobbyists will find this frustrating. At the same time, I am a professional taxonomist—I really do identify organisms for hire—and believe me, if I can’t come to a conclusion on the basis of a photograph or other data that are available, neither can the average hobbyist.

There is a further complication in the identification of aquarium animals. In many cases, we have no way of knowing where the animals originated geographically. Even if dealers tell us where the animals in a shipment come from, those animals are often placed in tanks where cross-colonization from other sources may occur. Many scientific species descriptions are written discussing how a given species differs from its nearest local neighbors. Species living in distant parts of the world may often look surprisingly similar. Without knowing areas of collection, certain animals, particularly corals, hydroids, sponges, and some echinoderms, simply cannot be identified to the species level.

For those animals you can’t identify with certainty, there is certainly nothing wrong with using common names—after all, a scientific name incorrectly applied is nothing but a different “common name.” When I illustrate an organism that you cannot definitely identify with a photograph, that organism or group of organisms will be referred to by some name other than a specific one.

Conditions of Maintenance

Tropical reef animals have a particular, and reasonably precise, set of conditions in which they will do best. Relatively few of the animals we keep are found at conditions outside these normal ranges, and relatively few of them do well at conditions outside these ranges. Because of this, a word about marine animal distributions is in order. Marine animals often have a larval dispersal phase that lives, swims in, or is distributed in the plankton. Because of this, they have the potential of spreading very widely.

Most animals have a range of conditions under which they may survive. For example, some corals can survive from temperatures of 68°F (20°C) to 90°F (32°C), and they are often found in areas where the temperatures reach the extremes or are predominantly shifted to either the colder or the hotter parts of their range. Animals living in such marginal environments are not thriving, they are simply surviving. The larvae that gave rise to these adults settled out of the plankton at such a site because other cues were acceptable. Once there, the animal is essentially stuck—literally, as it cannot get up and move. In the marginal environments, animals such as corals survive, but they reproduce poorly, if at all, and they grow slowly.

Most coral reefs have temperatures that range between 78°F (26°C) and about 86°F (30°C). The average temperature of about one thousand coral reefs spread throughout the world is about 81°F (27°C).

Virtually all coral reef animals will do best within this range, and I consider the temperature range of 81 to 84°F (27 to 29°C) as the optimal and normal temperature to maintain all reef animals.

There is a similar range for acceptable salinity, but it is a narrow band, around 35 to 36 parts per thousand (ppt). At normal reef temperatures, this is a specific gravity of about 1.025 to 1.026.

On coral reefs, the pH will vary from about 8.0 to about 8.5 depending on the time of day and current patterns. The stability of pH on a reef and in aquariums is due to the stability-enhancing effects of dissolved calcium and carbon dioxide. These materials are assessed in aquariums by the monitoring of both calcium and alkalinity.

Depending upon the animals kept in a given tank, it will be necessary to supplement these materials regularly. This will replace the critically important dissolved calcium and will ensure that the pH remains within a normal range.

Lighting is more variable and dependent upon habitat. This will be discussed within the context of each animal group. Unless otherwise noted, the basic conditions for all animals discussed are listed in the paragraphs above. To provide a quick method of assessing suitability of the animals discussed in this guide, I have used two, very subjective, indices.

Aquarium Suitability

This indicates whether an animal is suitable for a reef tank, or not suitable at all. Note that animals may be easy to keep, but not suitable for captivity for a number of reasons. As an example, the textile cone shell, Conus textile, is quite easy to keep in an aquarium. It also can sting and kill people. I consider this an attribute that disqualifies the animal as a “good” aquarium pet.

Reef Aquarium Compatibility

This category assesses the ability of the organism to survive in reef aquarium conditions. Many animals, such as the beautiful Dendronephthya species, would be suitable for reef aquariums, if only we knew how to keep them alive in the tank. However, as we cannot maintain the animals, they should be left on the reef, and are given a poor compatibility rating.

—Ronald L. Shimek, Ph.D.

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