A Spineless Column by Ronald L. Shimek, Ph.D.

You Can't Tell Your "Pods" Without A Program...

Figure 1. The definitions of "pod." From, Merriam-Webster's Collegiate Dictionary, 10th ed.,
Merriam-Webster, Inc., Springfield, Massachusetts.

Over the last couple of years, one term, "pods," has become almost ubiquitous in discussions of reef aquarium animals. Almost any online forum discussing coral reef aquaria contains statements about "Pods doing this, that, and the other thing." Unfortunately, upon even cursory examination, it soon becomes apparent that most of the people writing about "pods" don't have a clue as to what animals they are discussing, except, of course, that they are discussing "pods."

When discussions involve a large number or a wide variety of people, there is a tendency either for specific terms to be replaced by generalized ones, or for the specific terms to acquire a generalized meaning. As a result of this process, everybody can participate in such discussions, and everybody then knows precisely what they mean. Unfortunately, the odds of any two people in the discussion utilizing the same meaning for any single term tend to become vanishingly small. Such is the case with the term "pod." I would be willing to bet real money that none of the above definitions of "pod" would be applicable to any of the uses of that word in any of the recent aquarium discussions.

So What Has Been Meant By The Term "Pod?"

My best guess is that "pod" is an informal derivative of some unambiguous and descriptive biological term. If so, the question then becomes, "A derivative of what term?"

Here are a few of the marine biological terms that use the terminal syllable "pod" as part of their name. In all of these terms, that syllable's meaning is as in the last definition in the box above: "foot, or resembling a foot." Incidentally, all of these terms can and have been used in conjunction with animals found in reef aquaria.

The use of the term "pod" is probably meant as a diminutive of one or more of these terms. Given that of and by itself, the term "pod" is meaningless and confusing, I thought it might be useful to present a quick guide to the various potential meanings using the above list as a basis.

Amphipod - A member of the crustacean group referred to as the "Amphipoda." These animals are commonly, but incorrectly, referred to in marine aquaria as Gammarus and Grampus, among some other terms. Amphipods are small crustaceans that lack a carapace, are flattened from side to side, and whose appendages have two different shapes. This characteristic gives the group its name, amphi (= two) + pod (= feet). They often scurry around on their sides or balanced on longer legs that almost give them the appearance of being on training wheels. Amphipods are common in marine aquaria, and the term "pod" may, in some cases, refer to them.

Figure 2. Left: A diagram of an amphipod. Right: A photograph of a gammarid isopod
similar to those commonly found in aquaria.

Arthropod - A member of the huge group of animals having an exoskeleton made of protein, chitin and, in some cases, mineral salts. These animals have appendages constructed of several jointed segments, and the name "arthropod" means "jointed feet." This group includes all crustaceans, insects, arachnids, and a host of smaller groups. Many of the crustaceans are found in marine aquaria, and the term "pod" probably refers to one of them. Which group it refers to is, of course, the question.

Brachiopod- A member of the group of animals commonly called lamp shells. These animals occur in marine aquaria, but rarely. They enter as hitchhikers on live rock and it is unlikely that the term "pod" refers to them.

Figure 3. Two brachiopods (Terebratalia transversa)
(center and upper left) on a rock.

Branchiopod- A member of the crustacean group of animals containing the fairy shrimps, brine shrimp, clam shrimp, and water fleas. Although commonly used as aquarium food, it is unlikely that any of these animals will be found living in marine aquaria. Consequently, they are probably not "pods."

Figure 4. Some branchiopods. Left: Podon, a marine water flea. Right:. Diagram of brine shrimp or Artemia. Note the swimming and feeding appendages are phyllopods (see below).

Cephalopod- This term refers to the squids, octopods, and a few other types of interesting mollusks. Cuttlefishes and octopuses are relatively commonly kept in marine aquaria. Cuttlefishes are referred to as cuttlefish. Octopuses are referred to as either octopuses, or commonly and incorrectly, as octopi. They are probably not considered to be "pods."

Copepod- A member of the immense crustacean subgroup called the Copepoda. Copepods of one sort or another are found in all marine habitats. Many subgroups of copepods are parasitic, but the ones most commonly found in aquaria are free-living. Aquarium copepods tend to be small and hard to see clearly. They may be the most common animal described by the term "pod."

Figure 5. Left: A pelagic calanoid copepod from the N.E. Pacific. Right: A bottom-dwelling
harpacticoid copepod from a coral reef aquarium.

Decapod- There are two distinct and different kinds of animals referred to as decapods. Squids and cuttlefishes have ten appendages (typically eight shorter arms and two longer tentacles), and as "decapod" means "ten feet," are commonly referred to by this term. A large group of crustaceans is also referred to as decapods, and these are animals with a pair of pincher claws at the front end, and four pairs of walking legs. Together, these five pairs of appendages constitute the ten most obvious "feet" of these animals, so they are also given the term "decapod." Decapod crustaceans consist of all the crabs, shrimps, lobsters, and their kin. These animals are common in marine aquaria, but are generally referred to by their group or common names, such as crab, "hairy crab" or cleaner shrimp. It is unlikely that any decapods would be called simply "pods."

Gastropod (1,2,3)- Gastropods (or sometimes spelled gasteropods) are members of the large and successful group of mollusks known commonly as snails, nudibranchs or slugs. The name means "stomach foot" and refers to the fact that they slide along on their "stomach" or ventral surface. Generally referred to by aquarists as snails, slugs, or nudibranchs, it is unlikely that any of these creatures are commonly called "pods."

Hexapod- These animals are jointed-legged animals with a hard proteinaceous and chitinous exoskeleton. They have six legs, hence the name hexa (= six) pod (= feet). These are insects; they are rarely, but occasionally, found in marine aquaria. The most commonly found marine aquarium insects are the water-skimming flightless gnats in the genus Pontomia.

Figure 6. A specimen of a marine insect, Pontomia, from a coral reef aquarium. These skim the water's surface and are quite small, about 1 to 2 mm long. They are naturally found in shallow water regions of the Southwest Pacific Ocean.

Isopod- These animals are crustaceans, like amphipods or copepods. Unlike amphipods, they are flattened from top to bottom. All of their appendages look alike, hence the name iso (=same, or alike) + pod (= feet). These animals may be predatory or parasitic such as the cirolanids, or they may be harmless scavengers. Many of the latter are small and may be the "pods" to which some aquarists refer.

Figure 7. Two herbivorous isopods similar to types occasionally found in marine aquaria. Left: an Idotea found on sea grasses. Right: A munnid found on rocks.

Octopod- The group that the various octopuses belong to may be called the Octopoda, and thus any single member of that group can be called an octopod. Octopods are generally rather distinctive and I think it is unlikely that any aquarist would refer to these animals as "pods."

Pelycyopod- Clams, or bivalved mollusks, have been referred to as pelecyopods, a name meaning "hatchet foot." This name refers to the shape of the burrowing foot of such animals as the soft-shelled and steamer clams. This name was more in vogue from about 1950 to about 1975 than it is now, and used mostly by American biologists. It seems to have fallen out of common usage in recent years. In any case, I think it is unlikely any aquarist would refer to a clam as a "pod," but stranger things have happened.

Periopod- This term refers not to an animal, but rather to an appendage. Any of the appendages of the middle or "thoracic" part of a crustacean may be called periopods. Such appendages would include the walking legs and pincher claws of shrimps, and all of the "scuttling" appendages of small copepods. As it is simply a term for appendages, it has probably not been used as the basis of "pod," although such a use would be more correct than would be the use of "pod" for a whole animal.

Figure 8. A shrimp, Pandalus danae, with the five pairs of periopods labeled. These are also stenopods and may also be called thoracopods.

Phyllopod- Another type of crustacean appendage is the phyllopod, or "leaf foot." These are the swimming appendages characteristic of animals such as brine shrimp. Instead of being used for walking, they are used as oars or paddles for swimming and food collection. It has probably not been the direct source of the term "pod" as used by aquarists.

Figure 9. A phyllopod from a water flea is seen here, but one of the many appendages of a brine shrimp would appear the same.

Pleopod- Another type of crustacean appendage named for its position on the body rather than by its function or shape. Pleopods are found on the abdomen or rearmost section of crustaceans. These are the small paddle-like appendages found under the tails of shrimps which allow the animals to swim slowly or which may be used to hold the eggs.

Figure 10. A diagram of the external anatomy of a basic shrimp-like crustacean. The swimmerets may
also be called pleopods, and the thoracic legs may be called thoracopods.

Pseudopod - Amoebas and similar one-celled organisms move by the extensions of their body walls. These extensions are called pseudopods or pseudopodia, a name that means "false foot." Occasionally, amoebas may be seen in aquaria, but they are uncommon, and I don't think pseudopods are seen frequently enough by most aquarists to be the basis of the term "pod."

Rhizopod - Foraminiferans are common in reef aquaria, particularly the red spiked Holotrema rubrum that live on rocks looking like a miniature stony coral. These organisms catch their planktonic food by extending thin pseudopodia out from the ends of their shell. These particular types of pseudopods, found also in the foraminifera common in sand beds, are called "rhizopods," a name meaning, "root foot," as the ones in sediments tend to look like very thin and tiny tree roots. Although these may be quite commonly seen and photographed by aquarists, I doubt that they are the basis for the use of the term "pod."

Stenopod- Stenopods, meaning "narrow foot," are the typical appendages of most of the larger crustaceans such as crabs, shrimps, lobsters, amphipods and isopods. These appendages are tubular and have seven internal segments. In contrast to the phyllopods which are used for swimming, stenopods are generally used for walking or manipulation of food when they are modified into claws or pinchers. This type of appendage characterizes shrimps, and has been used as the basis of the scientific name of the genus of the banded coral shrimp, Stenopus (a name which literally means "narrow feet"). As stenopod is a term for an appendage, as with phyllopod, it would likely not be used as a basis for the diminutive "pod."

Figure 11. A stenopod from a shrimp or crab; these are simply
the walking legs of these animals.

Stomatopod- Stomatopods are the crustaceans that are the "boogey monsters" for many reef aquarists. These are the mantis shrimps. Mantis shrimps are among the more interesting and actively predatory of crustaceans, and make delightful aquarium pets. Unfortunately, some species can deliver a nasty cut and they all may eat fish or other animals on occasion. Most aquarists fear and loathe these animals, but that seems to focus attention on them, so they probably wouldn't be called "pods."

Thoracopod- This word is a synonym of periopod, and means "thoracic foot." As with the other appendage names, "thoracopod" is unlikely to have been used by aquarists as the basis for the diminutive "pod."

There are yet other biological terms and names using the term "-pod." Generally, these are names of animals seldom or never found in reef aquaria such as heteropods (swimming snails) and scaphopods (tooth or tusk shells), so I haven't included them in this missive.


The use of the term "pod" is apparently becoming a part of the aquarists' lingo, and as such I suppose we had better get used to yet another useless name, such as bristle worm or "SPS" corals, entering our communication. Pod is being used in a manner analogous to the phrase "small crustacean" or the term "bug." I would hazard a guess that most uses of "pod" refer to small amphipods, isopods, or harpacticoid copepods, although I have also seen it used to mean mysids and flatworms(!). Used in such a manner, it does convey some information, although not very much.

If you have any questions about this article, please visit my author forum on Reef Central.

References to more detail about the various pods mentioned above:

Bliss, D. E. (Ed.): 1982-1985. Biology of the Crustacea. 10 volumes. Academic Press, New York.

Kozloff, E. N. 1990. Invertebrates. Saunders College Publishing. Philadelphia. 866 pp.

McLaughlin, P. A. 1980. The Comparative Morphology of Recent Crustacea. W. H. Freeman and Co. San Francisco. 177 pp.

Ruppert, E. E, R. S. Fox, and R. D. Barnes. 2003. Invertebrate Zoology, A Functional Evolutionary Approach. 7th Ed. Brooks/Cole-Thomson Learning. Belmont, CA. xvii +963 pp.+ I1-I26pp.

Schmitt, W. L. 1971. Crustaceans. University of Michigan Press. Ann Arbor. 204 pp.

Schram, F. R. 1986. Crustacea. Oxford University Press. New York. 700 pp

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You Can't Tell Your "Pods" Without A Program by Ronald L. Shimek, Ph.D. - Reefkeeping.com