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