Figure 1. A Gammaridean amphipod clinging to
a group of hydroids.
groups of predominantly marine animals are hard to discuss
without resorting to the overuse of superlatives. Adjectives
such as "greatest," "most successful,"
and "most important" tend to dominate discussions
about these animals. Primarily, such superlative animals tend
to be either large, evident, ecologically successful or the
particular author's favorites. Many of them are abundant,
and some of them draw our attention because of their activities.
In many ways, the animals I will discuss in this column, the
amphipods, should fit into those animal groups discussed with
the use of superlatives. Amphipods are certainly "ecologically
successful;" additionally, they are very successful evolutionarily,
a fact reflected in their abundance in almost all marine environments.
They are also, however, generally small, obscure, and often
cryptically colored. These latter "attributes" have
worked together to keep their natural history poorly known,
especially in the tropics where there are many other more
beautiful or striking animals to study. Nevertheless, even
in these areas the amphipods are diverse,
abundant, and ecologically important. It should be evident,
then, that some of the most common inhabitants of coral reef
aquaria are amphipods. It turns out that in most cases they
are also some of the most desirable animals to have in those
So, what is an amphipod? Well, according
to tradition, the answer to the old English riddle, "Why
is a duck?" is, "Because one of its legs is both
the same." Following the same logic, the answer to, "Why
is an amphipod?" should probably be, "Because two
of its legs are both different." The name "amphipoda"
is derived from "amph" (from the Greek amphi=amphis
meaning "both sides of, double; also apart, asunder,
or round about"), and "pod" (from the Greek
pous or podos meaning "foot," as in
podion, "a small foot," or podotēs,
“footed”), and refers to the superficial appearance that these
animals have two distinct types of appendages or feet (Jaeger,
Taxonomically, amphipods belong to the
Order Amphipoda, of the Superorder Peracarida, of the Class
Malacostraca, in the Subphylum Crustacea of the Phylum Arthropoda.
To an invertebrate zoologist, this listing of terms gives
a great deal of information. I suspect, however, that after
reading that list of multisyllabic words the average reef
aquarist will be pretty much left thinking that the old English
riddle about ducks makes more sense.
Breaking all of the taxonomic terms down
into their meanings with regard to amphipods does tell us
a lot about the animals, however. Amphipods are arthropods,
and as such they possess the arthropodan features of a segmented
body with jointed, segmented, appendages and an external skeleton,
or integument. The integument is composed of the outer epidermis
or skin of the animal, plus many chemicals secreted by that
epidermis. This exoskeleton is flexible, but not stretchable,
and in order to grow, the animal must frequently molt or shed
its "skin" and grow a new one. In reality, the skin
itself is not shed, but rather the old external chemical layers,
or cuticle, are partially broken down chemically and resorbed.
What remains, a thin remnant of the original integument, is
shed. The skin or epidermis underlies the cuticle, and remains
with the rest of the animal. A new, larger, cuticle is formed
under the old one as part of the molting process, and becomes
exposed only after the integumental remnant is shed. Although
at first the new cuticle is soft, it hardens up in a short
Each of the major arthropodan groups, the
Crustaceans (shrimp, crabs, amphipods), Uniramians (insects,
centipedes), and Chelicerates (spiders, mites, horseshoe crabs),
is very different from the others, with each exhibiting many
unique characteristics. While they all share a common ancestor,
probably something like a trilobite, that ancestor lived about
half a billion years ago, so much divergence and many modifications
of the animals in each lineage have occurred since then. Many
of the differences that separate them are obvious. For example,
while crustaceans, including amphipods, always have two pairs
of antennae, uniramians have only one pair and chelicerates
lack them altogether. Additionally, crustacean appendages
generally consist of two separate parts or branches. For example,
each of a crab's walking legs consists of two branches, the
first being the visible part of the leg, and the second being
the gill found branching off the leg's base. No arthropods
other than crustaceans have branched appendages. Crustaceans
are predominantly animals of the marine realms, and that group
is moderately successful on both the land and in fresh water.
Insects and chelicerates are predominantly terrestrial or
fresh water. Although some chelicerates are found in marine
habitats, few insects are found there.
Even though they superficially appear to
be quite different from crabs and shrimp, amphipods are considered
to be relatively closely related to both groups. As such they
are placed in the group called the Peracarida, or "near
shrimps." All of these types of animals share many similar
structural characteristics such as the same number of appendages
found in each body region, and the general body form. However,
the amphipods and several other groups of mostly small animals
lack the distinctive carapace or shell found in the crabs
and shrimps. Another important difference is that amphipods
have a brood pouch on the female's ventral surface while true
crabs and shrimps both lack this structure. After copulation
the amphipod female places the eggs in the brood pouch and
cares for them until they are ready to leave the pouch as
fully functional little amphipods. This means that amphipods
lack free-living larvae and have to disperse by actual migration
of the adults, primarily the females.
There are three major, and one very minor,
groups of amphipods, but representatives of only one of them
are likely to be commonly found in our tanks. The common type
of amphipod is called "a Gammaridean amphipod,"
and belongs to a grouping named after one of the more common
amphipodan genera, Gammarus.
Such animals are characterized and differentiated from shrimps
by the absence of a carapace or shell over the front part
of the body. The separate anterior body segments are easily
seen. This condition may be contrasted with that seen in the
shrimps or crabs, whose separate segments are covered by the
carapace or fused to it.
amphipods are typically flattened from side-to-side and they
also usually possess large compound eyes on either side of
the head. Unlike the eyes of shrimps or crabs, these eyes
are not on stalks, but are imbedded in their head. To make
things a bit more interesting as well, although the name "amphipod"
refers to two types of legs, these animals actually have a
variety of leg structural types on any given animal. Most
of the visible segments are found on the middle body region,
referred to as the thorax, and each of these segments bears
one pair of appendages. The two most anterior pairs of visible
thoracic segments have modified appendages called gnathopods.
"Gnath" means "jaw" and the gnathopods,
literally "jaw feet," are thought to be used in
feeding, but relatively little close and careful observation
has been done on animals in this group. What these appendages
actually are used for is largely open to question. In any
case, gnathopods typically, but not always, end in recurved
jack-knife-like claws called "subchelae" (See Figures
2, 6, and 7).
Behind the two pairs of gnathopods are two pairs of shorter
legs followed by three pairs of longer legs
to the rear. The longer legs tend to splay outward, giving
the animal a recognizable and typical posture, rather like
a bicycle supported by training wheels.
Figure 2. External anatomy of a generalized Gammaridean
amphipod. The head region is shown in red, the thorax
in purple and the abdomen in yellow.
Figure 3. Internal anatomy of a common Gammaridean
amphipod. The heart is brown, the various parts of the
gut are green, the nervous system is blue, the gonad
and gonoduct yellow, and the kidney, or renal gland,
internal anatomy of a typical amphipod is shown in Figure
3. As in all arthropods, the nervous system
lies along the middle of the ventral surface. There is a ganglionic
swelling in each segment. The large supraesophageal ganglia
are located above the esophagus and together with the nerves
running around the esophagus, constitute the brain. The eyes
communicate directly with these ganglia by large optic nerves.
The antennae, or feelers, are sensory and large nerves run
out to them as well.
The mouth is located near the base of the head, and leads
to a short esophagus passing vertically to the stomach, located
just behind the head. Inside the stomach are some chitinized
plates lined with ridges that serve to grind food. Because
their mouth size is constrained by the more-or-less rigid
exoskeleton, most of the food that enters is either liquid
or is torn into small bits by the appendages around the mouth.
A long midgut passes though the majority of the body. A series
of pouches or caecae arise from the midgut. One very short
caecum rises from the top of the gut and runs forward a short
distance toward the head. Two to eight pouches arise from
the sides and bottom of the midgut, right behind the stomach.
These pass to the rear and extend almost to the abdomen. Both
digestion and the secretion of digestive enzymes and "juices"
occur in the various caecae. A single similar pouch arises
from the end of the midgut and runs forward up over the midgut
caecae and gonads. Its function is unknown. Posterior to the
origin of this caecum, the gut is referred to as the hindgut.
A long tubular heart is suspended from the dorsal body wall
in the thorax. Three pairs of valved openings, called "ostia,"
allow blood to flow one way into the heart from the surrounding
space. When the heart contracts blood is pumped forwards and
backwards through the anterior or posterior aortae, respectively.
Lateral blood vessels lead from the heart to the body wall.
The blood leaves these blood vessels and flows around and
through the tissues in channels. The blood flow is rapid and
completes a complete circuit in just a few seconds in a small
amphipod. The blood is loaded with various types of corpuscles,
but we really don't know how most of them function. Gammaridean
amphipods have no specific respiratory organs, and gas exchange
probably takes place over the entire body surface.
Amphipods have a rather straightforward life cycle that allows
them to reproduce well in our systems. They generally have
separate sexes and hermaphrodites are rare. The genders are
easily distinguished. The oviduct opens at the base of the
legs of the sixth thoracic segment, and the vas deferens opens
at the base of the legs of the eight thoracic segments. The
males have a pair of penes, modified thoracic appendages,
and fertilization is internal. Shapes of the eyes, antennae,
and second gnathopods may also vary between the sexes. Additionally,
females have the brood pouch discussed earlier. It is formed
under the female's bottom surface by a series of plates originating
from the inside edge of each of the thoracic legs. The plates
extend over the ventral midline and overlap with plates from
the other side, creating a chamber between them and the ventral
surface. After copulation, the fertilized eggs are deposited
in the chamber and are held there as they develop. When the
larvae finally mature to juveniles, the female releases the
brood and they disperse in the local area. Amphipods lack
a free-swimming or free-living larval stage. There is not
much chance of larval mortality in this type of reproduction
unless, of course, the female is eaten. Normally, when the
populations build up and become dense, pregnant females emigrate
to establish a new population.
gammaridean amphipods are regarded as looking pretty much
like the diagram in Figure 2 above, or the
photo below (Figure 4). This is, however,
a large group, with probably well over 5,000 species, some
of which are weird
The group displays a significant amount of variety of shapes
and sizes (Figure 5). It goes without saying
that these animals occupy a wide variety of ecological niches.
Gammaridean amphipods may be predators, herbivores, detritivores
or bacteriovores. A few are omnivorous. Many are commensal
and are found living in or on other animals,
and a few are parasites.
Figure 4. Typical Gammaridean amphipods.
These two specimens, both about 4 mm (5/32") long,
both found in aquaria and show the characteristic amphipod
Figure 5. Diagram of some gammaridean amphipods
found in one small geographical region (central California).
Note the variety of shape and structure. These animals
are not the same size and are not shown to scale. Modified
from Bernard, 1975.
The most important part of any animals'
natural history is their need to obtain food, and the variety
of ways in which amphipods feed may be reflected in differences
in their basic feeding appendages. In general, the gnathopods
are considered to be involved with prey capture, food manipulation
or feeding. The basic morphology of the gnathopods is shown
in the diagrams in Figures 2 and 7
and exemplified in the photo, Figure 6, but
there is a significant amount of variation in what the actual
structures are. Some of the various shapes of gnathopods are
shown in Figure 7. Each of these changes
from the simple subchelate appendage shown in Figure 2,
reflects a difference in diet and lifestyle. Consequently,
it should be obvious that the many types of Gammaridean amphipods
have radiated into a great many niches, and
occasionally we might encounter some rather odd ones in our
Figure 6. The subchelate first gnathopod
of an amphipod. This appendage is transparent,
and the relatively massive muscles used to close the dactyl
against the propodus are
labeled, as are blood corpuscles in the blood channels.
Figure 7. Some of the many modifications seen in the
shape of the first (mostly) gnathopod. In the "simple"
form the appendage is basically a walking leg. In all other
forms the appendage bears a flexible pinching or cutting claw
on its end. The pink segment, the propodus, is the basal part
of the claw. The yellow segment or dactyl is the moving part
of the claw. Modified from Staude, 1987.
One very small group of oddball amphipods
is called the Ingolfiellidea.
animals normally live between and on sand grains, and to the
best of my knowledge, they have never been seen in aquaria.
The other two groups of amphipods are both ecologically important
and diverse. They are the Hyperiidea
and the Caprellidea. The hyperiids live in the plankton, and
many are predatory or parasitic on gelatinous zooplankton.
Some hyperiids are rather bizarre, and one, Phronima,
, has been rumored to have been the model for the Alien in
the movie of the same name. Phronima
is pelagic and lives in pelagic tunicate houses.
The other group of amphipods, the Caprellids,
are fascinating animals. One lineage of caprellids live on
whales and are called "whale
lice." The second lineage is called skeleton shrimp,
which is an apt name. They look like small multi-armed skeletons
and are found clinging to algae, the backs of sea stars or
gorgonians. They are really very common, but seldom make it
into our reefs.
To a person used to considering the Gammarids
as typical amphipods, the first time a Caprellid is examined
it is very difficult to consider it an amphipod. At first
glance, and probably second and third glances as well, Caprellids
look NOTHING like a Gammarid. They have a long tubular body
terminating in a small but bulbous head that sprouts two pair
of long antennae, which may be as long as the rest of the
body. They have relatively large and pronounced appendages
that look like they are made of sticks, but that end in claws
that close like a jack-knife. The name of skeleton shrimp
is apt. They look like some sort of weird crustacean skeletons
that have come to life.
Although large ones may be several centimeters
long, most of the ones that make it into reef tanks are on
the order of a centimeter in length or less. They often can
extend their anterior appendages to a span as wide as they
are tall. They have the habit of sitting on a rock promontory
or a piece of algae, which they grip with their hind legs.
The body extends vertically up into the water and the front
appendages are spread wide as they wait for something to pass
them in the water. They will reach out and grab food drifting
by, or they may work along an algal frond, gorgonian branch,
or some other substrate gleaning food from it.
As in the other amphipods, females have
a brood pouch. In this case, it is located in the middle of
the body as the animal stands up, and is often visible as
a small white spot in the middle of some of the animals. They
are typically pale amber or white, but may display other colors
as well. Many of these are harmless or beneficial herbivores
or scavengers, but some are carnivorous, and they could eat
small, soft coral polyps, and other small animals.
As with so many animals, caprellids may
enter our systems as hitchhikers on live rock, algae, corals,
or in live sand. Generally, they are fairly harmless, and
are so much fish food. Occasionally, some are found that will
cause some minor problems. Desirable and generally herbivorous,
caprellids may be cultured in a refugium, sump, or occasionally
in the main tank, provided there is an appropriate food, generally
some alga. Culturing consists of providing light and algae
and letting the animals do their thing. Carnivorous forms
may be removed with forceps, if they are causing problems.
Generally, in the main tank, fishes remove them before the
aquarist was aware that they were even there.
Figure 8. Caprellid amphipods. Left: a
male. Right: a Female (note the large brood pouch
with developing juveniles in it).
Aquarium Behavior and Care
is generally very easy to recognize the majority of amphipods
in our aquaria; they lack the carapace, or shell, of crabs
and shrimps, and tend to be laterally compressed. The appendages
of the posterior thorax that extend laterally are also distinctive.
Most aquarium species are small, seldom reaching more then
a few millimeters in length. They normally vary little in
gross structure from the illustration shown in Figure 2.
These bugs are also often referred to simply
as gammarus shrimp, which is a misnomer as most of the species
found in our tanks are not in Gammarus proper. Occasionally,
as well, we hear various vernacular corruptions of Gammarus,
such grampus or gramus, used to describe these animals. Most
aquarists think that all amphipods are pretty much like the
standard gammarid amphipod that they see in their tanks, and
that all of these animals are herbivores or detritivores.
Within the limited world of reef tanks, this is more-or-less
true; however, not all gammaridean amphipods are herbivorous,
some are very decidedly carnivorous.
Nonetheless, the most common amphipods
found in aquaria are either herbivores or detritivores. They
tend to eat either plant or algal material preferentially
and either graze on algae or eat debris of plant or algal
origin. Generally, they don't eat much in the way of animal
flesh, although occasionally we do get some predatory amphipods
in our systems. It is difficult to distinguish between any
of these species without specific microscopic examination,
so the only way most hobbyists have of differentiating between
the two types (and keep in mind there are several hundred
potential species in each type) is to watch them feed. In
our aquaria amphipods are typically part of the cleanup crew.
In addition, they are good food for whatever fish can catch
them. Altogether they are a beneficial and interesting component
of our systems' fauna.
Man-Eating Amphipods and Other Oddities
Figure 9. Some oddball amphipods. Left:
A stenothoid amphipod, similar to the "coral fleas"
of some aquarists, living on a cerianthid, or tube anemone,
tentacle. The tentacle is about 1 mm in diameter. Center:
A Gammaridean amphipod, Dulichia, living on a
strand of its own feces on a mud bottom. It builds the
fecal strand and then crawls up it to suspension feed
in the water currents. Right: A Dulichia
female and her progeny. In this case, she has built
the fecal strand on the tip of a spine of the red sea
urchin, Strongylocentrotus franciscanus.
Carnivory in amphipods
is not uncommon, and occasionally some truly carnivorous forms
hitchhike into marine aquaria. In the deep sea, or even in
many shallow water areas below the photic zone, carnivorous
amphipods are either dominant members of the scavenging guild
or predatory in their own right. I even know of one species
that on occasion is a man eater, being the man it was documented
eating! This particular amphipod species, Chromopleustes
pugettensis, is strikingly colored (Figure 10).
The body is a rich dark brown with a brilliant white saddle
and longitudinal gold stripes. The eyes are lavender and the
legs are blue. Regular readers of this column will probably
recognize this coloration as warning or aposematic coloration.
This is coloration of animals that are dangerous, and this
amphipod is decidedly dangerous - both to its predators and
its prey. Additionally, this is an amphipod
species that does not hide. The animals are highly visible
on the bottom in daylight and do not flee when approached.
This behavior is also an indication that the animal is protected
Figure 10. Chromopleustes pugettensis,
the "man-eating amphipod" of the north Pacific.
Chromopleustes pugettensis is patchy
in its distribution. It is generally uncommon, but when it
is found in an area, it tends to be found in large aggregations;
I have observed several swarms of more than 20,000 individuals
on a single dive. During most of the year Chromopleustes
pugettensis appears to be associated with sea cucumbers.
Detailed ecological information is lacking, but it is likely
eating small cucumbers or portions of larger ones. Sea cucumbers,
like many echinoderms, contain toxic chemicals called saponins.
Saponins are thought to confer freedom from predation on many
echinoderms, and generally predators on echinoderms are very
rare. It has been presumed that somehow, Chromopleustes
has adapted to eating sea cucumbers and has concentrated their
saponins in its body. This has never been tested; however,
in a series of trials it was evident that most fish in the
area will not eat the amphipod. Many years ago, I did some
trials trying to feed the amphipod to various predators. In
only one case did a fish eat the bug, which it promptly spit
back out. The next morning the fish was dead. Anecdotal evidence,
to be sure, but certainly interesting anecdotal evidence.
On April 2, 1983, I was diving in an area called Pole Pass,
in the San Juan Islands of Washington. During this dive, my
dive partner and I came across a large sea star, Pycnopodia
helianthoides, that was spawning. It was absolutely covered
by a swarm of the amphipods and they were tearing off pieces
of its upper surface. As we attempted to get close, the swarm
rose and a portion of it settled on my face and before I knew
what was happening the bugs were biting my face and lips.
I rapidly "back pedaled" and managed to brush them
all off, but by the time I had done this, they had managed
to break my skin in several places and I was bleeding quite
profusely. NASTY LITTLE BUGS!!!
Fortunately, most reef aquarium amphipods are much more benign.
The above experience, however, ought to convince most aquarists
that they cannot take the non-predatory nature of amphipods
for granted. On the other hand, amphipods that are reclusive
and which lack warning coloration are probably quite safe
and beneficial to our systems.