Ah... Reef aquarium keeping, the hobby
where beautiful animals live in artful simulations of nature
and provide enjoyment and pleasure for their owners, as long
as those beautiful animals are well fed... by feeding on the
life blood of those same reef keepers.
Here are links to a couple of accounts
of aquarists becoming food: 1
Recently, a number of reef aquarists have
discovered the "delightful" situation wherein THEY
have become the food of choice for some of their pets. This
was not a situation that they planned for, nor was it one
that they anticipated, but it occurred nonetheless. When we
bring some reef rubble into our systems, we drag along any
of the animals in that rock at the same time. Most of these
animals are either helpful or at least benign. A few cause
And, those problems may be very serious,
indeed, as will be illustrated by examining representatives
of one of the most successful groups of marine animals, the
isopods. Isopods form a group of about 5,000 species, in what
taxonomists call the Order Isopoda, of the Superorder Peracarida,
of the Class Malacostraca of the Subphylum Crustacea within
the Phylum Arthropoda. Whew...
Commonly called isopods, pill bugs, fish
lice and rolly-pollies, these animals are found in all parts
of the marine environment. While many isopods are free living
and harmless or beneficial to reef tank denizens, a large
number are either predatory, or parasitic, or dangerous to
other reef aquarium animals.
To put the group in its proper perspective
and to comment on it in a meaningful manner, I must present
some information about Isopods. Taxonomists place these particular
animals within the animal group characterized by having an
exterior skeleton and jointed legs. This large group of animals
is called The Phylum Arthropoda, and includes such varied
animals as crabs, insects, and spiders. This is an immense
group with well over 1,000,000 species, and as result of the
human desire to put everything into pigeonholes, quite an
impressive amount of nomenclature has been developed so that
various researchers may discuss these animals without having
to relate to cumbersome descriptions. Instead, we all have
to use a lot of cumbersome terms...
Our sea-going bugs are placed within the
Arthropod subdivision called the "Subphylum" Crustacea
because they are animals that have two pair of antennae, a
particular kind of larva called a nauplius, and at least some
of their legs have two main branches. Insects with only one
pair of antennae and spiders with no antennae at all, for
example, are not Crustaceans.
Like insects, isopods have a body divided
into three major regions, the front region or head, a middle
region called the thorax, and a posterior region referred
to as the abdomen. However, unlike most insects, these regions
may not be easy to distinguish, particularly when looking
down on the animal from above. Isopods often appear to be
relatively similar from front to back, and often it is only
by examining the animals from the underside that the regions
become clearly evident.
The head is a single structure, but it
is composed of the fusion of several segments and bears six
pairs of appendages. The often-fearsome jaws and the two pair
of sensory antennae are the most obvious of these. Most shallow-water
marine isopods have a pair of compound eyes located at either
side of the head, on the upper surface; unlike the compound
eyes found in shrimps and crabs, these eyes are not on stalks.
Very deep-water forms, or species that live in caves, are
The thorax, or middle body region, consists
of eight segments, with the male's genital opening on the
eighth, and the female's on the sixth segment. The limbs of
these thoracic segments are very different from those of the
last body region, which is called the abdomen. Thoracic appendages
are generally narrow legs used only for walking, and food
manipulation, not for swimming or other forms of locomotion.
The abdomen has six appendage-bearing segments. The appendages
found under the abdomen are typically used for swimming and
as gills, and often are fan or paddle-shaped. The abdomen
has six segments, all of which may be more-or-less fused with
the final "extra" segment called the telson, depending
upon the particular subgroup of isopods. Gills are typically
found under the abdomen, and the heart is located in the abdomen
above the gills. The basic segmentation patterns, including
the number of appendages and segments in the various body
regions, and the position of the gonadal openings, are the
same in isopods as in shrimps and crabs, and serve to unite
the isopods with them and several other groups in the taxonomic
group called the "Class Malacostraca."
The Class Malacostraca is subdivided yet
further, and the isopods belong in the group called "The
Superorder Peracarida." This very large group combines
several forms of very different appearances; for example,
the shrimp-like mysids, the laterally flattened amphipods,
the cylindrical tanaids, and the isopods, which are flattened
from top to bottom, are all peracarids. None of these animals
has free-living larvae. They all develop directly from eggs
into small juveniles. The females possess a brood-pouch-like
structure, called the marsupium, located on the bottom of
the thorax. Additionally, unlike shrimps or lobsters, most
of the animals within this group lack a carapace or shell-like
extension covering the head and thorax region.
For comparison, here are links to
The isopods are grouped to form their own
group of peracarids, called the ORDER ISOPODA. The
name "Isopoda" means "similar feet' (iso
= alike, podus = foot) and refers to the walking legs
and feet of the thorax, and indeed there is very little variation
between all walking legs on any given isopod. This characteristic
alone can be used to separate them from the other peracarids,
but there are other differences as well.
Within this general description, the isopods
have radiated into all sorts of odd shapes and species with
all sorts of life styles. Isopods are one of the few crustacean
groups to become successful in terrestrial environments as
well as marine and freshwater ones, and many aquarists will
recognize them as "pill bugs" or "rolly-pollies."
Nonetheless, most of the Isopod diversity is within the marine
realm, and as reef aquarists, we occasionally run across a
bit of that diversity
The Isopoda is a huge and diverse assemblage
including many parasitic and terrestrial forms, as well as
an amazing diversity of free-living marine species. Although
most are minute, some very large isopods are found in the
deep ocean. Individuals over 30 cm long are routinely collected
from the Gulf of Mexico, and rumors persist of sightings of
animals over 1 m in length.
More information on Isopods may be
found by following this link.
Reef Aquarium Isopods
|Figure 1. A composite diagram
of the common reef aquarium species of free-living isopods,
shown from above, without their legs. Sand skaters are
not included. The relative sizes are correct, although
many of the Munnids and Idoteids seen in tanks are smaller
relative to the other types. The largest of these would
be about 2 inches long, but smaller individuals of even
the largest species are occasionally found. Identifying
characteristics are indicated by the arrows. These are
the shapes of the body segments in the Munnids, the size
of the eyes and shape of the final segment in the Idoteids,
the size and shape of the eyes in Cirolanids and Aegids,
the two pairs of large and evident antennae in the Aegids,
and the body shape and male tail appendages in the Sphaeromatids.
Drawings modified from Brusca, 1980; and Kozloff, 1996.
At least seven different general kinds
of isopods are found in coral reef aquaria, and if the frequency
of reports is any indication, their abundance has increased
drastically within the past couple of years. Biologists often
classify similar species together in a genus, and similar
genera together in a family, and each of the main types of
isopods is from a different family. The isopods in the family
Idoteidae of the suborder Valvifera are characterized by having
a long narrow body, often with a long tapering final segment
or telson. Idoteids are generally considered to be algae-eating
animals, and many are often colored to match the algae they
are found on. In reef aquaria they are benign or helpful algal
grazers. Large Idoteids are rare in aquaria. The specimens
I have seen in aquaria have been very small, most likely juveniles
of larger forms, around 1/10 of an inch or less in length.
Adults of most reported species are much larger, up to a couple
of inches long. It is possible that these small aquarium specimens
are introduced with live rock, and most simply do not survive
in our systems.
More information and pictures about Idoteids
may be found by following this link.
Some very tiny isopods appear to be relatively
common in aquaria, and possibly are very widespread, although
unreported in the hobby. These are members of the family Munnidae
of the Suborder Asellota. Some Asellotids get large, and intertidal
ones that act much like non-flying cockroaches are found in
the high intertidal zone of areas in Florida and along the
Pacific Coast of Vancouver Island. However, the ones found
in aquaria seem to be tiny little bugs, and are quite likely
mis-identified by many aquarists as amphipods, even though
the body is quite characteristically flattened in the isopod
manner. They are distinguished from other isopods by having
both pairs of antennae enlarged and evident and by rather
"separate-appearing" segmentation when viewed from
Some detailed diagrams of Munnids
are found here.
Individuals from another group of tiny
isopods are sometimes found, but they are very hard to see,
and may be fairly common. These are members of the Family
Serolidae. They are tiny flattened bugs that are often about
the size of a large grain of sand. They appear to flit effortlessly
over the sand surface and, in fact, almost appear to be skating
on it. This locomotion gives the group its common name of
More information about sand skaters
is found here
Figure 2. Female Epicaridean
isopods, Bopyroides hippolytes on the sides of
two candy striped shrimp, Lebbeus grandimanus,
from the N. E. Pacific. The large white lump on each
animal's left side is the female parasitic isopod fastened
in the shrimp's gill chamber sucking its blood.
The rather odd epicaridean isopods are
rarely, but regularly, reported from aquaria. "Epi"
means "on" and "carid" means shrimp,
so these animals are aptly named, as they are isopods most
frequently seen as parasites on shrimp. The males are very
small, look like normal isopods, and are almost never seen.
The parasitic females may be quite large, and look like large
lumps or tumors found on the side of a shrimp. The female
lives fastened onto her host's gills or upper leg segments
and sucks the host's blood. It lives under the carapace, which
becomes deformed over the parasite. The parasite is generally
not recognizable as an isopod, but rather looks like a large
white lump on the side of a shrimp.
Epicarids appear to be rather well-adapted
parasites, and do not seem to harm their host much, in spite
of their rather ghastly appearance. They may be found occasionally
on the sides of all of the various shrimp imported for the
hobby. They appear to be most frequently seen in peppermint
shrimp, Lysmata wurdemanni.
More information on Epicaridean isopods
is found by following this link.
The three remaining types of isopods are
all highly mobile, very active animals. One group is comprised
of the harmless scavengers in the group called the Family
Sphaeromatidae. One very large group of isopods, the Family
Cirolanidae, is comprised of carrion-eating scavengers and
parasites. The parasites may prey on and suck the blood of
some fishes. Both types of Cirolanids may be found in aquaria,
and actually they are quite common. The final type of isopod
that we need to be concerned about will kill and eat fishes.
These are animals from the Family Aegidae.
Sphaeromatids are small bugs, generally
less than a centimeter in length. They are common scavengers
in many shallow marine environments, including coral reefs,
and they are harmless to reef aquarium inhabitants. They can
be recognized immediately by a couple of distinctive characters.
First, each individual has the capability to roll into a ball-like
terrestrial pill bug. None of the other isopods likely to
be found aquaria will be able do that. Second, when examined
with a hand lens or magnifying glass, the last pair of appendages
of the males are expanded and extend to the rear, like small
rudders or the fins on a 1959 Caddy. Females lack these extensions,
but if some of the isopods are seen with them, that is usually
a good indication that the rest of them are also Sphaeromatids.
More information about, and some gorgeous
pictures of, Sphaeromatids may be found by following these
Figure 3. Comparison of Sphaeromatids
(top) Cirolanids (bottom), views from the side and top.
The easiest distinguishing characters are the large
obvious eyes on the Cirolanids, and the large obvious
tail appendages on the male Sphaeromatids.
These final two groups of isopods, the
Cirolanids and Aegids are so similar in appearance that it
typically takes an expert to distinguish them on the basis
of a few minor structural details. Not being an isopod expert,
I will treat them together. These are flattened, streamlined
crustaceans with a smoothly rounded or tapered and pointed
front end. Their very large eyes are found laterally on the
head and may occupy as much as half of the space of the head.
They are strong, very fast swimmers and have no obvious trailing
appendages. The thoracic legs are robust and end in very sharp
gripping claws. Aegids differ from Cirolanids in several small
details; probably the most evident is that both pairs of antennae
are often evident and visible, IF you can find one that holds
still long enough to observe that trait.
The taxonomic Family Cirolanidae is huge,
consisting of several dozen genera, and probably many hundreds
of species. Many of them are generally benign animals that
are obligatory carrion-feeding scavengers. Some of these scavengers
have been found in aquaria, and appear to be very well adapted
to being part of "the clean-up crew." Many of the
rest appear to be capable of scavenging when such food is
available, but they will occasionally swim up into the water
and attack fish, fastening on and sucking their blood. Finally,
several species appear to be more-or-less obligate blood-sucking
parasites of fishes, although some may be able to live for
extended periods by scavenging some dead food. Within the
last couple of years, some of these latter species have been
seen in aquaria with alarming frequency. In many cases, these
infestations appear to be the result of a pregnant female
that enters the aquarium and then drops her brood of 10 to
30 young, all of which are immediately hungry for a nice meal
of fish blood. A hobbyist will see the alarming sight of one
or more fish with from one to twenty blood-sucking parasites
on it. Often the isopods are nocturnal, and unless the aquarist
is alert, they may not notice the parasites, as the bugs drop
off the fish shortly after the lights go on and find shelter
in the rocks. Prolonged exposure to such densities of blood
suckers WILL kill fish.
The only way to rid an aquarium of these
animals is to catch them all, which although tedious, is possible.
Generally, this involves using a sacrificial fish, usually
something easy to catch and moderately large. A yellow tang
is a good choice for this because the fish's color pattern
allows easy determination of the presence of the parasite.
The other fish in the tank are collected and removed to a
quarantine tank, and the "bait" fish is introduced.
This fish is checked periodically and, if the parasites are
seen on it, it is netted and removed to a flat surface where
it may be immobilized with a wet paper towel. The parasite
is removed with a pair of tweezers or forceps. The fish is
then returned to the tank, and the procedure repeated. Often
the parasites are nocturnal; consequently, the fish will have
to be examined before the lights come on. The aquarium is
probably free of the parasites if none are seen on the fish
for a month or so after the last one has been collected.
More about Cirolanids may be found
by following these links: 1
A nice picture of a scavenging Cirolanid
is found here.
Aegids are "bugs from hell" as
far as the aquarist is concerned. They are like predatory
Cirolanids, only more so. Large Aegid isopods in the North
Eastern Pacific have been seen to wait on the bottom until
an acceptable fish, such as a small salmon, swims overhead.
The isopod then swims rapidly up and fastens on to the fish,
and proceeds to eat its fins and tail. The bug then slices
open the fish and eats all its blood, proceeding then to eat
the lateral muscle bands and, when they are done, they discard
the guts and skeleton.
Figure 4. This is an Aegid
similar to those collected in aquaria photographed in
nature in the waters of northern Puget Sound, Washington,
USA. This animal, Rocinela belliceps, was about
an inch long and is capable of killing small salmon,
and making the lives of some of my students miserable
The same species will fasten onto larger
fish and eat its way into a major blood vessel where it will
remain for some time sucking blood and eating tissue. When
sated, it will excavate its way out of the host and swim away.
Tropical species show up somewhat frequently
in reef tanks either riding on a fish or in a piece of live
rock. Often the first the aquarist knows of them is when they
see the isopod on a fish. Murphy's Law is active here; the
bug will never be on a cheap or expendable fish. The problem
is how to remove the isopod from the aquarium. If the bug
stays on the fish, the fish needs to be captured. This happened
to me several years ago, and the fish it was on was a Mandarin
dragonet. Imagine trying to catch this fish in a fully set
up 100 gallon aquarium! Of course, I couldn't catch the fish
until the next day, and the isopod was still on him. There
wasn't much left of the fish. Even I, with my notable lack
of coordination and dexterity, can catch a fish that is half
If you can catch the fish, the isopod may
be removed with a pair of forceps. Carefully!!! Upon removal,
the fish should be isolated in a hospital tank, and treated
with antibiotics until the wound heals. The bug may be disposed
of. Carefully!!! About 15 years ago, I had a student who was
holding in her clenched hand a 1.5 inch long Aegid. The bug
cut through the flesh of her palm, dug in, and started to
eat HER. Her vocal response was rather impressive.
So was the tenacity of the isopod, it was HARD to remove!
If you notice one of these animals in your
tank, and it leaves the fish, there is almost nothing that
may be done to catch it. They are very fast and quite capable
of avoiding a net. And if it is a pregnant female (and remember,
all female isopods have brood pouches), and the brood hatches,
you have REAL problems. There are only three courses
of action in this situation; and I truly am not jesting about
these responses. The first is to remove all the fish from
the tank and wait the two or three months until you are certain
that all the isopods have died from starvation. The second
solution is to effectively nuke the tank. Remove all live
rock and discard it as the isopods may hide in it and, as
some of the isopods bury in the sand, you should also remove
and discard the sand.
You may, of course, take the third option
and do nothing. The most likely outcome in this situation
will be that the isopods will kill your fish one by one. These
isopods are masterfully designed predators. Hope fervently
that you never have to deal with them.
Here are some more
data on Aegids.