Well, it got your attention, didn't it?
Nudibranch means "naked gills" and this month I
thought I would briefly review these gorgeous and wonderful
animals; animals that, almost without exception, have no place
in aquaria, but which find their way there anyway.
Now, I will freely admit, right up front,
I don't have many pictures of beautiful tropical nudibranchs,
and so I won't put any pictures of them in this column. However,
I will refer you to a site that has a lot of good images;
tropical eye candy, if you will. Nudibranchs are exceptionally
interesting creatures, and I will concentrate my efforts in
this column discussing how to recognize them. In doing so,
I hope to give you enough information that you can recognize
a nudibranch when you see one, which is not a difficult proposition,
and decipher a bit of its biology simply from its appearance,
which is a bit more difficult.
For much information and a lot of good
images, and references, go to The
Sea Slug Forum.
Nudibranchs are mollusks. Following from
last month's column on molluscan diversity, they therefore
must have all those typical molluscan characters such as a
radula, a foot, and specific types of gills, guts, and nervous
system. This, they do, and possibly more importantly for the
topic at hand, they reproduce in a standard molluscan manner.
All nudibranchs are hermaphrodites. This means they have the
sexual organs and plumbing of both genders, and they are frequently
simultaneously active. During copulation, they both give and
receive sperm. They cannot, however, fertilize themselves.
After mating, they lay eggs, often by the
millions, all encased in some sort of jellylike mass. All
of these develop into embryos, and these embryos develop into
small larvae, which hatch from the egg mass. These larvae
are called veligers, and each of them has the beginnings of
small snail shell. The shell is transparent, because at this
stage of the animal's life it can't secrete the calcareous
shell of an adult snail; nevertheless, it is a perfectly good
little snail shell. Nudibranch larvae look like any other
good planktonic snail larvae, and live much the same way.
They swim in the plankton, and feed on phytoplankton.
If they get enough food, and if it is the
correct food, they will grow, and develop further into recognizable
snails, which do have a calcareous shell. They are normally
still planktonic when this occurs. During this period they
are getting larger, and their internal structure is developing
as they get more complex. Eventually, they become able to
live on the bottom in the benthic environment. Biologists,
who study larvae, refer to them at this stage as being "competent"
Competent larvae don't just rain out of
the plankton down to the ocean floor and change into small
snails. Instead, they swim down until they encounter the substrate
and then they chemically analyze it. Or, to throw in a highly
technical, jargon term from biology, they "taste"
it. These larvae will not settle from the plankton until they
find a suitable substrate, and if they don't find one, they
perish. What constitutes an appropriate substrate varies with
each nudibranch species. Often, it is something that is correlated
with their adult prey. So, for example, a coral-eating nudibranch
may regard a species of coralline algae typically found near
the corals on which the adults feed as the appropriate substrate,
as it provides a cue for future food.
If they find the substrate, they settle
or touch down on the substrate. Following this they metamorphose,
or change their body, into a small juvenile nudibranch. In
this process, they discard the shell; it is simply shed into
the water. They also generally reabsorb the larval feeding/swimming
organ called a velum. Typically, the gut reorganizes; the
cells capable of digesting phytoplankton die or change, and
from this point forward the animal can only digest animal
prey. All nudibranchs are carnivores. This process takes from
a few hours to a few days, and during this time, the little
nudibranch lives on fat reserves it accumulated during its
larval planktonic period; it can't begin to feed until the
gut has completed its changes. Additionally, during this time
in the life of most nudibranchs, gills develop from the upper
surface, which in other snails is covered by a shell. Consequently,
as the shells have been shed, these animals have "naked
gills" exposed to the environment.
See The Slugs, Er, Nudibranchs, Er, Sea
. What Are These Things, Anyway?
Humans look at the world and from this
looking, they make decisions. We are predominantly visual
animals and we share, with the other higher primates, a visual
system that is probably the best in the natural world. So,
it stands to reason that we often make sweeping generalizations
based on exterior appearances. For example, politicians look
like humans, close enough to fool some experts, but examination
of their internal morphology will disclose they belong to
a different sort of species. Snails without shells all really
look alike. Basically, there is this blobby body which may
or may not have any permanent appendages on it, and underneath
is a muscular foot. And they secrete mucus. Always. Lots of
We call such animals, slugs, and recognize
they all basically look alike. Unfortunately, the appearances
of similarity are only skin deep, and actually in many cases,
not even that, as their skins may be different, too. If all
snails are in one group, the group we call the Class Gastropoda
of the Phylum Mollusca, nudibranchs and all marine slugs,
belong to a subdivision of that large group. They are put
in the sub-grouping called the opisthobranchs. Branch,
pronounced "brank" which rhymes with "bank,"
is a derivative of Branchia, a Greek word for the gills
of fishes. We ignore the niceties that snails are not fishes
and use the term anyway. Opistho- is a derivative of
opisthen, meaning "at the back" or "behind,"
so this group of marine snails, the Opisthobranchia, are those
snails with the gills behind, or at the back. Actually, they
are the snails with their gills behind or to the rear of their
hearts. Terrestrial slugs, by the by, don't belong to this
group. They are all air breathers and have a lung. However,
in their favor, even if they aren't as colorful as the nudibranchs,
some terrestrial slug species are close to being the champion
mucus producers of the animal kingdom.
Not all opisthobranchs are slugs; many
of them have shells. As an aquarium example, the Pyramidellid
snails that parasitize Tridacna, are also opisthobranchs,
as are the "bubble shells." The groups of snails
characterized by the pyrams and bubble shells have representatives
that may have good shells. However, in the bubble shell group,
the "head-shield snails" or Cephalaspidiceans, there
appears to be a trend toward the loss of the shell, with some
species having only a small internal shell, and looking quite
So, what is a nudibranch, and what isn't,
and how can you tell? More importantly, why should you care?
I will address the second question first.
Many sea slugs (notice I was careful not to use the term "nudibranchs")
are quite beneficial, as they may be predators on some of
the algae that become problems in aquaria. An example of one
such animal is the lettuce slug, Elysia cristata, which
may eat various green algal pests. A similar situation is
found with the sea hares. These beneficial animals all look
"nudibranchish," but they are not, belonging to
different groups, the Sacoglossans and the Aplysiids, respectively.
Nudibranchs, fortunately, are grouped together
in what taxonomists call, the "Order Nudibranchia."
Among some other things, this means that all of them have
some characteristics in common that will help in the determination
of whether or not a newly found slug becomes a pest or a pet.
Unfortunately, the Order Nudibranchia is huge, with several
thousand species having been scientifically described, and
more being found regularly that are not yet described. Some
of these undescribed species may be quite uncommon or very
cryptic. I have personally found an example of one undescribed
species, which has been subsequently described and named by
a nudibranch taxonomist using the specimens I sent to her.
I found this species in a very well-studied locality near
a marine laboratory, and it has subsequently been found elsewhere,
but all of these other collections have only been through
environmental grab samples. As far as I know, I am still the
only person to have seen a living example in its natural environment.
It is a small, very cryptic, species, and looks like a small
piece of organic leaf litter. It may well be common, but if
so, it is commonly overlooked. Other scientifically undescribed
or "new" species may be quite large and often very
strikingly colored. During my stay, in the early 1970's, as
a graduate student at the University of Washington Laboratories,
several large and obvious species were found and named from
localities around that marine station. That is the largest
marine biological laboratory on the west coast of North America,
and has been operational since early in the twentieth century,
and yet new species of nudibranchs are still being found in
So, what can an aquarium hobbyist do when
confronted with a sluggish blob? How can such a person identify
it? Well, identification to species is likely impossible,
even for the experts, as in many cases you have to know where
it was found in nature to be able to separate it from dozens
of other look-alike species. However, to identify the unknown
critter as a nudibranch or some other snail, and then (if
it is a nudibranch) to be able to identify it to a major group
is certainly something that an observant aquarist should be
able to do.
Probably the first thing you should do
if you find an animal like this in your aquarium is to remove
it from the system and put it in a clean container with some
tank water. Once it relaxes enough to move around, use a good
digital camera to take some pictures. Don't bother taking
pictures with a camera that has less than about 3 Mpixels
if the slug is small, as the enlarged image will be too blurry
to decipher. You can often send good pictures to an expert
and get a valid name for the animal.
Once the animal is moving around, examine
it. Does it look like it has a hard lump in the middle or
does the middle of the body appear swollen? If so, gently
touch it. If you feel any hardness or resistance to your touch,
it is likely a shell, and the animal isn't a nudibranch, and
you probably have a bubble shell. Check references on the
Sea Slug forum for more precise Cephalaspidacea identifications.
A diagram of a bubble shell or cephalaspidean.
A small bubble shell with an exposed shell. In most
bubble shells, the shell is completely internal.
A small, 1/8th inch long, bubble shell found on a reef
If the animal lacks an internal shell,
or you don't feel one, examine the head as it is moving. There
will be two large tentacles or projections pointing up and,
often, forward from the front end. These tentacles are typically
larger than other tentacles in the "head" region,
if there are any. These are the main sensory organs for the
animals, and are called rhinophores (in nudibranchs) or cephalic
tentacles (in most other groups). Use a magnifying glass to
examine them, and note their structure. If they appear to
consist of a fold of tissue rolled into a cylindrical shape,
the animal is a sacoglossan, such as Elysia, the lettuce
slug, or a sea hare, such as Aplysia. Check other references
for these animals.
Most sacoglossans are green and eat algae. I wanted
to show you something different. This tiny slug, Olea
hansinensis, about 1/50th of an inch long, finds
its way to bubble shell egg masses and eats the eggs.
I took this photo in nature using a 35 mm camera, and
the image on the slide is in crisp focus, but even scanned
at 4500 dpi I couldn't get a crisp image here. The faint
dots on the back of the slug are individual cells in
If the back of the animal is covered by
a more-or-less smooth surface (there may be small bumps on
it), and if the two sensory tentacles arise from under the
front of this surface, not through it, you may have either
a side-gill slug, or a nudibranch. Use a medicine dropper
tip and gently lift up the dorsal surface on the right side
of the animal. If there is an elongate structure covered under
the flap of the dorsal surface, the animal is a side-gill
slug, or Notaspidean. Remove the animal to another aquarium
and consult other references for the biology of these animals.
Most of them are predatory, and if bothered some will secrete
sulfuric acid as a defensive agent. If there is no elongate
structure along the right side, although there may be warts,
bumps or circular depressions present there, you have an arminid
nudibranch. Arminids eat soft corals, and will not be reef
Berthella californica, a side-gill slug, or
Notaspidean. The gill, peeking out from under the dorsal
mantle, is normally invisible, being covered by the
flap of tissue on the animal's right side.
An arminid nudibranch, Armina californica. Animals
essentially identical to this individual, except for
slight color differences, are found in the tropics.
If your specimen is unlike the examples
covered so far, gently touch the tentacles with the dropper
tip or some other glass probe. They should retract into sheaths
of one sort or another. If so, you have one of the several
other types of nudibranchs. None of them are generally reef
safe, although a few are useful as they may prey on undesirable
reef-aquarium animals such as Aiptasia.
Examine your critter again, this time concentrating
at the general shape of the beast, and the number and kind
of projections off of the top of the animal. If the animal
looks rather like the slug version of a small brick, more-or-less
rectangular in cross-section and with defined sharp right-angled
corners at the front end, it is likely a "Dendronotacean."
These often have two rows of projections called "cerata"
on the back. Each row is found on the edge of the back where
it meets the side of the animal, and these projections generally
have small branches off of them. Dendronotaceans eat soft
corals, sea pens, sea anemones, and jellyfish polyps. They
are definitely not reef safe, and they appear in aquaria with
some frequency. The largest dendronotaceans are found off
the Pacific Coast of North America, where Tokuina tetraquetra
may reach wet weights exceeding 20 pounds, and Dendronotus
iris is commonly seen at lengths exceeding 16 inches.
The Tritonia festiva in the slide
show in this issue of Reefkeeping Magazine is also a dendronotacean.
Tokuina tetraquetra, the largest nudibranch
in the world, is a dendronotacean. Note the small frilly
gills along the sides of the flattened dorsal surface.
This animal, a small individual, was about 15 inches
long, and what is not readily apparent is that the top
of the animal is about three inches above the substrate.
An unidentified dendronotacean, possibly a species of
Doto, eating an aquarium coral. Photo courtesy
of Skip Attix.
Upon examination, if the body is not rectangular
in cross-section, and if any projections along the back are
not found in linear rows along the back, then you have one
of the other kinds of nudibranchs, most likely either a dorid,
or an aeolid. Examples of each of these kinds of nudibranchs
are found rather frequently, actually, in marine reef aquaria.
Aeolid nudibranchs are a bane for some
aquarists, particularly those involved with stony coral culture,
as they eat the corals. Identifying them is, fortunately,
an easy process. The top surface of aeolids is covered by
unbranched projections, also, as in the dendronotaceans, called
cerata. These cerata may be arranged in linear rows, running
from the front to the back of the animal, or in paired groups
or clusters, one of each cluster on each side of the midline
on the back of the animal. The smallest aeolids may have only
one or two cerata, and are found living on, or between, sand
grains. The largest ones cruise tropical and temperate seas
in search of their prey. Aeolid cerata, or gills, are distinctive.
If you have a "furry" nudibranch, you almost certainly
have an aeolid. Examine one of the cerata, and you will truly
see one of the wonders of nature.
The cerata have at their center a brown,
tan, or beige colored tubular core. Occasionally, in some
species a different color is found, but only rarely. This
is an extension of the digestive gland in the gut running
up through the gill to its tip. The digestive gland is the
part of the molluscan gut which actually digests and assimilates
food. In some ways it is an analog of the human small intestine.
Aeolids are generally predators on corals, anemones, hydroids,
and other cnidarians. Each nudibranch species is probably
specialized to eat only one or a few species of prey. When
they eat their prey, they typically bite off rather large
chunks; large relative to the nudibranch, that is. In their
digestive process, all the flesh is moved to the digestive
gland where it is digested and assimilated. That is, with
the exception of the stinging capsules or nematocysts, which
are moved up the digestive gland tube in the center of each
gill and come to reside in the tissue under the bright white
tip, in what is called the cnidosac. The tips of the gills
actually are usually the first thing one sees, when an aeolid
Flabellina rufobranchialis, a common aeolid
from the Northeastern Pacific. Note the brown digestive
gland cores in the cerata, which are also tipped with
the bright white cnidosacs.
The nematocysts in the cnidosac remain
fully capable of discharging and are oriented so that their
threads will fire through the epidermis covering the cerata.
The nematocysts are provided with a nutritive environment,
and immature nematocysts will mature in the cnidosac. Old
nematocysts, those incapable of extending firing, are extruded
from the cnidosac tips. Nematocysts in the cnidosac remain
capable of firing for several days to a couple of weeks after
the animal that secreted them has been eaten. If the aeolid
is threatened by some potential predator, the nematocysts
can be discharged, stinging the potential predator, and presumably
deterring predation. As a result of this, relatively few animals
will eat aeolids. Consequently, the prey of an aeolid not
only nourishes it, but also protects its killer by providing,
in the form of nematocysts, protection against predation.
Over the past couple of years, aeolid nudibranchs
have been increasingly found in aquaria on Montipora
colonies and other small-mouthed stony corals. These tiny
aeolids are white or translucent gray, small, about one millimeter
long to maybe about one centimeter, and are easily overlooked.
They appear to rapidly pass totally through their larval stages
in the egg mass, and hatch as crawl-away slugs. As such, once
they are found infesting a tank, it may be very difficult
to get rid of them, and if they become established in some
dealer's tanks, they may be spread far and wide very rapidly.
Others have been found just "roaming" tanks. Other
aeolids, such as species of Berghia, have been used with varying
success in aquaria to eradicate pest anemones such as Aiptasia.
These species die when individuals of their prey become so
rare that the slugs starve before finding more food. If this
starvation occurs when they have eaten all of the pests, then
they have succeeded. More often, it simply means they can't
find the last of the pests, and with the last nudibranch gone,
the pests return.
The remaining nudibranchs typically found
in aquaria are in the group called the Doridacea, or the dorid
nudibranchs. These animals are characterized by having their
anus on the top midline of the body, about two-thirds to three-fourths
of the distance from the front to the back. The anus is surrounded
by a tuft of gills, which when extended look rather like a
feather duster (the implement, not the worm). The rest of
the top surface of the dorid may be either smooth, often to
the point of "glossiness," or bumpy like the surface
of a raspberry, or with appendages that look like the cerata
of an aeolid. In these latter species, the cerata are not
typically arranged in rows or clusters, but rather are scattered
randomly over the back of the dorid. In all cases, however,
the anus will be on the mid-line of the dorsal surface, and
if the animal is "relaxed" will be surrounded by
its tuft of gills.
montereyensis, a typical dorid nudibranch. The
front of the animal is at the lower right, and the
two rhinophore tentacles are visible. The tuft of
gills at the upper left surrounds the anus. All dorids
have this basic gill and tentacle structure.
Dorids are often the most colorful of the
nudibranchs and are often purchased by well-meaning aquarists
who bring them home, only to watch them die. Dorids generally
eat one of three different types of sessile or non-moving
animals: sponge, tunicates, or bryozoans. Often the nudibranch
species can only eat one or two closely related species of
prey, so it is effectively impossible to keep dorid nudibranchs
alive for extended periods. Not because aquarists can't provide
the physical conditions necessary, but simply because the
food is impossible to get. Few tropical dorids have been well
studied, and in most cases we don't even know what they eat,
so it would impossible to import food for them, even if we
Once you have a dorid in a tank, however,
you really often want to keep it alive, if for no other reason,
that they are often so filled with toxic chemicals that they
can kill a tank upon their death. This is especially true
for some of the beautiful brightly patterned blue, black and
gold Phyllidia. (Check these out on the Sea
Slug Forum). The bright, beautiful colors of nudibranchs
are some of the best-known examples of aposematic or warning
coloration. Warning coloration patterns are found on animals
that in some way have a good defense against predators that
hunt using vision, such as fishes or crabs. These bright colors
and striking patterns are present to be visually obvious and
thus to warn the predators away.
The system of dorid nudibranchs and their
prey forms an interesting "double warning system."
Sponges, tunicates, and bryozoans, the foods of dorid nudibranchs,
are also often quite toxic, and brightly colored, providing
a signal to fend off their own visually oriented predators.
This is the first warning component, and in most cases, these
poisons and bright colors work. Very few predators will eat
these animals. However, one group of predators that has species
that do eat them is the Doridacea, which specializes on these
prey. Coincidently, the nudibranchs are blind and can't see
the warning color on their prey. But that doesn't matter,
as the poisons don't have any effect upon them.
When nudibranchs eat their toxic prey,
they often modify the prey's own internal toxins. In many
cases this makes them even more toxic. Dorid nudibranchs are
typically animals that marine predators such as fish sample
only once in their life. If they survive, they never eat or
attempt to eat a nudibranch again, and the nudibranch's bright
colors are there to tell them what to avoid eating, so this
is the second warning in this particular system of predators
and prey. As many marine fish live several years to several
decades, this is a lesson that, once learned, is of significant
benefit to the snails.
Aquarists often see some beautiful nudibranchs
in their local fish stores, where they have been starving,
and bring them home as an addition to the tank. Generally,
the slugs continue to starve in the aquarists' tanks and within
a few weeks they die. The toxins that they have accumulated
during the life of the nudibranch may be released into the
tank water and cause serious problems. These are animals truly
best left in the wild, or if inadvertently collected, they
are best left for someone else to deal with.
I have tried here to give you a little
bit of background on many nudibranchs and how to identify
them to their major group. Once identified, you may follow
links online or do some research at a library about them and
try to determine if you really want to tackle the problems
of trying to maintain them. They are truly beautiful and easy
to maintain, once you have accepted the cost of that maintenance,
which is, of course, the cost of their foods. Although often
attractive, they are destructive of other decorative animals
such as corals, soft corals, anemones, sponges, tunicates,
and bryozoans. They are not reef safe unless your reef is
very big indeed.