Of the common reef aquarium animals, only
the polychaete annelids, or so-called bristle worms, are more
misunderstood and badly treated than the common snails that
many aquarists put into their systems to graze on or remove
algae. Relatively little thought generally goes into the choice
of these animals and, judging from the questions asked in
my online forum, even less thought goes into their care. This
maltreatment often results in significant mortality. But who
cares, they're only snails, right?
The moral and ethical issues of treating
living organisms as "disposable" aside, I care.
Snails are fascinating, complex, and often beautiful animals.
The evolutionary radiation of body form in animals that can
be called "snail" is one of the life's most successful
stories. Depending on which way the taxonomic estimate is
made, there are an estimated 35,000 to 150,000 living species
of snails. This makes the "lowly" snails the largest
animal group other than insects. Reflecting this amazing diversity,
they have occupied every terrestrial and aquatic habitat on
I will devote this and my next two columns
to briefly discussing some of the major groups of the common
grazing snails that are kept in aquaria to assist in the control
of algae. In this initial column, I will cover a rather large
assemblage of similar-looking snails commonly sold in the
aquarium hobby. Their similarity of shell shape and internal
structures is an indication that they are closely related
and that they share many attributes in common. Their structural
similarities allow an author to cover a lot of ground by generalizing
and focusing on like attributes. However, in such situations,
it is always worth the reader's time to remember that no two
species have the same requirements. Each species is special
and unique in its own attributes, and discussions of generalities
can go only so far. So, read what I have to say with a critical
eye. I will also cover several of these animals more specifically.
This first major group contains those animals, often referred
to by aquarists as Turbo, Trochus and Astraea
grazers, is called by the scientists who study these animals,
the "Superfamily Trochoidea." A superfamily is a
large, inclusive, group containing many smaller groupings
of superficially similar animals differing in characteristics,
at the family level, that are generally considered to be relatively
minor. Within the snails, such minor characters are often
manifested in small differences in shell shape, operculum,
and shell ornamentation.
The evolutionary interrelationships within
mollusks have been the subject of much research within the
last twenty or so years, and as a result the taxonomy of the
whole phylum has been undergoing a set of major changes. Presently,
these animals are taxonomically
classified as being in species that taxonomists have placed
in the Families Trochidae and Turbinidae of the Superfamily
Trochoidea. That Superfamily is, in turn, placed in a larger
group called an Order. The Ordinal name is uncertain at the
present time as it is undergoing revision, but it is one of
several orders within the Superorder Vetigastropoda of the
Subclass Orthogastropoda. All of these, of course, are within
the Class Gastropoda, which contains all the snails. One can
refer to all of the grazers I will discuss in this column
as Trochoidean snails.
Trochoidean gastropods include all the
Turbo, Astraea, Trochus and related animals
such as Margarites, Stomatella, Norrisia,
and Tegula. There are many other snails that eat algae
in aquaria, consequently the Trochoidea is not all-inclusive
of grazers. It does not include the nerites, abalone, limpets,
the ceriths, conchs, and some cowries. These latter animals
are perfectly good algae-eating animals, but they are only
very distantly related to the trochoidean grazers, and I will
cover them in future columns.
Most, but not all, of the Trochoideans
have shells that look rather similar; as they should if they
are all closely related. It is, in a sense, this similarity
that prompted this column, as these animals are often difficult
for hobbyists to tell apart. Their similar shell shapes may
sometimes be deceiving, though. Stomatella varia has
a small, flattened, cap-shaped shell and tends to look like
a slug. It is, however, a good Trochoidean snail. Nevertheless,
the shell is decidedly an odd one for the group.
Malacology, or the scientific study of
mollusks, began in the sixteenth century among the shell collections
or "conchological cabinets" of the "gentlemen
naturalists" of that period. These were individuals who
had the considerable wealth necessary to indulge their whims
by collecting sea shells and other marine oddities to include
in their displays, which were often kept in large mansions.
Incidentally, the study of corals started at the same time,
as coral skeletons were among the items included in these
collections. In time, these collections became the basis for
many modern museum collections. At the time of the collection,
however, the collectors had no consistent way to describe
the shells that made up their collections, and over the course
of about a century, malacology arose primarily as a way of
identifying these shells. In the process, a rather complex
and descriptive terminology developed, which allowed the discussion
of the various aspects of snail shell shape. I will try to
ignore most of this terminology, but some of it is inescapable
in a discussion of closely-related snails, because in general,
the most visible differences between species are related to
differences in shell features.
I think it is useful to start a discussion
of a snail shell by considering the basic animal to be a slug,
without a shell. Such an animal is essentially a mound of
flesh that creeps along on a broad, flat foot. Now, consider
that slug with a shell over the top. The shell would likely
be rather shield- or cap-shaped, probably something like that
found on a limpet. One of the characteristics of snails is
that they grow primarily in a top-to-bottom, rather than a
front-to-back or side-to-side, direction. In other words,
as they add mass to their body, they get thicker, not longer
or wider. This means they become taller, and higher off the
substrate. Given such growth, the flat cap shell would change
to look something like the shape seen in a flat, broad, ice-cream
cone. Further growth would tend to make the shell have a truly
conical shape, much like an ice-cream cone. If one pictures
a small slug carrying a sharply pointed ice-cream cone shaped
shell oriented vertically on its back, it is probably obvious
that such a shell would be unstable. The way in which this
instability was solved appears to have been by coiling the
shell. Coiling makes many snail shells doubly conical. They
are cones coiled in a conical manner. This coiling pattern
creates a shell that is helically coiled; in other words,
it's basically spiral in shape, but occupies three dimensions.
Each complete 360° "lap" of the coil is called
a whorl. The largest and last whorl contains the aperture
into which the animal can withdraw, and is called the body
whorl. The relative shape of the animal depends on three main
factors: the shape of the aperture (which defines the shape
of the whorl itself), the rate at which the whorl gets bigger,
and the rate at which the aperture moves away from the coil's
center. Varying those three factors together can produce all
possible snail shell shapes. Fortunately, in the Trochoidean
snails, there is not a lot of diversity in shell shape (Figure
1). In some respects, this is a problem, as they all tend
to look a lot alike. On the other hand, it makes recognition
of the basic Trochoidean pretty simple.
Figure 1. The basic shell forms found in trochoidean
snails. Turban snails tend to have rounded whorls, while
trochids tend to have smooth-sided shells. Although
I have illustrated them using only the trochid shells,
either shell type may have an umbilicus, a "belly
button" or a pit on the underside of the shell
caused by the coiling, or a callus, a calcareous
plug filling the hole. The auriform shell is formed
when the whorl enlarges very rapidly relative to the
coiling rate and is found in animals such as Stomatella.
It is not for the shells, however, that
we aquarists put these animals into our tank. To be succinct,
we buy these snails for their grazing ability and dietary
preferences. Snails, in general, and Trochoideans, in particular,
are called "microphagous" feeders. This means they
feed on small particulate materials. They get such materials
by sweeping or rasping them off the substrate. This rasping
ability is related to, and determined by, a structure found
only in mollusks called the "radula." The radula
(plural = radulae or radulas) is a projection arising from
a pit in the bottom of the front part of the digestive tract,
just inside the mouth. One side of the radula is covered with
spiny projections. In their simplest forms, these spines are
made of cuticular material, basically protein and chitin.
Vertebrate teeth are also basically cuticular and are found
in the mouth and, because of these similarities, the spines
on a mollusk's "tongue" are called "radular
I have drawn a simplified diagram of the
structures inside the front part of a snail's head, as if
it were cut down the middle and viewed from the side (Figure
Figure 2. Diagram of the radula of a common snail shown
in longitudinal section.
The radular teeth (red) sit on a radular
ribbon (pink) and both are formed in the inside of a radular
sac that projects from the bottom of a cavity just behind
the mouth. The teeth and ribbon are secreted in the bottom
of the radular sac and used teeth are sloughed off into the
mouth. The teeth and ribbon sit on cartilage, and the cartilage
may be pushed out and pulled in through the mouth by the action
of muscles. The radular teeth are relatively complex structures
(Figure 3) and there may be several hundred rows. The snails'
rasping may wear out several dozen rows of teeth per day.
Figure 3. Scanning electron micrographs of the
working surfaces of the radulae from three different
Trochoidean snails. These are the rasping surfaces of
the snail's radula. Each photograph is from an individual
of a species in the genus indicated, and each shows
a view about 1-2 mm (0.04 inch) wide. Tectus
has a radula similar to Trochus and is included
here to show the general type of radula found in that
genus. Each yellow band highlights a single row of teeth.
Note the wide diversity of tooth shapes. Photos are
modified from Hickman and McLean, 1990.
All of these snails feed by moving along
the substrate (Figure 4) and periodically "licking"
the surface with their radula. In Figure 3, you can clearly
see that the design and number of radular teeth is different
in each genus or group of species. This difference in the
teeth HAS to be reflected in feeding differences - different
species will feed on either different algae or different types
of substrate (discussed below). The teeth are probably optimized
for some algal types, but the feeding behavior and natural
history of the Trochoideans is poorly known. It is presumed
that most are herbivores, but evidence suggests that a significant
number are at least partially carnivorous.
Figure 4. Diagram of a typical Trochoidean grazing
snail in a common feeding posture on aquarium glass.
Acclimation Problems, Or Why Do They Die?
The complexity of internal anatomy in snails
is one major reason why some of the Trochoideans seem to die
easily, or otherwise not live through their full life spans
in our tanks. Although most folks probably never give it a
second thought, snails have a complex internal morphology.
In many ways, the internal structures that the snails possess
have allowed them to be very successful. It is not, however,
a particularly "rugged" morphology when subjected
to some specific stresses. Snail tissues often seem to be
composed of thin layers of tissue that are very filmy and
diaphanous. Consequently, they often suffer significant damage
during water changes or during transport from a dealer's to
an aquarist's tank. The circulatory system of snails may be
very complex (Figure 5), and many of its vessels and channels
can rupture under stresses caused by changes in salinity.
The vessels in the kidney are numerous and delicate, and may
rupture if the animal is not slowly acclimated when being
moved from one set of water conditions to another. If the
acclimation is too fast, the animal will die in a few minutes
to a few weeks. If the snails are drip acclimated, the acclimation
time may need to be on the order of five to ten hours for
Figure 5. This diagram shows the circulatory system
of one type of snail, showing the major blood vessels. The
shell has not been drawn (Modified from Fretter and Graham,
1994). Arteries are shown in blue (Unlike the red blood of
vertebrates, snail blood has hemocyanin as the respiratory
pigment and oxygenated blood is blue; deoxygenated blood is
colorless); veins are shown in green. Snails don't have capillaries
connecting the articles and veins, instead blood flows directly
through the tissues between the end of the artery and the
beginning of a vein.
Another factor in their long-term survival,
of course, is the appropriate food. The diversity of radular
structures that I illustrated above ought to convince most
folks that these animals are not created equal with regard
to their diets. Or to put it another way, they eat different
things... This is a real problem for aquarists, who often
perceive the various grazing snails as "interchangeable,"
when in fact they may require decidedly different conditions
or foods for good health. Unfortunately, the real problem
with most of the various grazers is that we haven't a clue
what their requirements really are. There have been precious
few studies in the natural world of the reefs that assess
the determining factors of their existence, and consequently
we are stuck with a "trial-and-error" response.
This is further complicated by the fact that as hobbyists
we really don't know, in most cases, where the animals originate,
what microhabitat they inhabit, or even what species they
A Trochoidean Primer
It is relatively easy to distinguish between
some of these grazers and I have cobbled together some illustrations
to help identify them to genus. I have included some illustrations
or links to those snails commonly offered for sale to hobbyists.
Some of them are decidedly inappropriate for reef aquaria,
and I have indicated those specifically.
Although determining the genus is not terribly
difficult, identification to species will take some expert
consultation, as there are often several dozen to several
hundred species in each of these genera. In all cases, however,
there are several characters critical to identification.
First, note the shape and composition
of the operculum, or trapdoor, which plugs the animal's
hole when it retracts (and with the exception of Stomatella
and its near relatives, they all have one).
Second, notice the shape of the shell,
- Is it tall or “squat?”
- When viewed from the side,
are the edges of each whorl smoothly rounded, or more
Third, does the animal have any sculpture
or ornament on it, such as ridges or lines?
Fourth, does the animal have an open
"umbilicus" (a pit in the bottom of the shell),
or is that pit partially or completely filled by a calcareous
All of the above characters are used in
assigning the animal to some of the various genera below.
Be forewarned, most dealer identifications are wrong.
and Tectus [Link
species (Figure 6)
These are animals with either a tall or
squat shape. The aperture to the shell has an angulate, wavy
or toothed appearance due to folds in the shell at the margin
near the center of the shell. If the shell is looked at from
the bottom, there is a hole in the center. The operculum is
typically protein or horn-like material and is generally amber
or brown. These are typically good grazers for reef aquaria.
They are sold under several names by various dealers. Surprisingly,
on some occasions they actually get named correctly, for Trochus,
anyway. These genera are closely related and differ mostly
by soft part anatomical and radular differences, and so the
average hobbyist, and particularly the dealer, may be unable
to distinguish them. Trochus or Tectus species
seldom have any pronounced bumps, warts, or ridges on the
Figure 6. Two different species of Trochus.
The aperture is outlined in black in the right views. Note
curves of shell material near the center of the shell, and
the umbilicus at the bottom center of the shells.
Turban shells, as their name implies, have
a more rounded shape, like that of a turban. They have a large
and prominent calcareous operculum. They may or may not have
sculpturing. Turban shells may have a small umbilicus in the
bottom center of the shell, but in most cases the hole is
filled with a calcareous callus.
7. A large smooth turban shell without any pronounced
sculpturing; note the
rounded shape of the whorls and the large, white, calcareous
(Figure 8, 9)
Astraea (and yes, the spelling includes
the second "a") may be thought of as "angular
Turbo." The outer edges of the coiled whorls are
not rounded or inflated as in Turbo, but are often
straight or flattened. When viewed from the aperture side
of the shell, Astraea tend to present the appearance
of a smooth conical shell without the rounded edges visible
in Turbo. Many Astraea have very prominent axial
sculpture, so that when the animal is observed from above
it appears star-shaped. That shape, in fact, gives the genus
its name; "Astraea" means star, and they
are commonly called "star shells." However, the
Astraea species offered for sale in the aquarium hobby
are generally not heavily sculpted, although various species
may have small bumps or ridges. Most snails sold as Astraea
in the marine reef hobby are trochid snails, although they
are seldom from the genus Trochus. Many of the actual
individuals of Astraea offered for sale in the reef
aquarium hobby come from the cooler waters of Baja California.
These animals are not from areas that have sand substrates
or, really, much of anything else in the way of flat surfaces.
If they get dislodged from rocks, they tend to fall into crevices
or crannies where they can reach a rock with their foot and
attach to it. Consequently, they have never developed a "righting
response." This means that if they fall from the aquarium
walls onto the sand, they will not be able to turn over, and
will die there unless somebody or something turns them over.
Given that they also are not warm water animals, they tend
to die young and leave a good-looking corpse in the aquarium.
Purchase them if you wish. I would spend my money on tropical
Figure 8. Astraea, like Turbo,
have a calcareous operculum, and unlike trochids they
seldom have an open hole in the bottom center of the
Figure 9. A small, 2 cm high, species of Astraea
with prominent ridges.
Collonista are "mini-Turbo
snails." They are seldom purchased by hobbyists, but
are relatively common in reef tanks anyway, because they appear
to hitchhike in on live rock or in some live sand. When hobbyists
first see them, they presume them to be "baby" grazers
of some sort. Instead, they have some of these animals. They
reach a maximum adult size of about 1/4th inch (6 mm) in height
and diameter. They are often tan to white and have mottled
brown color patterns on the shell. They can be distinguished
from all other Trochoideans by their small size and the presence
of a small pit or hole in the center of the calcareous operculum
that plugs the aperture. They reproduce well in aquaria, and
are quite good grazers. If present in large numbers, they
may effectively replace all other grazers in our systems.
Figure 10. Collonista. These small Trochoideans
are distinguished by their minute size and
by the small pit in the center of their operculum.
varia (Figure 11)
Almost slug-like, but possessing an auriform,
cap-like shell, individuals of Stomatella seem unlikely
to be Trochoideans. However odd their appearance, their internal
anatomy puts them squarely in this group. Stomatella
individuals may reach lengths of about an inch and a quarter
(3 cm), and as their specific name of "varia"
implies, they may be almost any color, although gray, black,
and mottled colors tend to predominate. They are quite good
grazers, and also reproduce well in aquaria lacking hermit
crabs and shrimp predators. Their shell shape precludes their
retraction into the shell, but evolution has given them a
rather neat way to escape many predators. The rear portion
of their foot can break off, much like some lizards' tails,
when they are threatened by a predator. The detached portion
then writhes, and twists and otherwise distracts the predator
while the snail, presumably, makes good its escape. Additionally,
they also tend to be nocturnal and avoid a lot of visually-oriented
predators in this way.
Figure 11. A Stomatella varia about half an
inch long. This individual's foot is retracted and the shell
is quite evident. The foot commonly extends far behind the
shell and, additionally, often obscures
part of the shell. The odd shell shape, for a Trochoidean,
Caveat Emptor!! Let The Buyer Beware!!
A large number of Trochoideans are sold
to reef aquarists despite having little or no chance of long-term
survival in a reef tank. Most of these are collected from
the northern shores of Baja California. Although it's south
of the United States, the marine environment of the northern
part of this peninsula is anything but tropical. Its Pacific
side is bathed in waters that are quite cool, and is the home
of several Trochoideans that are collected for the reef aquarium
hobby. These animals typically have a tolerance for warm conditions,
and they are often intertidal animals and can withstand quite
hot water - for a while. Prolonged exposure to warm conditions,
however, kills them.
Among their many interesting attributes,
many marine snails have impressively long life spans. I have
counted over 120 annual growth rings on some specimens of
funebralis, a temperate water species. This species
is one of the three or four species of Trochoideans collected
from cool water areas of Baja California and unethically sold
to gullible, or informed, aquarists as a reef aquarium animal
under the delightfully ambiguous name of "margarite or
margarita snail. Tegula funebralis has a high thermal
tolerance for an animal that lives
in cold water areas (it ranges northward from Baja and
is common in the British Columbian and Alaskan intertidal
zones). They normally live a small fraction of one percent
of their normal life span, or only a few months, in reef aquaria.
Putting these animals in a reef aquarium is both unethical
12. Tegula, two species; Tegula funebralis,
left, and Tegula brunnea, right. These
cool water species of Tegula are harvested from
the coast of either Baja California or California and
sold under the name of "margarita" or "margarite"
snails. Shells are about ¾ inch (2 cm) in diameter.
Image modified from Hickman and McLean, 1990.
This confusion is complicated by various
distributors and dealers who just can't seem to grasp "The
Phenomenon of a Name," and blissfully attach names seemingly
at random to their livestock. A good example of this is in
the common name, "Margarite snail." Well folks,
I have been studying snails for a long time, and to me a "Margarite
snail" is a snail in the genus Margarites. These
are small snails, similar in many regards to the grazers we
put into our tanks. They are even found in the tropics. However,
when they are found in the tropics, they live several thousand
feet down in water whose temperature is 39° F. or lower.
In other words, there are no snails of the genus Margarites
that are found in warm tropical waters. One finds examples
of the common North Pacific species, Margarites pupillus
(Figure 13), offered for sale and this invariably leads to
a quick death for this species if kept in reef aquaria. As
with the species of Tegula, with which they share the
common name "margarite" snail, they have no place
in reef tanks. Still vendors persist in selling them. Go figure...
13. Margarites pupillus, from Northern Puget
Sound, photographed in water that was about 45°F.
This species is a cool water animal and should not be
purchased for a reef aquarium.
norrisi (Figure 14)
The red turban or red-footed turban shell,
Norrisia, is yet another cold water animal offered
for sale to reef aquarists. As with individuals of the various
Tegula and Margarites species, Norrisia
individuals effectively get cooked slowly to death in reef
aquaria. Norrisia individuals generally die faster
than those of the various Tegula or Margarites
species, however, because they not only get slowly poached,
they also starve. This animal lives
on the giant kelps off the central and southern California
coasts, and grazes on the surface layers of the kelp. It really
is not a diatom feeding animal so much as it is a feeder on
kelp mucus and surface cells. Such food normally is lacking
in reef aquaria, so while Norrisia looks nice in the
aquarium, for a while at least, it has nothing to eat there.
14. Norrisia norrisi, the Red-Footed Turban
or the Red Turban Shell, found off the California coast,
and sold to reef aquarists.
Many of the various Trochoidean snails
are effective grazers and thrive in coral reef aquaria. The
key to finding the right species, however, is in finding species
that are actually tropical, rather than temperate. Almost
all the tropical species do well in reef tanks, while virtually
none of the temperate forms persist. By and large, all of
the species imported for the aquarium hobby feed primarily
on diatoms, and their longevity in aquaria is directly dependant
upon having sufficient food. Although one often sees advice
to add "x number" of these snails per gallon, such
advice generally results in the death of most or all of the
snails, as they generally exhaust their food source and starve
to death. It is better to add them to a tank a few at a time
until there are sufficient grazers in the tank so that at
the end of the day there is a barely visible film of diatoms
on the rocks or aquarium walls. Such a film will be gone by
the morning, and the snails will keep the diatoms in check
henceforth. I will close with an image (Figure 15) of one
cold-water Trochoidean thankfully never offered for sale to
reef aquarists. The animal illustrated is Calliostoma annulatum,
and I think it is one of the most beautiful snails. They reach
about an inch and a half in diameter, and unlike their tropical
cousins they are mostly, if not entirely carnivorous, eating
hydroids and bryozoans.
15. The ringed top shell, Calliostoma annulatum,
found throughout the shallow-waters of the North Pacific.