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Introduction:
In last month's column I wrote about the
group of snails generally referred to in the aquarium hobby
as "Turbo Grazers." Commonly called "top"
or "turban" snails these animals are fundamentally
similar in shape and in natural history. In this column, I
will be discussing other algae-eating snails, specifically
the Abalones, Limpets, and Nerites. Unlike the trochoideans,
these animals are not closely related and don't form a nice,
coherent group with common characters. Rather, they are from
several distinct evolutionary lineages. Their generalized
internal anatomy and physiology, however, are the same as
in the trochoideans, and I refer the reader to May's column
if there are questions about the animals' basic needs and
the necessity of slow acclimation to differing conditions,
particularly salinity. Feeding in these animals follows the
basic invertebrate pattern; the animals "lick" the
surface of the substrate with a structure which has been called
a "rasping tongue," and which biologists call a
"radula." On the other hand, while there are similarities
in feeding between all of these various groups, there are
also differences. The number and shapes of radular teeth as
well as, in some cases, their compositions vary between these
groups and are different from the trochoideans as well.
The gastropods, or snails, constitute a
huge array of species; there are well over 40,000 named snail
species and estimates of species numbers range up to 150,000.
The number of actual species may be significantly fewer than
that, but nevertheless that number will still be very large.
The soft parts of snails are largely hidden out of sight inside
their shells, so consequently, snails are distinguished based
on differences in the shapes of their shells. So far, so good;
their identification sounds like it should be easy. However,
the typical snail shell is basically a long slender cone wound
into a helical shape, and that is a simple shape. There are
fundamentally very few variations that can be made in this
structure. This, in turn, means that identification of species
may be very difficult as there are not a lot of differentiating
characteristics that can be seen. Given the number of different
species, it is highly likely that there is a large number
of distinct species with exceedingly similar shell shapes.
Consequently, to identify them requires examining characteristics
that are often tiny, obscure or uncertain. To further complicate
matters, given that the shells are made of calcium carbonate,
the identifying characters may simply be worn away or used
as a substrate by other encrusting forms inextricably attached
to the shell. Additionally, even with shells in the best of
condition, it may be impossible to distinguish some species
for the simple reason that their shells are identical. In
one snail group I worked with, one researcher described two
species, one with a radula and one without, but the shells
of the two species were about as close to being identical
as they could be (Smith, 1967). For these reasons, I generally
do not recommend that aquarists try to identify snails to
the species level; it is sometimes possible, but for a great
many species, it is simply not worth the effort.
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Figure 1. An abalone, Haliotis kamtschatkana,
grazing on some algae. Note the tentacles protruding
from the holes in the shells. The head is to the left.
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The trochoideans that were the subject
of the May column all have a relatively similar appearance.
In fact, they are so much alike that most aquarists have problems
telling them apart. This ambiguity of identification is an
advantage in that it allows collectors to provide us with
usable animals without having to be very discerning in their
collection. In the aquarium trade, the terms Turbo,
Trochus, and Astraea (generally misspelled "Astrea")
are all applied to any species of those genera. In other words,
if you want to know what Trochacean you have, you are pretty
much on your own for identification, because many common names
used by dealers are unreliable to the point of uselessness.
This is largely immaterial, however, as the animals do tend
to have similar requirements and attributes.
Unfortunately, the grazers that are the
subject of this column have no such similarity of shape. While
the consumption of various types of algae is widespread in
the gastropods, the number of those species that are suitable
for reef aquaria is really surprisingly small, and with few
exceptions there are few similarities in shape to help distinguish
them. It is up to the individual hobbyist to be sure of the
identification of these beasts. Three references may be of
significant help in this regard. As aquarists we can, in this
case, benefit from the hobby of shell collecting. Shell collectors
greatly outnumber reef aquarists and their hobby supports
the publication of many shell identification guides. The following
three references, while not written with the aquarium hobbyist
in mind, will help with the identification of many mollusks
including the snails. They have the added advantage of being
in many local public and university or college libraries,
and so may be readily available for consultation.
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Identification References:
Abbott, R.
T. 1974. American Seashells. Van Nostrand Reinhold
Company. New York. 663 pp. This reference will allow
you to make tentative identifications of about 30% to
50% of the Caribbean reef snails.
Abbott, R.
T. and S. P. Dance. 1982. Compendium of Sea Shells,
A Color Guide to More than 4,200 of the World's Marine
Shells. E. P. Dutton, Inc. New York. 410 pp. This
reference is good for identifying snails from marine
environments around the world, but is missing a lot
of, mostly smaller, species.
Keen, A. M.
1971. Sea shells of tropical west America. Stanford
University Press. Palo Alto, Ca. 1064 pp. This reference
is excellent for Eastern Pacific and Gulf of California
animals, and is good for some other tropical Pacific
animals.
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Abalone
Abalones, or snails in the genus, Haliotis,
are some of the classic grazing snails. Unfortunately, they
tend to be too large for reef aquaria as some of them reach
diameters of eight inches (20 cm) or more. When viewed in
profile from the side, abalones are basically wedge-shaped
with the sharp point of the wedge at the front end. Many of
them are the preferred prey of some very diligent and persistent
visually-oriented predators such as fishes, birds and sea
mammals, so the snails have been forced to evolve the behavior
of hiding from their predators in cracks and crevices where
they are difficult to see and even more difficult to remove.
Practically speaking, this means that in aquaria, they often
tend to push their wedge-shaped shell under rocks or between
the larger pieces of rock. With their large broad feet and
tough shells they can exert a significant amount of force
on the rocks. In nature, where the rocks are part of Mother
Earth and don't move, this behavior ensures that the animal
is well protected. In an aquarium, such behavior may significantly
rearrange a tank. A four or five inch long abalone can easily
move and shift rocks weighing twenty pounds (9 kg) or more.
Few aquaria can withstand this sort of perturbation, and as
a consequence not many aquarists want to deal with abalones.
We are fortunate, however, as not all abalones
will cause this rearrangement. One of the best types of grazing
snails for cleaning diatoms and other adherent algae from
smooth surfaces, such as aquarium walls, is a species of tropical
abalone, Haliotis
asinina, commonly called "the Ass's ear abalone."
The common name comes from its elongate shape, and to someone
who may have never seen a donkey's ear, it probably looks
like one. Abalone shells are coiled, but they don't really
look like it as the coil expands so rapidly that the shell
looks almost like the bowl of a spoon, or an ear. In fact,
the scientific name, Haliotis, means "sea ear."
Viewed from the top, the shell can be seen to be in the form
of a rapidly expanding coil, with the more pronounced whorls
being located at the animal's stern. Additionally, abalone
shells may be positively identified by the series of small
holes running in a line near the left edge; no other flattened
type of snail has such a row of holes. When the animal is
grazing, a sensory tentacle will often be seen extending from
one or more of these holes. The shell color is typically mottled
greens and reds to yellows; good camouflage coloration for
a reef animal. Much of this variety of color is the coloration
of the shell proper, but a lot of it is due to the growth
of various algae on the shell. Haliotis asinina reach
a maximum size of about four inches (10 cm), but grow slowly
in our systems. The ones available for the aquarium trade
are generally from aquacultured
stock and are often about one to 1.0 - 1.5 inches (2 -
3 cm) long. One specific caution is necessary for their care:
they need to be acclimated very slowly to salinity changes,
and it is best to err on the side of caution. As with other
snails, they use their radula
(scroll down on the linked site) to graze. Generally, they
prefer to graze on glass, and often will not go on the rocks
at all. They are nocturnally active and often will seek a
dark space in which to pass the brightly lit hours. Often,
they will return to the same "home" space for many
months.
Limpets
The generalized limpet body form is that
of a snail bearing a conical, uncoiled cap-shaped shell and
possessing a broad foot. This type of morphology is found
in a number of snail groups that are only distantly related.
Consequently, it is difficult to generalize about or predict
the behavior of any particular limpet species. The limpet
shape is well adapted to withstand wave stress and the pounding
surf, and most limpets are animals of the intertidal regions.
Intertidal tropical limpets are common, and are quite good
grazers, but they are not good reef tank inhabitants as they
tend to climb out of the tank. Often they climb up and out
of the tank and then fasten themselves down, presumably to
wait for high tide; which, of course, never comes. This results
in the death of the limpet and the concurrent creation of
limpet jerky.
There are a number of types of subtidal
limpets occasionally available for the coral reef aquarium.
Generally, they are not particularly good animals to have
in aquaria. Those shallow water forms that graze on algae
seem to have a decided tendency to eat coralline algae. These
limpets are exceptionally well equipped to eat these algae.
They have a radula
with teeth made of a mixture of iron salts (primarily hematite)
and silica (in the form of opal). This gives them a rasping
organ with a value of 7 or 8 on the Mohs scale of mineral
hardness. Additionally, the opal is deposited in the teeth
as small inclusions that tend to abrade away slightly faster
than does the hematite. This turns the tooth into a self-sharpening
rasp; the more it is used, the sharper it becomes until the
tooth abrades away completely and another replaces it. The
limpets possessing such a rasping apparatus can cut through
and remove coralline algae like it was butter, and some of
them can do the same to acrylic aquarium walls. Unlike sea
urchins, which typically eat a small patch of algae and move
off some distance until they feed again at some later time,
the limpets
are pretty much constant grazers. There are some other
species of small limpets, occasionally available from some
vendors, which are relatively benign and good grazers on microalgae.
Unfortunately, these species are seldom offered for sale even
though at least some of them appear to reproduce in captivity.
It is essentially impossible for a novice to distinguish between
these species as some of the identifying characteristics are
on the internal surface of the shell, so one has to have a
dead shell to examine to determine the identity of the animals.
Unless the vendor can vouch for the dietary preferences of
the animal that he is selling, it might be best to pass by
limpets on the way to the checkout counter.
Additionally, there are some limpets that
generally do not graze on algae. The ones that we are primarily
concerned about are the keyhole
and slit limpets. These are limpet-shaped snails with a perforation
on the top of the shell (keyhole limpets) or a slit on the
front shell margins (slit limpets). These animals are generally
carnivorous, and will eat sponges, soft corals, and other
sessile animals. I have not heard of any specific reports
of them eating stony corals, but I suspect that they would.
Some of them, however, may be very useful for the control
of some low growing colonial nuisance hydroids. Unfortunately,
little work has investigated the specific diets of tropical
keyhole limpets, so we don't know the names of any beneficial
species. Consequently, unless you are willing to put up with
some predation on coralline algae or some of the animals in
your system, I would leave most limpets to the sea.
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Figure 2. A keyhole limpet, Diodora aspera.
The fuzz on the limpet shell is comprised of hydrozoans,
and they are probably as safe as they can be anywhere
in the vicinity of the limpet since it can't feed on
them up there.
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Two types of limpets that appear to be
not only beneficial, but attractive, are ones that really
don't look like limpets. These are the shield limpets and
fleshy limpets. When fully active neither of these types of
animals has much of a resemblance to the common limpets, but
both are related to them. Both of them have a large fleshy
structure, the mantle, which extends up over the shell and
largely obscures it; in essence the animal looks like some
kind of slug or nudibranch.
Shield limpets, Scutus
unguis, are commonly collected on Indo-Pacific live
rock. They may reach lengths of an inch or a bit larger (up
to about 3 cm). Most of the size is due to the fleshy black
mantle that covers the shell. These animals are nocturnal
and will hide under and between rocks during the day. At night
they come out to graze on microalgae such as diatoms on the
rocks. They appear to be blackish blobs, although the white
shell can often be seen peeking through the folds of the mantle
covering the animal. If disturbed so that the animal retracts
the mantle, the ordinarily appearing limpet shell
will be seen attached to a rather large body. These are good
grazers, and are quite beneficial animals to have in a reef
tank. Unfortunately, they don't seem to reproduce in reef
tanks, and generally don't seem to persist for more than a
few months.
Fleshy limpets, Lucapina
species, are commonly found on Caribbean rock, and there are
several species of them. The largest are about the size of
shield limpets, but many are smaller. They also have a mantle
that extends up over the shell, but in this case, the mantle
is brightly colored and ranges from yellow to red. These
are keyhole limpets, and as such are probably omnivorous,
but from the reports I have received, they don't seem to be
eating the decorative livestock. Generally, they seem to be
grazing on algae. Like the shield limpets, they are largely
nocturnal and are generally out of sight and inactive during
the day. When the mantle is retracted, they will be seen to
have a small shell on their back and its center will be perforated
with a hole. As with the shield limpets, they don't appear
to reproduce in aquaria and generally only survive a few months.
Nerites
Numerous
species belonging to the genus Nerita
make good herbivores for many reef tanks. These snails are
recognized by the rounded shell which, although it is coiled,
has a low spire. The aperture from which the body extends
is basically "D-shaped" with several large calcareous
bumps on the inner edge. The outer edge is also often marked
by similar calcareous bumps. I have seen at least four species
of Nerita in reef tanks, and there are undoubtedly
more than that. They tend to be dark; browns and blacks are
the predominant colors, but there are several species that
are white with rich brown markings.
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Figure 3. Diagram of a Nerite shell showing the
aperture and some of the diagnostic characters used
in identification.
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Some of the ones collected for sale in
reef tanks are really intertidal marsh animals and have no
business in a reef tank. I am not the only one who thinks
so, by the way. The snails agree with me, and vote with their
feet, moving out of the tank and into the wilds of the adjacent
rooms. These animals will live in the tank, but seem to have
a physiological need to move above the water line. At least
in many cases, unlike some of the limpets, they also move
back down to the water. Unfortunately, there is no good way
for a hobbyist to determine if the animals offered for sale
are intertidal or subtidal as the shells are quite similar.
There are several other Nerita species
that don't have the vertical wanderlust, and these make good
grazers on the glass and elsewhere. These animals seldom harm
other species in our systems, although they may occasionally
bulldoze some unattached structures around. Nerita
females lay egg masses frequently, but the larvae seldom pass
through the larval stage and juveniles are seldom seen in
our systems. The largest nerites in our systems are about
2 cm (0.75 in) across, and the smallest are adult at 1-2 mm
(1/12th to 1/25th of an inch) across. Well-fed Nerites lay
eggs almost continuously on the aquarium rocks and walls.
The white eggs are enclosed in a protective covering and develop
from egg to larvae within it for about a week or so. The egg
capsule then opens and the larvae are released into the tank's
water. This feeding larval stage is prolonged and it is unlikely
any larvae will survive to settle and metamorphose in a reef
aquarium.
Conclusion:
With the exception of the Nerita,
the snails that I have written about in this column are not
often seen in reef aquaria. Haliotis asinina are fine
grazers, as well as attractive animals, and even though they
are aquacultured and have been marketed to hobbyists, their
availability is spotty. Relatively few species of limpets
suitable for the hobby are available and not many vendors
have them. The shield and fleshy limpets are neat additions
to reef tanks, but I know of no vendor that specifically and
consistently offers them for sale. Next month, I will conclude
this short series on the shelled snails that eat algae with
some information on some animals that are readily available:
ceriths, strombids, cowries, columbellids, and some bubble
shells.
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