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Introduction:
The focus of this third column devoted
to grazing snails is on some of the less traditional algae-eating
snails; specifically, the Conchs, the Ceriths, the Cowries
and one species of a group called the Columbellids. Unlike
the trochoideans, but like the animals that I discussed last
month, these animals are not closely related and are from
several distinct and different evolutionary lineages. This
means that they don't look or behave alike. Their basic snail
features, however, are the same as in the trochoideans, and
I refer the reader to May's column for the discussion of anatomy
and physiology and how they relate to the necessity of slow
acclimation to differing conditions, particularly salinity.
As with the other snails, these animals all do best at normal
reef physical conditions where salinity remains in the 35
ppt to 37 ppt range, and the temperature is between 80°F
and 84°F. The basic method of feeding also remains the
same; the animals "lick" the surface of the substrate
with a structure which has been called a "rasping tongue,"
and which biologists call a "radula." The radula
in these snails differs significantly from the rasping organ
found in the trochoideans. For example, it has a lot fewer
teeth; however, the teeth are generally more robust and structurally
specialized, and their basic function remains unchanged.
There are a LOT of different snail species,
and as I mentioned above, the grazers that are the subject
of this column have no similarity of shape. Not only that,
but with most of these useful species, similarly appearing
animals often have decidedly different habits. Not only are
these look-alike species not benign grazers, but instead they
may prey on desirable reef aquarium species. So, it pays to
closely examine the animals that you intend to purchase or
receive from dealers. A few suppliers do a good job of separating
these dangerous species, but I have seen a number of them
surfacing at various places, and this is mostly due to faulty
collection and identification.
As I have mentioned in the previous columns,
it is up to the individual hobbyist to be sure of the identification
of these beasts. I again list the following three references,
which can be very useful in determining which snails have
appeared in our tanks. These bookshave the added advantage
of being found in many local public and university or college
libraries, and so may be readily available for consultation.
These references are:
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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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Ceriths
Individuals of a few species from the immense
group that taxonomists refer to as the Superfamily Cerithiacea
are becoming relatively commonly found in the reef aquarium
hobby. The Cerithiacea consists of several large subgroups,
called "Families." The most common of these are
the Family Planaxidae, which contains the genera Planaxis,
Hinea,
and Quoya; the Family Modulidae, consisting of the
genus Modulus
(1,2)
whose species look quite trochoidean; the Family Batillariidae,
containing the genus Batillaria;
the Family Potamididae containing the genera Rhinocoryne,
Pyrazus, Terebralia,
Telescopium
and Cerithidea;
and the Family Cerithiidae, containing the genera Cerithium
(1,2,3,4,5),
Bittium,
Rhinoclavis
(1,2,3)
Pseudovertagus(1,2),
Clypeomorus,
Gourmya,
Colina,
Trochocerithium, and Campanile. The animals
constituting the species in all of these genera are relatively
similar in basic appearance and are characterized by having
a shell with a small, almost circular, aperture which has
a small siphonal canal or "spout" which projects
at an oblique angle to the long axis of the animal. Additionally,
the shell is typically quite long relative to its width, and
mature animals often have six or more visible whorls. They
typically have a proteinaceous or horny covering, or operculum,
that plugs the shell's aperture when the animal is withdrawn.
In a few species the shell may be smoothly rounded, but typically
the shell has some ridging that is parallel to its long axis
or spiraling around it, or both. The ceriths typically found
in the aquarium hobby generally come from the genus Cerithium,
Clypeomorus, or Batillaria, but really almost
any tropical cerithiacean would do well in aquaria provided
there is an appropriate substrate, and given the non-specific
nature of the collectors in the field it is likely that many
others will eventually show up in reef tanks. Most of these
species are found in or on unconsolidated sediments, although
many others, such as those in the genus Planaxis, are found
on rock. In most cases, they have been shown to be sand-swallowing
organisms. Such animals eat the sediment and digest its organic
material, mostly encrusting bacteria and microalgae. Those
found on hard substrata are generally grazers that feed similarly
to the trochoideans. Individuals from several species are
commonly found in aquaria as part of the sand bed fauna. They
will also venture onto rocks and aquarium walls from time
to time, and use these sites to lay their rather distinctive
egg masses.
For more images of ceriths, see here,
here,
and here.
Conchs
Conchs, or members of the species in the
gastropod group called the Family Strombidae, are strictly
herbivorous. Within this family are several genera, including
Strombus
(1,2,3,4,5)
(true conchs), Lambis
(1,2,3)
(spider conchs), Tibia
(1,2)
(tibias), Varicospina (beak shells) and Terebellum
(terebellum), that differ significantly in shell form and
structure. Their internal anatomies, however, are similar.
Unfortunately, many of these animals, particularly those in
the genus Strombus, have a basic shell shape that most people
think is more representative of the carnivorous whelks. This
type of shell is tapered at both ends with the front end elongated
out into a calcareous spout or siphon. In some cases, ignorant
or malicious dealers have sold some of the predatory whelks
as various conchs. There is, however, a surefire way of discerning
whether you have a real conch. When the animal extends from
its shell, look closely at its eyes. In all of the animals
in this group, the eyes are large and evident, located at
the end of a long stalk, and they possess an evident eyeball
with a visible pupil. They are visually oriented animals and
will watch you. In the predatory whelks the eyes are typically
just a small black dot with no discernable structure and are
located at the base of the tentacles coming off the top of
the head. The conchs have a long and evident proboscis which
they use to bite off chunks of algae. In a very real sense,
they are not rasping grazers but biting eaters; however, they
are superb at eating algae.
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Figure 1. A partially buried Florida fighting
conch, Strombus alatus, showing one eye extending
from the shell aperture.
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All strombids are animals of the sand and
have a small muscular foot which is not adapted to crawling
on rocks. They actually move by lurching or jumping, in what
has been called "saltutory locomotion" by the researchers
who have studied it. Saltutory is a $5.00 word for "jumping."
Never let it be said that invertebrate zoologists would use
a simple word when an odd or complex one was available. This
mode of locomotion is good for sand substrata, but is inefficient
and largely ineffective when moving on rocks, so the snails
tend to stay on the sand once they have reached a shell length
of an inch or so.
A good-sized sand bed is needed to keep
these animals successfully. Generally, as a rule of thumb,
there should be about one to two square feet of open, algae
covered sand per inch of snail shell length. Conchs will move
around the bases of the rocks and remove algae from quite
high up on the rocks with their amazing proboscides. They
are interesting animals, and a valuable addition to a tank
that can support them.
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Figure 2. An eight-inch long Strombus gigas
feeding in one of my experimental tanks. I didn't bother
to clean the diatoms off the walls of this tank, and
this animal fed well upon those algae. Note how the
proboscis can reach some distance away from the animal.
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Some of them, such as the Queen Conch,
Strombus
gigas, grow to quite large sizes. Adult queen conchs
may reach 16 inches in length and are really too large for
any home aquarium. The ones for sale in the aquarium hobby
are aquacultured and are small juvenile individuals, often
about one or two inches long. However, if given enough algae,
they do grow well; I have had one that added an inch of shell
length per month for about a year. In doing so, they will
get too large for a small tank and will then starve to death.
Other smaller conchs are equally good at grazing on algae,
and should be purchased instead of Strombus gigas.
Several of the smaller conchs, such as
the Florida fighting conch, Strombus
alatus, have the habit of disappearing under the sand
for extended periods. In some cases, they actually will move
along under the surface of the sediments, eating algae; in
other cases, they will stay in one place but use their proboscis
to forage in the sediments up to a couple of inches from the
animal. Although they appear to be invisible, they often extend
their eyes to or slightly above the sediment's surface, and
careful observers can find them by searching the sediment
for the watchful snail.
For more images of strombids, see here.
Cowries
Cowries are in the genus Cypraea
(1,2,3).
This is another of the many huge groups of snails. There are
about 500 species described in this genus, and some of them
are quite excellent herbivores. Cowries are easily distinguished
from all other snails. Their shell is basically ovoid, with
the aperture on the bottom. This aperture is a slender opening,
and lined on both sides by calcareous bumps or nodules. The
shells are highly polished and often brightly colored, but
the mantle that comes out and covers the shell when the animal
is moving is often even more brightly colored. Simply put,
they are beautiful animals worthy of investigation to find
some that are appropriate as grazers for our aquaria.
Although seldom seen in the U. S., the
money cowrie, Cypraea
moneta, is both abundant in nature and an excellent
herbivore. A similar species, the ring cowrie, Cypraea
annulus, is likewise exceptionally common in many
shallow reef environments, and is also herbivorous. These
two species are small, about an inch in length, and do very
well in marine aquaria. Tiger Cowries, Cypraea
tigris, are also good herbivores, but they reach lengths
of about four inches and are quite capable of rearranging
unstable aquarium rock work. Many of the other cowrie species
also may be good herbivores, but unfortunately we have little
real data about their diets, and only a few non-herbivorous
or very large species are commonly available. Many of the
few cowries whose diets have been examined feed on sponges,
tunicates, or soft corals, but the diets of most species are
not known. These are beautiful animals, and as our information
base grows, many of them should be acceptable as herbivores
for our systems.
For more images of cowries, see here.
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Figure 3. Two cowries that are good herbivores
for the marine reef aquarium. Both species reach lengths
of about one inch. Left: Ring cowrie, Cypraea
annulus. Right: Cypraea moneta.
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Columbellids
Dove shells, or snails in the family Columbellidae,
are common in all marine environments, and they form another
exceptionally diverse group. Some of the genera in the family
are Amphissa,
Alia,
Euplica,
Strombina,
Anachis, Columbella,
Parametaria, Pyrene,
and Mitrella,
among many others. They tend to be small animals and are nearly
ubiquitous in shallow water marine ecosystems, including reefs.
These are animals that are generally considered to be either
predators or scavengers, but as a whole the group is poorly
studied, and the natural history of most species is simply
unknown. Interestingly enough, at least one species has been
marketed quite successfully as a herbivore, and indeed this
species is a good algae-eating snail. This species, although
sold as the Pacific spotted conch, Strombus
maculatus, is not that species, and actually
is a columbellid snail, probably in the genus Euplica
or Pyrene. The taxonomic status of that particular
genus is uncertain, but in any case, it is a columbellid,
similar to what is illustrated as Pyrene
versicolor (Note: the linked image is reversed, the
aperture should be on the right) by Abbot and Dance, 1982;
p161. This particular species reproduces well in aquaria by
laying capsules on the walls of the aquarium. Successful reproduction
in aquaria is frequent, and these nice little snails are becoming
quite commonly found in marine aquaria throughout the United
States.
For more images of columbellids, see here
and here.
Figure 4. The columbellid offered for sale as
Strombus maculatus. This animal seldom exceeds
one half inch in length. Note the eyes at the base of
the tentacles in the right image. Compare with these
images of actual Strombus
maculatus.
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Conclusion:
The number of snail species sold in the
reef aquarium hobby as algal grazers has increased dramatically
over the last decade. Some of these animals, such as the Norrisia
and Tegula species discussed in the May column, are
unsuitable for reef aquaria and their continued marketing
is really a symbol of the irresponsibility of some dealers
and distributors. Additionally, many of the truly subtropical
or temperate animals sold in the hobby are also inappropriate
for reef aquaria. Unfortunately, these latter species are
still commonly available and probably constitute the majority
of snails sold as grazers.
On the other hand, the marketing of such
true coral-reef animals as Hawaiian Trochus, the money
and ring cowries, several species of conchs, and the small
columbellid species known as "Strombus maculatus"
is quite indicative of either fortunate happenstance or the
application of reasonable natural history information to develop
alternatives to some of the unsuitable animals often sold
as "reef grazers." These true reef animals add a
natural component to our aquaria. They also live
long and prosper in our systems, while providing a stable
means of controlling various types of algae.
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