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Like corals, tridacnids
can bleach for a number of reasons, losing their complement
of energy-producing zooxanthellae, and are susceptible to
attacks by numerous bacteria and other organisms. Of all the
things that can trouble them, however, I think the two problems
that most commonly plague hobbyists' clams are infestations
by parasitic snails and pinched mantle disease. I will provide
in this article some information about both problems, and
discuss the best ways to treat the conditions. Fortunately,
both conditions can be prevented and treated effectively if
you can properly identify the signs or the organisms that
cause them.
Pyramidellid Snails:
A few types of generalist
predatory snails will attack and eat the flesh of tridacnids,
but these are very uncommon in aquariums as far as I am aware.
For example, some snails of the genera Chicoreus and
Cymatium are known tridacnid predators (Govan 1995),
but I don't recall ever seeing one of these snails for sale
at a shop (they're predatory, after all), and have never seen
one in an aquarium even after being in the hobby/business
for almost 15 years. In fact, with all the years I've been
photographing marine invertebrates, I don't even have a photo
of one (but an Internet search for them using the "images"
feature at www.google.com
will work if you'd like to see what they look like).
To the contrary, the parasitic snails that attack tridacnids,
however, are actually quite common and are, thus far, more
troublesome for hobbyists (and clams) than the above mentioned
generalist predators. These parasites are called the pyramidellid
snails, better known as pyrams to hobbyists, and include members
of the genera Pyrgiscus, Turbonilla and Tathrella
(Cumming 1998). These parasitic varieties aren't just common,
they also are very small, difficult to see and can reproduce
prolifically.
Pyramidellid snails are actually much smaller than the generalist
predatory species, and rarely reach a maximum size of around
7mm in length (Cumming 1988, Boglio & Lucas 1997). They're
also lightly colored, and usually hide under a clam's shell
in the petal-like scutes present on some species' shells,
or in the sand/gravel substrate during the day, and thus are
very hard to see. Finding them is easy enough if the tridacnid
specimen isn't attached to the substrate, but far more difficult
when a clam is firmly affixed to a piece of rock that prevents
its removal. If a clam is attached, the best method to find
them is to watch for the snails at night using a flashlight.
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Pyram snails hiding on the underside of a small Tridacna
derasa. The right photo shows a close-up of the
snails.
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Rather than munching on a tridacnid's tissues, these parasites
use a trunk-like snout called a proboscis to puncture the
soft, extendable mantle near the shell's upper edge, and then
feed on the victim's bodily fluids (Cumming 1988). So that's
where you need to look for them if a clam can't be removed.
It's also interesting to note that by some means the snails
can prey on a clam without eliciting a defensive response
from the clam. As such, any effects of their activities may
not be detected until a clam is severely weakened and starts
to gape or bleach. Gaping means its shell is open wider than
normal and its mantle tissue is not extended, or is lying
flaccid within the shell, while bleaching can be seen as a
loss of color due to a loss of zooxanthellae.
Pyram snails can also move from one clam to another if they
choose and, as mentioned, they can reproduce rapidly if left
alone. On tridacnids' shells they can produce numerous small,
gelatinous, egg masses, which are transparent and, again,
difficult to spot (Cumming 1988). Each egg mass can produce
up to a couple hundred baby snails within just a few days
and they are direct developers on their host (Cumming 1988).
As if that's not bad enough, the rate at which they can produce
these egg masses will make you cringe, as a mature snail can
produce one of these egg masses every few days (Cumming 1988)!
The newly-hatched offspring can then feed on a clam and start
to lay their own egg masses in as little as a few weeks (Knop
1996).
In the wild, many of these snails die and/or get eaten by
other reef animals before reaching maturity. In an aquarium
that lacks numerous natural predators, however, many, many
more will survive and live to bear their own offspring. Such
unchecked reproduction can lead to an absolute explosion of
these snails in a tank and any and all clams present can be
killed within a matter of weeks even if only one pair of snails
is inadvertently introduced and begin mating. Larger clams
can deal with these little parasites far better than small
clams can, but as you can imagine, if there are hundreds of
hungry snails in a tank, even a big clam will eventually succumb
to them.
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An easy and effective way to get rid of pyrams is to
break out an old toothbrush and start scrubbing.
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So, what can you do about these snails? The first thing
to mention is quarantine. Keeping clams under close watch
in a quarantine tank allows you to be sure that any snails
and/or egg masses have been eliminated, rather than adding
a new clam to your tank and worrying about the possibility
of two undetected snails going in with it. But, I know a lot
of hobbyists, and I know good and well that many still won't
quarantine clams anyway…
Regardless of whether or not you quarantine a specimen, the
most effective thing you can do to eliminate snails and egg
masses is to vigorously scrub a tridacnid's shell with a toothbrush
or some other brush with firm bristles with care to ensure
that you don't scrub any of the soft tissue that's exposed
on the underside of most tridacnids. Injuries to this tissue
can lead to an infection that can kill a clam faster than
the snails.
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The Sixline wrasse is a natural predator of pyram snails,
which can be added to a reef aquarium in order to help
get rid of the parasites.
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If you scrub a clam but somehow miss some snails or eggs
and they do get into your tank, you can also try adding a
Tail-spot wrasse (Halichoeres melanurus), a Green wrasse
(H. chloropterus), a Sixline wrasse (Pseudocheilinus
hexataenia), a Fourline wrasse (P. tetrataenia)
and/or a Red Coris wrasse (Coris gaimard) (Knop 1996,
Neigut 2005, personal observation), all of which are the snails'
natural predators. Still, this may not be entirely effective
since these fishes tend to feed during the day and may not
reach all the snails in a tank if they are too well hidden
and protected. Thus, it's also a very bad idea to impatiently
skip the scrubbing and quarantine protocols, thinking that
you'll just add a new fish if problems arise later. There's
also the question of incompatibilities with other invertebrates
you might have as these fishes eat more than pyram snails.
Size is an issue, too. Some of these fishes may be acceptable
in size when they're young, but it may not be too wise to
have an adult 40cm wrasse in your tank as the fish matures.
Thus, you'll need to do a little homework on a species that
you might be thinking about adding before actually doing so
(researching them at Reefkeeping
Magazine, wetwebmedia.com
and/or fishbase.org
is a good place to start).
Another effective predator of pyramidellids is the crab,
Thalamita
sima (Cumming 1988), if you can find one. These grow
to only about 2.5cm in size, and they have been known to eat
small tridacnids along with the snails, but generally only
prey on small specimens that are much smaller than what are
typically available in the trade; so, no worries there. Still,
I've never tried one of these crabs as a snail predator, and
don't know of anyone else trying one in a reef aquarium, either.
Thus, there may be some unknown incompatibilities with other
invertebrates, as well.
Regardless of what biological controls you try, you still
should carefully watch any new tridacnid(s) for a period of
several weeks just to be sure that no snails are present.
If possible, it's a good idea to lift a clam up and look at
its underside too, because the snails also like to hide there
more than anywhere else. Start scrubbing if you find anything,
but avoid the soft tissues!
Pinched Mantle Disease:
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An example of pinched mantle.
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Unfortunately, at this time no conclusive
evidence for the cause of pinched mantle disease has been
found. However, tridacnids can be plagued by a number of microorganisms,
and the pinched mantle disease may be the result of an attack
by an unidentified protozoan, according to Barry Neigut, owner
of Clams
Direct. Hopefully, a definitive answer will be found in
the future.
Protozoans are a mixed bunch of single-celled organisms,
and as is the case with bacteria, lots of these organisms
occur in marine environments and can live in/on tridacnids.
It seems that, in some cases, these protozoans that live with
tridacnids are parasites that don't cause any serious trouble
when present in low numbers, but when pinched mantle disease
strikes, the end result is almost always death unless it is
treated in a timely manner.
This condition causes the mantle's smooth, curving edges
to be pinched and contorted. A specimen may look like it is
doing its best to stretch out its thin mantle tissue, but
the margins just won't extend fully the way they should. According
to Neigut, it affects T. crocea most often, with the
other species being more resistant, but not immune. And, it
can spread to other clams at times and is nearly 100% fatal,
usually within a week or two of the first signs. Very bad,
indeed! Fortunately, an easy treatment actually works well:
an affected clam is given a freshwater dip for approximately
30 minutes in freshwater of the same temperature and pH as
the tank from which it came. The only other thing to do is
to thoroughly shake the clam for a few minutes while it is
submerged in the freshwater to make sure that the water makes
its way into all areas of the clam's body and into the spaces
between the mantle tissue and the shell.
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More examples of pinched mantle disease.
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If done correctly, it may take a clam a day or two to show
any signs of recovery and re-extend its mantle, and maybe
a couple more days to regain a completely normal appearance.
This treatment is highly effective, and while it may be stressful,
it shouldn't kill any clams that aren't already seriously
compromised. It might sound hard to believe that a tridacnid
can survive such a long period in freshwater, but you should
note that in their natural habitat many live in very shallow
water and often become exposed to air at low tide and can
end up sitting in the rain for up to several hours before
the tide comes back in. Dick Perrin, owner of Tropicorium,
also told me a story about putting a few clams in a bucket
of freshwater and actually forgetting about them for several
hours, but they all survived, too. The pinched mantle condition
is consistently fatal unless treated, so even if a freshwater
dip is stressful to a clam, you've got nothing to lose by
trying.
The freshwater dip works, but Neigut also experimented with
the medication metronidazole, a drug usually employed to kill
protozoan and bacterial infections in fishes, but apparently
it can also treat clams. However, he did find that the clams
temporarily lost their bright colors for some unknown reason.
The colors reportedly came back eventually, but for a period
of several days the clams looked very dull (but not bleached).
Very strange, but thankfully it's only temporary.
In any case, metronidazole
is readily available to hobbyists, and each product has its
own usage instructions. I don't add any sort of drug directly
to a reef tank unless there's no other option, whatsoever.
However, its use in a quarantine tank, on the other hand,
should pose little problem (other than the possible loss of
color), and I should also mention that any drug should be
inactivated by adding some bleach to the treatment tank after
the treatment is finished, before the water is discarded.
Again, pinched mantle is almost always fatal, so action of
some sort must be taken if you expect a clam to survive, regardless
of possible side effects.
For more information about tridacnids and their care, including
a whole chapter about tridacnid troubles, watch for my new
book, "Giant Clams in the Sea and the Aquarium,"
due in October of 2006.
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