I
decided to continue on with the subject of sea cucumbers for this
article, because there are many subjects that I mentioned but never
really discussed at any length in the last article about medusa
worms (Toonen 2002).In
fact, I am going to do one more article on sea cucumber after this one,
but I’ll focus specifically on the colorful and popular sea apples (Pseudocholchirus
spp.) in that next article.
So,
let’s start off by backing up a step from my introduction
last time and actually explaining what a sea cucumber is.Well, sea cucumbers are members of one of the most
familiar groups of marine invertebrates, the Phylum
Echinodermata.I say that echinoderms are one of the most familiar
groups of marine invertebrates because some members, in
particular the sea stars (Class Asteroidea), have become
common symbols of life in the sea throughout popular culture.In contrast to the sea stars, their less attractive and
well-known cousins, the sea cucumbers (Class Holothuroidea)
are largely unknown in popular culture.There are roughly 6000 known species of Echinoderms
alive today, and approximately 900 of those described species
are sea cucumbers (Ruppert and Barnes 1994).The Class Holothuroidea is further divided into three
main groups, although only the first two are commonly sold in
the pet trade: the Dendrochirotacea (or filter-feeding sea
cucumbers), the Aspidochirotiacea (or deposit feeding sea
cucumbers), and the apodacea (or medusa worms).The medusa worms were the subject of my last column
(linked above), and will therefore not be covered again here.Instead I will discuss the other two groups of sea
cucumbers that are more common in the trade, but focus
primarily on the deposit feeding Aspidochirotiacea in this
article.
Sea
cucumbers can vary dramatically in size, with some of the
smallest animals reaching a maximum size of less than a 2 cm
long, and the largest can reach more than a meter (3 ft) in
length and be nearly 1ft in diameter (24cm)!A meter long cucumber is a big animal, and obviously
anything that reaches more than 3 feet in length is not very
practical for a miniature reef aquarium at home.Regardless of whether the animals look it or not, there
is a ‘belly’ (ventral side) and a ‘back’ (dorsal side)
to a sea cucumber, and the animals will always attach with the
‘belly’ side toward the substrate: if you flip a sea
cucumber onto the side opposite that with which it is
attached, it will right itself before reattaching.This is easier to see in some species than others, but
for the most part sea cucumbers are motile enough that even if
they are placed into an aquarium upside down, they will
quickly right themselves and find a comfortable location to
feed.
A close-up of the broad,
palmate feeding tentacles of this Holothuria sea
cucumber. The cucumber mops the surface on
which it is feeding with its mucus-covered tentacles
before drawing them back, one at a time, into the
mouth to be sucked clean of any particles. The
larger a cucumber is, the more food it will need, and
these animals are not generally suitable for very
small (< 20 gallon) reef tanks. A reasonable
rule-of-thumb is to stock no more than ~3" of
cucumber per 20 gallons of aquarium to avoid
starvation.
Photo: Greg Schiemer
A
comfortable position in which to feed will vary depending on the species
of sea cucumber you are planning to add to your tank.Although there is an enormous amount of variation within the
broad categories of deposit-feeding and suspension-feeding sea
cucumbers, most of the animals that are commonly imported for sale will
generally perform a similar function in the aquarium.So, with the full acknowledgment that there are many exceptions
to the rule that are not commonly imported at this time, I will treat
each of these groups on the whole in this article and discuss some of
the more common species in detail later on.I will discuss some of the specifics of suspension-feeding
cucumber husbandry in the next article in which I go into detail about
the captive care of sea apples.In this article, I will instead concentrate on some generalities
about deposit-feeding sea cucumbers and end with a discussion of what
risk a sea cucumber poses to a reef aquarium.One of the most common deposit-feeding sea cucumbers found in the
pet shops these days are the ‘tiger-tail cucumbers.’This is one of the many cases in the pet trade where a well-known
common name is applied to a wide variety of species, many of which
don’t act or even look similar.In this case, however, most of the cucumbers that I have seen
offered for sale under the name ‘tiger-tail’ fall into the same
general life history mode and will actually perform a similar function
in the aquarium: they are generalist detritivores that mop the sediment
surface for fine organic detritus (Erhardt and Moosleitner 1998).The majority of the species of deposit-feeding sea cucumbers that
are offered for sale in the petshops are usually members of the genus Holothuria.Perhaps the most commonly offered for sale under the name
tiger-tail are H. hilla from the Red Sea and Indo-Pacific, and H.
impatiens or H. thomasi from the Caribbean.Other commonly sold species include H. floridana (a
mottled and highly variable-colored Caribbean cucumber that is common
along the Gulf shores southward along the coast of Central America as
far as Colombia), H. atra (a black cucumber from the Read
Sea and Indo-Pacific), and H. edulis (the edible sea cucumber
which is black along the upper surface, and bright pink along the belly,
also from the Red Sea and Indo-Pacific).Each of these species performs a similar function in the
miniature reef aquarium: they consume organic detritus and ingest
fine-grained sands to digest off the bacteria, microalgae and diatoms
that cover the surface of each sand particle.In this way, they help to maintain a cleaner and more attractive
aquarium, which makes them a useful addition to your aquarium.Because these animals feed on the same food, however, it is
important that they not be overstocked in an aquarium, or they will
rapidly consume all the available food and then starve.Despite the fact that they don’t look like much, sea cucumbers
are relatively large animals, and they generally require a lot of
organically enriched fine detritus to survive.In the absence of sufficient food, they will of course starve to
death in an aquarium.
Feeding
and starvation in sea cucumbers
Like
most marine invertebrates, sea cucumbers are capable of going for long
periods of time without feeding before they show any outward signs of a
problem.Even when they do
show obvious signs of starvation, most people do not know what to look
for in these unusual animals, and will likely miss the warning signs.Because sea cucumbers are able to slowly digest themselves to
cope with periods of starvation, they are often able to last for many
months without any outward signs of starvation.The first time that people notice something is probably wrong
with their cucumber is usually when they realize that the cucumber is
much smaller than it was when it was first purchased.Slowly shrinking is a sure sign that the animal is not getting
enough food, and is bound to waste away in your tank if they do not get
more food.Deposit-feeding
sea cucumbers can be either selective or not.The non-selective species that make the best detritivores for the
aquarium feed primarily by eating sand ormud and digesting away the organic component before pooping out
perfectly clean sand castings (often looking almost like a spaghetti
noodle) behind them.If you
do not see these sort of castings from your sea cucumbers on a regular
basis, chances are that they are not getting enough food.
Holothuria
feeding
on the surface of a sandbed in the aquarium. The
cucumber feeds by mopping the surface of the sandbed
with its palmate feeding tentacles and lifting any
particles which are small enough to easily ingest.
The animal then digests the organic detritus and any
surface film (such as bacteria, diatoms, benthic
microalgae and cyanobacteria) from the surface of the
sand and help to keep the sediment surface bright and
clear of organic buildup. Photo: G Shiemer
There
are three common reasons why a sea cucumber may not get
sufficient food in captivity.The first is quite simply overstocking.As I mentioned above, these animals need a lot of organic
detritus, and an average reef tank can only support one or two
sea cucumbers. Sprung (2001) recommends a rule-of-thumb of three
inches of sea cucumber per 20 gallons of reef aquarium to
prevent overstocking, which seems like a reasonable level for a
well-established and stocked aquarium, but is probably still a
bit heavy for a sparse or newly setup reef tank.A second problem comes from placing sea cucumbers into
tanks with inappropriately-sized sediments.The deposit-feeding sea cucumbers seen in the hobby
typically come from areas of fine sands (such as mangrove swamps
or seagrass beds) rather than areas in which gravel predominates
(Erhardt and Moosleitner 1998; Hendler et al. 1995).Although the Caribbean tiger-tail cucumber (H. thomasi)
is capable of ingesting particles as large as 2mm in diameter
when fully-grown, they are restricted to much smaller particles
when they are small (Erhardt and Moosleitner 1998), and few
people have anything close to a fully-grown cucumber in their
tanks.Because
these animals often feed by simply ingesting everything of the
appropriate size and digesting what they can from those
particles, placing a small sea cucumber into a tank with coarse
gravel (the size of crushed coral rather than sand) often
prevents them from feeding appropriately.Thus, if your gravel is too large (if it can be called
gravel rather than sand), or your tank is too clean (new,
frequently vacuumed or bare bottom), a sea cucumber is almost
certain to be doomed in your aquarium.I generally suggest that people not add a sea cucumber
unless you have a well established tank – or at least lots of
organic detritus -- with sugar-sized or finer sand.The final reason is one which you as an aquarist have
little control of - the animal may not have a gut, and
regardless of how well-suited your tank is, a cucumber without a
gut will not be able to feed.
Some
of you may be asking yourself ‘how can the animal not have a
gut!?’Well, violently expelling part or all of the gut (evisceration)
is one of the defensive mechanisms of sea cucumbers (I’ll come
back to this in more detail in the section below), and if they are
not handled with great care during the collection and shipping
process, there is a good chance that the cucumber may have ejected
its gut in response to that stress.Evisceration is a common response among the sea cucumbers
to severe stress (Ruppert and Barnes 1994), and I suspect that
most tropical sea cucumbers are likely to experience some stress
in the course of being collected and shipped around the world.For example, during the time that I did research on sea
cucumbers (Parastichopus)in the Pacific Northwest,
I often found that after collecting many individuals on a dive,
the entire group would eviscerate when I brought them to shore to
be weighed and measured.This was a result of simply picking the animals up,
sticking them into a bag underwater, and then bringing them to
shore.If the tropical species that are imported for our reef
tanks behave anything like these animals, then it would not be at
all surprising if many of the sea cucumbers that slowly shrink and
eventually die in the aquarium despite the best efforts of the
aquarist have been roughly handled at some point and have
eviscerated as a result.Given the repeated insults of being collected, shipped and
placed into aquarium after aquarium during that time before
finally being added to a reef tank thousands of miles from where
the animal was collected, it would be surprising to me if the
animals did not experience extreme stress during that
time...
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However,
even if the animals have an intact gut, are housed in a tank that
is not overstocked, and has great water conditions and
sufficiently fine sands that the animal can feed, some of them
will still starve.This
may be because some species are actually specific feeders, and
although they may ingest foods, they may not be able to digest
them.For example,
the cold-water sea cucumber that I studied, Parastichopus
parvimensis, ingests a wide range of organic detritus but it
turns out that the plant components of that detritus remain
undigested even after passing through the animal’s gut (Ruppert
and Barnes 1994).Obviously, if you were to add a species such as this to your
aquarium, and the primary source of organic detritus in your tank
was from various micro- and macroalgae, then this cucumber would
not fare particularly well.Fortunately
for us as aquarists, the majority of species that are commonly
sold in the trade are relatively non-selective, and prefer to feed
on any nutrient-rich particles (Erhardt and Moosleitner 1998;
Hendler et al. 1995).
Thus,
for the most common species of deposit-feeding cucumbers in the
trade, the primary concerns are: 1) avoid overstocking, 2) provide
properly-sized sediments, and 3) try to obtain a healthy animal
when you purchase one...
Sea
cucumber defense responses
I would not be doing
anyone a favor if I did not point out that most popular aquarium
texts have dire warnings about the dangers of including one of
these animals in your reef tank (e.g., Delbeek and Sprung 1994a;
Delbeek and Sprung 1994b; Fenner 1998; Paletta 1999; Tullock
1997).So why is it that these animals are so feared by many
aquarists?Well,
the simple answer is that when they are injured in the aquarium,
some of these species are capable of releasing a toxic chemical
soup that can poison an aquarium (reviewed by Paul 1992; Pawlik
1993).The
possibility of having one of these animals poison an aquarium
(commonly called a ‘cuke nuke’ on the internet) is enough
for many people to shy away from ever adding one of these
animals to their aquarium.However, I think that this fear is somewhat misplaced.I’d like to take a bit of time to explain the chemical
defenses of sea cucumbers in some detail and hopefully dispel
some of the myths surrounding these animals.
Let’s
start at the beginning: when a sea cucumber is stressed, it can
react in a variety of ways. First, like other echinoderms, they
have a compound in their skin called catch collagen -
this tissue is under neurological control and is capable of
changing from a 'liquid' to a 'solid' form very quickly (Brusca
and Brusca 1990; Motokawa 1984a; Motokawa 1984b; Ruppert and
Barnes 1994). This is one of the coolest things about
echinoderms in general, and is one of the reasons that this
group has been so successful.The ability of the catch collagen to change from liquid
to solid form at will is how sea cucumbers manage to get
themselves into such tiny holes in the live rock structure -
they are able to ‘goopify’ their bodies (for lack of a
better description), literally pour themselves into the hole
they have chosen, and then solidify their skin to prevent
anything from being able to remove them (Motokawa 1984a;
Motokawa 1984b). The same is true of how urchins move and
‘lock’ their spines, or how sea stars can exert continuous
pressure on a clam to slowly pry it open without getting tired,
and so on. Sorry, I guess I’m getting off track...OK, so as I just said, the cucumber can change its
consistency, and many react to stress by either becoming flaccid
and goopy, or by ejecting all the water from its system and
becoming a small, hard turd-like lump (at least that seems to be
the description I hear most often from the spouses of aquarists
who own a sea cucumber). Either of these responses is typical of
animals that have been moved between tanks or harmlessly
disturbed (e.g., poked with a finger, nipped by a fish or such).
This
close-up shows the sucker-tipped legs that echinoderms
use to move around, and also includes an attractive
little commensal shrimp that lives among the tube feet
of this sea cucumber. Holothurians host a number
of fascinating commensals including shrimp, crabs, worms
and even fish, which makes them even more interesting
additions to the aquarium. Photo: G Schiemer
This
is a pile of cleaned sand recently left by a sea
cucumber such as H. atra or T ananas.
After these nonselective detritivores ingest particles
that are small enough to be swallowed, they digest any
organic detritus collected along with the surface film
of the sand particles. These piles quickly fall
apart and in this way, the sea cucumbers continually
turn over the sediment surface while simultaneously
providing the service of being a living gravel vacuum.
Photo: J Sprung
A
second and more drastic response is evisceration - the
violent expulsion of the gut I mentioned above.In this case, the cuke basically expels a portion of its
digestive system (guts) onto the substrate.It may be the fore or hind gut depending on the species
in question, and may or may not be a serious trauma to the
cucumber (Ruppert and Barnes 1994).Evisceration can be induced in a variety of ways
(including such factors as chemical stress, physical
manipulation, crowding, etc.), and in some species, every
individual in the population appears to be at the same stage of
regeneration, suggesting that evisceration may be a normal
seasonal phenomenon in some species (Ruppert and Barnes 1994).It is unclear why individuals go through the
evisceration, but possible explanations include periods of
inactivity when food is naturally rare, or the elimination of
toxic wastes that have accumulated in the internal tissues (Ruppert
and Barnes 1994).Whatever the cause, this ‘puking your guts out’
response usually includes some or all of the digestive system
(and in some cases other organs such as the respiratory tree and
gonads), but is not necessarily accompanied by chemical
discharge (Brusca and Brusca 1990; Ruppert and Barnes 1994).Because there is not necessarily any chemical discharge
that accompanies evisceration, even a highly stressed sea
cucumber that ejects its intestines may not have much of an
impact on your aquarium, depending on the situation. Despite the
fact that this stress response may not wipe out your tank, it is
certainly not trivial to the sea cucumber – the cuke loses its
digestive capacity in the process and although it can regenerate
the gut, it needs time, rest, and excellent water conditions to
do so.If the cucumber was stressed enough to eviscerate in your
aquarium in the first place, chances are slim that conditions
are ideal for them to regenerate their gut, either.
The
final and most drastic response possible is when a stressed cuke
expels its Cuvierian tubules.These tubules are a series of long, spaghetti-like tubes
which are associated with the hind gut of certain sea cucumbers,
and are thought to be primarily defensive in function.Not all species possess these defensive structures, and
even those that do, generally do not eject them without dire
provocation. The Cuvierian tubules are located near the anus,
and branch off from the base of the respiratory tree (the
branched ‘gills’ of a sea cucumber).Now, if you’re reading this carefully, you should be
wondering why the ‘gills’ of a sea cucumber are so near to
its butt.Well, the answer is simple, although unusual – sea
cucumbers actually breathe through their anus!That’s right – many people make the mistake of
watching a sea cucumber breathe (it is quite obvious as the anus
opens to allow water to flow in and then pinches down as the
animal ‘exhales’ the water it just ‘inhaled’) and
thinking that the opening that they are watching is the mouth.In cucumbers however, respiration is done through the
anus, and the respiratory tree is associated with the other end
of the digestive system.Why does any of this matter?Because when a cucumber is really threatened (it
thinks it is about to be eaten), it can respond by inhaling a
bunch of water and physically rupturing (literally exploding)
the hind gut to expel these tubules and a soup of defensive
chemicals that are intended to prevent the predator from ever
wanting to mess with a sea cucumber again.The amount of defensive chemistry and the specific
combination of chemicals that are produced varies by species
from just under two to nearly three and one-half percent of the
dry weight of the cucumber (Bryan et al. 1997).In general, the chemicals which appear to protect the sea
cucumbers from being eaten are saponins (soap-like compounds)
which are derived from triterpenoids (Ponomarenko et al. 2001;
Stonik and Elyakov 1988).These chemicals usually work well to discourage many
generalist predators, and unfortunately are also likely to
seriously impact, and potentially even wipe out all their
tankmates in a reef aquarium (the so-called ‘cuke-nuke’).
Holothuria edulis
is another species that tends to be more common in the
food market as trepang than in the aquarium market,
although this species is seen far more commonly for sale
in North America than T. ananas. H.
edulis lives openly on the sediment surface in sandy
regions and sea grass beds throughout the Indo-Pacific,
where it grazes on organic detritus and microorganisms
from the surface of fine sediments. H. edulis does
not tend to thrive in captivity unless housed in very
large (> 100g) reef tanks.
Photo: J Sprung
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Of
course the amount of this chemical soup and the exact identity and
toxicity of the chemicals a cucumber has vary from species to
species, which makes it hard to make reliable generalizations
about which cucumbers are safe for a reef tank and which are a
potential nightmare.As
for being poisonous, I would have to say that there is really no
such thing as a completely non-toxic sea cucumber, because nearly
every species that has been tested to date appears to possess
chemical defenses against at least some species of predators
(e.g., Bryan et al. 1997, reviewed by Paul 1992; Pawlik 1993).However, having said that, the defensive chemicals not only
differ among species of sea cucumbers, but the same set of
defensive chemicals also tends to have very different effects on
different species.For example, Bryan et al. (1997) found that incorporating the
defensive chemicals from the body wall of three different sea
cucumbers (Holothuria lentigenosa, H. thomasi & Isostichopus
badionotous) into food pellets led to all of them being spit
out by both pinfish (Lagodon rhomboides) and killifish (Cyprinodon
variegatus).However,
the exact same suite of chemicals incorporated into food pellets
offered to the arrow crab (Stenorhyncus seticornis) had no
detectable effect on the crabs appetite.Thus, there is variability both among species of sea
cucumbers in the exact chemicals that they use to defend
themselves, and the effect of each of those chemicals on potential
tankmates (e.g., Paul 1992; Pawlik 1993).
So
which cucumbers are safe for a reef tank?
Well,
to recap what we’ve learned so far, different species tend to
have different specific sets of chemical defenses, and those
chemicals tend to have different effects on different potential
predators.So,
obviously it is virtually impossible to make any real
generalizations about the specific toxicity of a given species of
sea cucumber or how their chemical defense is likely to impact
your reef tank.In
general, it is probably reasonable to say that sand-feeding turd
cukes are less toxic than the colorful filter feeders, or more
specifically, the concentrations and variety of toxic chemicals
found in the Aspidochirotiacea – the group that contains the
sand-feeding cucumbers – is generally lower than those of the
Dendrochirotacea – the group that contains most of the colorful
suspension feeding cucumbers (Paul 1992; Pawlik 1993).However, there are plenty of exceptions to that
generalization, and the fact that there is a strong behavioral
component makes it impossible to predict the effect of any given
cucumber in any given tank when something particular happens.Ultimately, the real question that most likely matters to
you is not which sea cucumbers are more or less toxic, but rather
how much of a risk is some particular sea cucumber to the rest of
the critters in your tank.
The
dreaded ‘Cuke Nuke’
Well,
as I’ve said now several times, there is simply too much variability
for me to make any realistic generalizations about the risk of any
specific sea cucumber.The
best generalization that I can make is that for most drab
deposit-feeding sand cucumbers the risk of a tank wipe-out is quite
low (although there are certainly some drab cucumbers, such as H.
atra, that possess particularly nasty toxins).In fact, even with the most toxic species of sea cucumbers, the
risk of having a serious episode in your reef aquarium is relatively
low as long as you take certain precautions to prevent the animal
meeting an unpleasant end.Why
is that?Well, as I
explained above, the sea cucumbers do not release their defensive
chemicals on a whim - they must be pretty threatened or stressed to
react in that manner.If
you take responsibility for the safety of your animals, and properly
cucumber-proof your tank, then there is no reason that any cucumber
should ever be in a sufficiently stressful situation that it would
release those defensive chemicals.Furthermore, cucumber-proofing your tank is relatively simply,
and we have no excuse for not doing so.For example, properly protecting pump intakes and overflows
before adding any crawling animal (such as a sea cucumber) seems like
a pretty reasonable request for our tanks, and in my opinion this
should never be viewed as a downside to keeping such animals.I would argue that it is simply our responsibility to provide
an adequate home for any animal that we purchase.The same precautions ought to be taken prior to adding an
anemone, snails or any other animals that crawl actively around the
aquarium and could blunder into the intake of an unprotected pump.If you plan to keep motile animals in your tank, it is your
responsibility to take adequate precautions that they cannot wander
into what amounts to a blender for them.
While
I stick by the assertion that equivalent precautions ought to
be taken with any motile animal, the fact that most sea
cucumbers possess toxins which could impact an entire tank
makes this concern that much more important with these
animals.A good strainer will prevent most animals from being sucked
into the intake of a pump, powerhead or overflow.What many aquarists find is that the problem with many
strainers is that they are either poorly designed (i.e., they
are too small, or they break), or they are difficult to keep
where we want them (i.e., they fall off or are dislodged by
the activity of the animals themselves).Ideally a strainer should be large enough to diffuse the flow
sufficiently that you cannot feel any suction at all, and it
should also have holes small enough to protect the animal that
you're trying to keep out of it, but at the same time, the
holes must be large enough that they do not become plugged.The strainer must also be able to be securely attached,
so that it cannot be knocked off accidentally by you or by any
of the animals in your aquarium.Personally, I hot glue my strainers on so that they
never come off by accident.Problems with strainers generally occur when either the
baskets are small enough that your animal can cover most of it
or flimsy enough that it can be knocked off by a roaming
animal.The
intake of a pump, a powerhead or an overflow can do some real
damage if they are unprotected and the animal gets into it,
and that is what leads to an animal being stressed enough that
they will release their defensive chemicals.
Although
sea cucumbers get a bad reputation because of the reports of
‘cuke nukes’ that are common on the boards, in reality
such events are actually pretty rare. They are reported a
couple of times a year on various newsgroups, but generally
seem to happen when people are away, or they didn't notice if
or when the cucumber disappeared, or the cucumber looked fine
before the problem but died along with the rest of the tank,
and so on...The fact of the matter is that many times it is
impossible to determine the exact cause of the problems in the
tank, but because so many people have heard the warnings about
sea cucumbers, they are often an easy target for unexplained
problems.In fact, I suspect that because sea cucumber don’t
generally move around very much, and are not considered the
most interesting or attractive pets by most people, they are
probably not watched quite as closely as they should be.The simple fact is that people probably don’t check
on their sea cucumber quite as often as they would a fish or a
coral in the tank.Because of that, many people may not necessarily know
the difference whether or not their sea cucumber is actually
alive...Obviously, any large animal dying in the aquarium and
left to decay will have a dramatic impact on water quality,
and I would be willing to bet that at least some reports of
‘cuke nukes’ could be simply explained by the animal dying
in a tank.Perhaps the tank did not provide the proper conditions
for it, or perhaps the animal was not healthy to start with,
but whatever the reason, because many people may not know what
to look for, they may not know the difference between a live
sea cucumber and a dead one until it begins to rot...
Holothuria
(Thymiosycia) impatiens,
a Caribbean species that is often offered for sale in
the hobby under the name Tigertail Cucumber, is an
excellent generalist detritivore that shows little
preference for substrate types, making it an ideal
choice for the aquarium. Being one of the most
active species of cucumbers, H. impatiens does
a great job of cleaning detritus in the reef aquarium,
but this activity level also makes it more likely to
encounter a pump intake or overflow, and because this
cucumber also possesses Cuvierian tubules, such an
encounter needs to be prevented by the aquarist.
Photo: A Nilsen
Holothuria
atra
is an Indo-Pacific species that is capable of asexual
reproduction (fission) when well-fed, but appear to
switch to sexual reproduction when stressed.
Thus, spawning in the aquarium for this species is an
obvious sign that they are not being kept under ideal
conditions. H. atra are found exclusively
in regions with fine sandy sediments and small coral
rubble, and are a nonspecific detritivore. They
ingest sand particles whole, digesting away the
organic coating and associated detritus before
defecating nice clean sand (see photo). Photo A Nilsen
The
bottom line is that most sea cucumbers won't cause a tank wipe
out even if they hit a powerhead (which as I said, is a pretty
easy thing to prevent and really shouldn't be a concern in a
well maintained tank).The
usual exception to the bad experience with a sea cucumber
being the obvious cause of a tank wipe out is when something
happens to a sea apples (Pseudocholchirus spp.),
because they are among the most potentially hazardous of the
sea cucumbers that we see in the aquarium hobby.Even if a cucumber ends up being damaged badly enough
to release their toxins in the aquarium, there is not
necessarily any serious repercussions if the aquarist is
attentive.The
primary danger from a cucumber releasing its toxins in the
tank comes from the fact that the defensive chemicals continue
to circulate, and remain undiluted in the aquarium.The die-offs that result from this situation can be
effectively avoided by water changes, heavy skimming and
carbon addition if the problem is caught immediately.In fact, many soft corals, sponges and tunicates kept in reef
aquaria are actually muchmore toxic than are
these sea cucumbers, but because these other species are not
crawling around and potentially hitting unguarded powerheads,
they pose less of a threat than do sea cucumbers.That is why I usually tell people that the biggest
danger of keeping sea cucumbers in our tanks comes from our
own negligence rather than from the sea cucumber itself...
Personally, I do not think that there is anything wrong
with keeping sea cucumbers in a reef tank as long as you take
the necessary precautions to protect them from an untimely
death.I keep at
least one sea cucumber in every one of my own reef tanks.
Ultimately, though, you are the one who has to decide
if you are willing to keep the animals in your tank, whether
your tank conditions are right for one, and whether the
benefits of keeping one are worth the potential risks...
Literature
Cited:
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J. C., and J. Sprung. 1994a. The Reef Aquarium, Vol. 1.
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Thelenota
ananas is
an Indo-Pacific species which is striking for both its
bright coloration and spiny skin. Although
rarely seen in the North American market, it is
sometimes offered for sale in the aquarium, but the
primary market for these cucumbers is the food market
for 'trepang' or 'beche de mer'. T ananas
is a nonselective detritivore that mops the sediments
to ingest particles from which it digests the organics
before defecating the cleaned sand. Photo: J Sprung
Motokawa,
T. 1984a. Catch connective tissue: the connective tissue with
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T. 1984b. The viscosity change of the body-wall dermis of the sea
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E. E., and R. D. Barnes. 1994. Invertebrate Zoology. Saunders
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V. A., and G. B. Elyakov. 1988. Secondary metabolites from echinoderms
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R. J. 2002. Aquarium Invertebrates: The Medusa Worms. AAOM
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J. 1997. Natural Reef Aquariums: Simplified approaches to creating
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