I have just
returned from MACNA XVI in Boston, and as usual, it was
a great opportunity to catch up with many old friends, and
meet some new ones. It was also a great to be able to finally
attach faces to many of the Email names I have corresponded
with over the years.
On my way back
to Hawaii after the meeting, I stopped off to visit my family
in Edmonton, Alberta. I was quite surprised to see how much
the reef specialty shops in town had expanded since I had
visited last, and was also surprised to see the variety
of animals that were now available for sale there. Not only
had the extent and sophistication of the hobby in Edmonton
expanded, but there was even a petshop selling marine fish
& invertebrates in my hometown of Devon, which has now
grown to a massive population of roughly 5,000 people!
Photo
by Greg Schiemer
Elysia
crispata (the lettuce sea slug) photographed in a
bucket during drip acclimation. These animals are
extremely sensitive and need to be acclimated very
slowly if they are to thrive in a new aquarium.
Photo
by Greg Schiemer
Elysia
crispata in the refugium of Greg Schiemer. These slugs
often do best in the lagoonal environment of an algae-filled
refugium such as this. In some cases, they will even
reproduce under these conditions although few people
have much success propagating these animals in the
aquarium.
Photo
by Julian Sprung
A
sea hare (Aplysia sp.) in search of fleshy macroalgae
upon which to feed. These sea slugs are one of the
few that are suitable for aquarium life, but should
still be attempted only by advanced aquarists.
Photo
by Julian Sprung
A
close up of the head of Aplysia californica feeding.
In addition to gaining nutrition from its macroalgal
food, these slugs also ingest a wide variety of defensive
compounds in their seaweed prey, which they concentrate
into a purplish dye that can be ejected when the slug
is threatened by predators.
Photo
by Julian Sprung
Photo
by Julian Sprung
Sea
hares have two large flaps that fold back on the body
to cover the internal shell plate. These wing-like
extensions can be used for short bursts of awkward
swimming, such as pictured here, when the slug is
threatened. If you look carefully, you should be able
to see the outline of the internal shell plate between
the extended "wings."
Photo
by Julian Sprung
Photo
by Julian Sprung
Photo
by Julian Sprung
Elysia
crispata is one of the most variably colored sea slugs
in the world, ranging from dull green, to bright blue,
to almost white, to red, yellow and blue banding on
a green background, and virtually every combination
in between. There remains debate among the experts
about whether the diversity of colors seen in this
slug is a consequence of extreme individual variability
or poorly understood taxonomy.
Photo
by Julian Sprung
Elysia
diomedea is another beautiful lettuce slug that is
extremely difficult to distinguish from E. crispata.
The difficulty in correctly identifying the species
of slug offered for sale in the hobby, combined with
the lack of knowledge about their basic biology and
needs in captivity result in a virtual death sentence
for the majority of the sea slugs in the trade.
This was both
encouraging and discouraging to me. I remembered it being
difficult to find a wide selection of reef animals and supplies
from many years ago. That had certainly changed! Unfortunately,
many of the animals that I saw for sale in some shops have
extremely low chances of survival in an aquarium, and I
wondered if people were being informed of that when shopping
for new additions to their tank. Many of the conversations
I had at MACNA involved the sustainability of our hobby,
education to prevent trade in animals that have low survival
success, supporting captive rearing of fish and invertebrates,
and trying to better educate newbies to avoid many of the
problems that we had inevitably encountered over the many
years of reefkeeping.
As I was cruising
the petshops in Edmonton, one tank in particular that caught
my eye contained about 50 sea slugs. As I crouched in front
of the tank, I couldn't help but notice that the primary
substrate for the animals was a large mass of plastic plants
that resembled Caulerpa algae. I was pleased to see
that the two species of sea slugs in the aquarium happened
to be 2 of the more likely sea slugs (sea hares and lettuce
sea slugs) to survive in captivity, and there were none
of the more colorful and infinitely more difficult nudibranchs
that are so often imported. As I was writing this article,
I was going to discuss sea hares, lettuce slugs and nudibranchs
all in the same article, but it ended up running so long
that I have split the discussion into two parts. I'll save
the discussion of nudibranchs for next time.
Getting back
to my story, I was very encouraged that the only slugs in
the shop were the ones that had a reasonable chance of survival.
I figured that meant that the staff must be pretty well
informed, and were making a conscious decision to only stock
animals with a reasonable chance of survival in tanks. Almost
as soon as the thought entered my head, I overheard a salesperson
telling a shopper that "these lettuce nudibranchs are reef-safe
and would be a great addition to your tank."
Now, as I said,
I do have to give the shop credit for stocking animals that
had a non-zero chance of survival in captivity (the sea
hare Aplysia and the lettuce slug Elysia),
but this proclamation bothered me for a couple of reasons.
First, neither of these slugs are actually a nudibranch,
and second, both species feed on specific algal prey that
were absent from any tank in the entire shop. The
fact that the salesperson did not ask the perspective customer
about their setup, the inhabitants, or tell them anything
about the biology or feeding requirements of the sea slugs
was a little disturbing to me. Couple that with the observation
that there was nothing in the shop that could have been
provided to these animals as food, and I had a sinking feeling;
either the effort of the person doing the ordering was being
undermined by a lack of knowledge on the sales floor, or
there wasn't quite the conscientious effort for responsible
stocking that I had given them credit for in the first place…
What is in
a name?
So what do I
mean that these animals are not really nudibranchs, and
who really cares? First, let's back up a step and define
the terms 'sea slug' and 'nudibranch'. Many people use the
terms interchangeably to cover the more than 3000 species
of opisthobranch sea slugs, which is simply incorrect. Nudibranch
is actually a proper taxonomic description - the naked-gilled
sea slugs. Sea slug, on the other hand, is a descriptive
term that can be applied to any of the shell-less gastropods
that live in the sea.
Together with
their shelled cousins, Opisthobranchs are members of the
Molluscan class Gastropoda. In the simplest terms, the subclass
Opisthobranchia is simply a group of snails that do not
produce a shell - unlike their more familiar cousins, the
snails. The subclass Opisthobranchia contains several different
groups of sea slugs, but the most diverse and spectacular
members are the Nudibranchia. Members of the Nudibranchia
also happen to be the most commonly sea slugs offered for
sale in the pet trade, so it is easy to see why some people
might make the mistake of considering 'nudibranch' and 'sea
slug' as synonyms. In addition to the order Nudibranchia,
the other common orders of sea slugs are the Sacoglossa
(also known as the ascoglossans), the Anaspidea, the Notaspidea,
and the Cephalaspidea (see Brusca and Brucsa 1990 or Ruppert
and Barnes 1994 for more detail). Because each of these
groups have the same taxonomic status (they are all orders),
it is technically incorrect to refer to a sacoglossan or
cephalaspidean as a 'nudibranch' and the generic term 'sea
slug' is preferable if you do not want to use the proper
name of opithobranch.
I make this
distinction for several reasons. First of all, I think it
is important to use the right names for the animals in our
tanks to avoid confusion. Second, there are actually some
researchers who do not think that all of these sea slug
orders are really all that closely related to one another,
and if those people are right (i.e., different snail ancestors
have converged on a slug-like body and they are not all
closely related to one another), then there may be at least
as much divergence between the different groups of 'sea
slugs' as there is between fish and mammals! People would
never lump all fish and mammals under a single common name,
and it is one of my pet-peeves that it is done with sea
slugs. However, it is more than simply a pet-peeve of mine:
the biology, life-style and aquarium care of groups that
are this distantly related is almost guaranteed to be very
different from one another. If no other reason, the similarities
among closely-related, and the differences among distantly-related,
groups should be sufficient reason for you to care about
the proper classification of these animals.
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Finally, and
most importantly, there are literally thousands of species
within the Nudibranchia, but every one of them that have
been studied to date are exclusively predatory (i.e., they
feed on only animal matter, not algae). Because all known
members of the Order Nudibranchia that have been studied
to date prey entirely on animal matter, and many of them
are so specific in their diets that they are able to recognize
only a single species or two of prey. If that prey (which
is usually a sessile invertebrate such as a cnidarian, bryozoan,
tunicate or sponge) is not available within the aquarium,
then the animal will simply wander endlessly around the
aquarium searching for prey, and slowly digesting itself
until it can no longer sustain its metabolic needs and finally
starves to death (this is usually a matter of one to several
months for a well-fed and healthy animal that is introduced
to an aquarium). Given these specific food requirements,
none of the nudibranchs can be considered either easy to
keep or particularly reef friendly. I will discuss these
animals in more detail in Part 2 of this article.
General advice
to avoid buying sea slugs
First of all,
I have to say that unless you are able to determine the
species of a sea slug and its exact requirements in captivity
from a reliable source, I would always recommend
against buying any of these animals. In their book
Reef Invertebrates, Anthony Calfo & Bob Fenner
(2003) have an entire section devoted to opisthobranch molluscs
and discuss the selection, care and feeding of many of the
species offered for sale in the hobby. They discuss the
members of these groups in more detail than is possible
in a short article here. Julian Sprung (2001) also has some
very nice photographs and descriptions of some of the more
common sea slugs to be sold in the hobby in his book, Invertebrates.
If you don't have one of these books, and are unsure of
the identification of the animals offered for sale in your
local shop, ask your dealer to show you their copy of the
books to compare. Although there are some spectacular pictures
in these books, there are only a few of the more than 3000
known species mentioned, and your best resource for up-to-date
information about the taxonomy and biology of an unidentified
sea slug will probably be Bill Rudman's Sea Slug Forum or Michael Miller's Slug Site. Despite
the fact that there are many beautiful sea slugs out there,
unless you know exactly what you are buying and their
feeding requirements, the rule of thumb is to always
avoid ever purchasing any sea slug. Sadly, the vast majority
of sea slugs are beautiful, but virtually impossible to
care for animals in the aquarium.
OK, so having
said that, I also wanted to give you a little more encouraging
news. If you are really interested in maintaining a sea
slug in your reef tank, there is a decent possibility that
an herbivorous species from one of the other Orders (such
as Sacoglossa or Anaspidea) could actually be kept successfully
in an appropriate aquarium. Whenever I am asked about sea
slugs in general or nudibranchs in particular, I almost
always give a blanket recommendation that they should be
avoided. I stick by that still unless you know a reasonable
amount of detailed information about the biology of animal
you intend to purchase, and what are the specific food(s)
that the animal requires. I am not going to change that
advice here - I still believe that no one should ever consider
buying one of these animals unless they know exactly what
the animals require to thrive in captivity.
I know that
I am repeating myself, but I am belaboring this point because
almost all sea slugs have extremely specific food
requirements, and they will inevitably starve to death while
searching if their preferred prey is not available. Frequently
the only nudibranch sea slugs that tend to thrive in reef
aquaria are those that are considered pests, such as the
soft coral specialistpredators of the genus Phyllodesmium
, whose members feed on corals such as Sarcophyton,
Briareum and Xenia. I will discuss this in
more detail in Part 2 of this article.
Sea
Hares
Given that
the tank full of slugs that got me started on this article
were both non-nudibranch sea slugs, I figured that I would
start out with a discussion of those two groups. The Anaspidean
sea slugs, often called "sea hares" or "sea bunnies" in
the trade, get their common name from the two long ear-like
extensions that apparently made them look like a rabbit
to someone in the distant past. I can't honestly say that
I see that resemblance, but maybe you can. Unlike nudibranch
sea slugs, that lack a shell, the anaspideans have a reduced
shell plate. This shell plate is not always present, and
even when it is, the shell is internal, making it seem like
they have no shell to most people. In general they also
have wing-like extensions that fold back over the reduced
shell, and which can be used for short bursts of awkward
swimming if the slug is in a hurry. Some of these animals
can get very large: in his book Pacific Coast Nudibranchs,
Dave Behrens reports that the largest sea hare, Aplysia
vaccaria, can grow to about 1 metre long and can weigh
nearly 14kg!
Most sea hares
tend to protect themselves by the exudation of a bright
purplish ink when they are alarmed. There remains some debate
about the exact function of the ink, but research suggests
that the purplish ink is a concentrated secretion of defensive
compounds isolated from their algal food (primarily fleshy
red or green algae or cyanobacteria, depending on the species).
The best data support the hypothesis that the ink functions
for defensive purposes. Observed behavioral changes from
potential predators included: (1) bristle erection by fireworms;
(2) increased mucus production by other opisthobranchs;
(3) reduced feeding behavior, increased grooming behavior,
and temporary pauses in heart and scaphognathite beating
by crabs; (4) reduced or increased activity by cryptic and
exposed sea urchin species, respectively; and (5) rapid
swimming by fish (Carefoot et al. 1999). When threatened,
these animals apparently eject their defensive ink to confuse
and irritate any potential predators while they make a hasty
retreat. This ink is not particularly toxic (although it
is certainly a stressful irritant to most species), but
it is a potential hazard for pollution and stress if not
removed from the aquarium. Fortunately, the ink is easily
removed with the use of activated carbon, or by water changes.
Like the sacoglossans
I will discuss later, the anaspideans are exclusively herbivorous,
but the degree of diet specialization depends on the species
in question. Some species (e.g., Bursatella) are
specialists on microalgae growing on the surface of sand
grains, and actually ingest mouthfuls of sand to digest
away the cyanobacteria and diatoms growing there. In this
sense, they function very much like the popular sand-mopping
sea cucumbers known as Tiger-tails (e.g., Holothuria
hilla). Obviously an animal such as this will be better
suited to a tank with an established deep sandbed than one
with a bare bottom. Also, Bursatella would be a much
better choice for inclusion in an aquarium than any of the
specialist predators that are frequently offered for sale
in the hobby, but unfortunately I have never seen these
animals in any petshop.
Chemical defenses
of sea hares
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These sea hares
are of interest to aquarists, because they are among the
few species that tend to feed on these highly distasteful
nuisance algae that are avoided by the majority of the animals
in our aquaria. It was previously thought that sea hares
fed specifically on these distasteful prey species to collect
and sequester the defensive compounds that prevent other
animals from feeding on these algae. However, more recent
research suggests that sea hares have generic mechanisms
for sequestering algal metabolites rather than mechanisms
that are tightly linked to particular compounds, that these
mechanisms do not differ dramatically between species, and
that sequestered secondary metabolites are not located optimally
for defense (Pennings & Paul 1993). Furthermore, bioassays
found that the distasteful compounds located in the skin
of sea hares was produced by the animals themselves and
not derived from their algal diet (Pennings et al. 1999).
Researchers found that algal-derived secondary metabolites
play a role in chemical defense of sea hares via the ink,
but are not responsible for the unpalatability to potential
predators on the reef of the skin or eggs (Pennings et al.
1999). They concluded that accumulation of dietary-derived
metabolites that function as defensive compounds in the
digestive gland may occur primarily to detoxify a chemically
rich diet, rather than to deter predators (Pennings et al.
1999).
Captive breeding
of sea hares
The animals
that were in the shop in Edmonton were a species of Aplysia.
As I said previously, I was encouraged to see these
animals for sale because this slug happens to be one of
the few that is well-suited to aquarium life. Their diet
is known, and most sea hares are generalist grazers on filamentous
and large, fleshy red or green algae. The techniques to
raise them are well-known; in fact Aplysia was one
of the first opisthobranchs to be reared from eggs to reproductive
adults in captivity (Kriegstein et al. 1974). That knowledge
was then scaled up to large-scale mariculture and Aplysia
mass production began in 1983. The NIH/University of Miami
National Resource for Aplysia Facility now ships
over 25,000 animals a year from their facility. Despite
the fact that all of these factors suggest that these animals
should be ideally-suited to the aquarium trade, my enthusiasm
was quickly dampened when I realized that there were no
algae in the entire shop that could be fed to these magnificent
animals. Furthermore, very few of the sea slugs for sale
in the aquarium hobby are captively-bred at this time, because
all the Alpysia raised at the University of Miami
are sold to researchers rather than hobbyists.
Furthermore,
depending on the species, feeding these animals in captivity
can be more difficult than it sounds. For some sea hares,
the dietary requirements depend to a large extent on the
age, because like many marine invertebrates, they can show
marked size-related shifts in their feeding preferences
(Pennings 1990). Thus, even knowing the species of the slug,
and the fact that it is herbivorous may not be sufficient
to provide a suitable environment for the animal if you
were unaware of the shifting requirements of the animal
as it grows. The biology and feeding preferences of these
animals have been the focus of a great deal of research,
and interested readers can find more information from the
compiled list of research papers on sea hare feeding posted
on the Sea Slug Forum here.
As I mentioned
above, the mating systems of these slugs is well-suited
to culture in captivity. Aplysia are simultaneous
hermaphrodites that mate repeatedly in the field acting
as both males and females (Angeloni et al. 2003). They lay
gelatinous egg masses from which free-swimming larvae hatch
out and spend several weeks feeding in the plankton before
they become competent to settle and metamorphose into the
adult body form. For this reason, they are not likely to
reproduce in the aquarium. However, it is obviously possible
to supply the hobby demand for these animals if the University
of Miami is able to sell tens of thousands of slugs they
raise each year. For the home aquarist, culture of larvae
is more difficult, because they produce long-term feeding
larvae although the techniques to raise planktonic marine
larvae are well-established (e.g., see Toonen 1996 for more
info). One of the major hurdles to face when culturing animals
such as these is knowing what cues they respond to when
they settle. Fortunately Aplysia tend to metamorphose
in response to a wide range of macroalgae (particularly
reds, see Pawlik 1988) and compared to their nudibranch
cousins, they are relatively easy to breed and grow in captivity.
Likewise, their preferred food item, the red alga Gracilaria,
is also relatively easy to culture and is readily available
in the trade.
Hopefully captive-bred
sea hares will be more widely available to aquarists in
the near future.
Lettuce
Sea Slugs
The slugs identified
in the shop as the "Lettuce Nudibranch" were most likely
the sacoglossan sea slug Elysia crispata, which was
known as Tridachia crispata until it was recently
renamed (Gosliner 1995). Even after Gosliner's recent re-examination
of the genus, E. crispata remains one of the most
variable sea slugs in the world, and there remains some
debate whether this is a consequence of extreme individual
variability or poorly understood taxonomy. Some experts
(such as Bill Rudman and Kathe Jensen) appear to believe
that these slugs are probably members of a morphologically-similar
species complex for which the taxonomy (and specific biology)
is not yet understood.
In support of
the hypothesis that E. crispata may be a species
complex rather than an extremely variable single species,
there is a considerable amount of variation among how these
animal appear to behave in both the aquarium and the wild.
There are reports of differences in feeding preference and
reproductive mode that may accompany the variation in color
seen among different varieties of this slug. If true, then
there are some major differences in the aquarium requirements
and proper care of the various color morphs that have been
reported. That would also make it more likely that there
are at least a couple of different species within this complex
that are waiting for a proper name.
From the hobbyist
perspective, however, even more important than knowing the
real species name for these animals, we need to understand
what exactly the natural history of each group is, what
they feed on, and what are their specific biological requirements
within the aquarium? Only after we understand these outstanding
questions and the specific site of origin for the animals
in the hobby can we reliably predict what they need to survive
in the aquarium.
Biology of
Lettuce Slugs
As I mentioned
above, these animals are not nudibranchs, either. They are
a Sacoglossan sea slug that makes a living by sucking the
contents from the living cells of their prey. Fortunately
for the adventurous aquarists among us who really want to
keep a sea slug, some of the members of these non-nudibranch
groups (especially sacoglossan, cephalaspidean & anaspidean)
sea slugs may actually be reasonably well-suited for certain
aquaria.
In general,
sacoglossans are among the more colorful of these slugs,
but none of the groups are typically as colorful or striking
as their nudibranch cousins, however, there are some spectacular
exceptions that could be perfectly suitable for a reef aquarium.
For example, some species, such as Lobiger are herbivores
that could actually provide a useful service to aquarists.
Although I say these animals are herbivores, there is really
a little more to it. They are actually specialist feeders
that have a modified radula (the hardened ribbon
of teeth used by most molluscs to feed) that can "surgically"
remove the contents of individual algal cells and allow
them to digest the cellular contents while saving the choloroplasts
(the photosynthetic organelles plants use to produce energy
from sunlight) to incorporate them into their own body.
By stealing these photosynthetic organelles to use for a
time, the slugs are able to take advantage of their function
for a short time much as they did in the plant from which
they were removed. This amazing ability actually allows
these animals to generate some of their own food by making
themselves "solar-powered sea slugs!"
In this case,
the slugs that I saw in the petshop, Elysia (formerly
Tridachia) crispata, happen to fall into the
"solar-powered sea slug" category. That was one of the reasons
why I was happy to see these animals being offered for sale
in the local petshop. Unlike their carnivorous cousins the
nudibranchs, the sacoglossans (of which E. crispata
is a member) are largely herbivorous, and you could think
of them as sort of a marine mosquito for algae. I know that
sounds a little odd, but how else do you describe an animal
that makes a living sticking a fine tube into their host
and carefully sucking out their fill of the contents before
moving on?
Most of the
lettuce slugs imported for the hobby appear to be the form
that specializes on the nuisance 'hair algae' Bryopsis.
However, as I mentioned above, there are reliable reports
of variation in feeding preferences and reproductive modes
reported among the various color morphs, and some lettuce
slugs will completely ignore Bryopsis in the aquarium.
For example, Elysia collected from the Great Barrier
Reef appear to specialize on the alga Chlorodesmis
rather than Bryopsis (Hay et al. 1989). Furthermore,
some slugs within this species appear to be direct-developers
that reproduce well in aquaria, whereas others produce planktonic
larvae that do not generally survive in the aquarium. Given
this degree of variability, and the fact that researcher
cannot currently tell these things apart in a reliable manner,
it is obviously going to be very difficult for you as a
hobbyist to make sure that you know what you're getting
when you buy one of these animals in the local pet store.
As I mentioned
earlier, the Lettuce Sea Slug (E. crispata) is among
the most variably colored sea slugs in the world, ranging
from solid pale olive to individuals with brilliant yellow,
red & blue highlights to solid electric blue animals,
and it seems likely that additional study will reveal that
some of this variation is a result of there being multiple
species in this complex. However, other species in the genus,
such as Elysia viridis, are quite cryptic, resembling
their food (primarily the algae Codium, Bryopsis
and Halimeda) so much that most people would miss
them even after they were told that there were sea slugs
in the aquarium!
Using Lettuce
Slugs for algae control in the aquarium
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Although these
sea slugs are often sold as a means to eradicate nuisance
algae from reef tanks, the simple truth is that they will
rarely perform that task. Very few sea slugs have been shown
to control population density of their prey in nature, and
efforts at biological control often have unexpected results
(reviewed by Secord 2003). In fact, some experiments at
using sea slugs for biological control have had exactly
the opposite effect that was expected. For example, in an
effort to control the spread of the invasive alga Caulerpa
taxifolia researchers experimented with the specialist
predator sea slug Lobiger serradifalci. Instead of
destroying the algal fronds as the researchers hoped, the
slugs actually fragmented them into tiny living portions
that were each capable of regenerating (Zulijevic et al
2001). Thus, the introduction of L. serradifalci
may actually facilitate the spread of C. taxifolia
rather than controlling it (Zulijevic et al 2001). That
is pretty much the exact opposite effect that we would want
for a biological control agent!
Despite our
hopes for using sea slugs to eradicate nuisance species
in the aquarium, I doubt that most will be any more successful
at accomplishing that role in the aquarium than they are
in nature. Lettuce sea slugs will definitely feed on Bryopsis,
but they will rarely provide a means to completely eradicate
this nuisance alga from an aquarium. Unless they are introduced
in very high density, they have little effect beyond drastically
slowing the growth of the algae. In fact, if you manage
to introduce enough slugs to actually eradicate the algae
from the tank, they will promptly starve to death once the
algae on which they feed has been eliminated. However, the
presence of a specialist sea slug (such as E. crispata
feeding on Bryopsis, E. viridis feeding on
Halimeda, or L. souverbiei feeding on Caulerpa)
will often slow the growth rate of the alga to the point
that they are less likely to become problematic in a reef
aquarium and are more easily controlled by a reasonable
effort from the aquarist. Almost any of these slugs would
be infinitely more appropriate for the average reef tank
than any of the true nudibranch species I will discuss next
time.
Chemical defenses
of Lettuce Slugs
Another interesting
note about Elysia is that the slug appears to be
well defended via distasteful chemicals (Hay et al. 1989).
The source of the antifeedon chemistry in these slugs
is not well-studied, but they appear to sequester a distasteful
compound called chlorodesmin from this alga that provides
some level of defense to the slugs. However, that cannot
be the complete story, because food that contained extractions
of the compounds found in Elysia (including the chlorodesmin)
was readily eaten by wrasses of the genus Thalassoma,
whereas the slugs themselves are not (Hay et al. 1989).
On the other hand, another common sacoglossan sea slug from
the region, Cyerce nigricans sequestered metabolites
from Chlorodesmis algae in relatively small amounts,
was much more distasteful to fishes: Cyerce were
never attacked by fishes and incorporation of the crude
organic extract of this slug into food strongly deterred
feeding by wrasses in laboratory assays (Hay et al. 1989).
Fortunately,
these chemical defenses tend to be effective only at preventing
the slugs from being eaten, and not as a toxin that may
affect tankmates. Provided that your pets do not try to
eat your Lettuce Slug, there is no risk of keeping one in
your tank.
Conclusion
& Summary
Well, that
is a bit of a diatribe that was started from my recent trip
home. I hope that I have conveyed that anyone but experienced
hobbyists should studiously avoid ever purchasing a sea
slug for their tank. Remember that unless you know the specific
identification and requirements of the animal that you are
looking to purchase, it is a good idea to leave them be,
because if you don't know the species, then just by chance,
you're most likely to get an animal for which it will be
impossible to provide appropriate care. I also hope that
I have given the adventurous and experienced aquarists who
read this a better idea of what to expect if you decide
to ignore my advice and get one of these animals for your
tank anyway. So, for those of you that have a good supplier
who provides you with detailed information about the animals
you plan to purchase, then you may be able locate one of
the species of non-nudibranch sea slugs that has a decent
chance of surviving (and maybe even thriving) in captivity…
References:
Angeloni,
L., J.W. Bradbury & R.S. Burton, 2003. Multiple mating,
paternity, and body size in a simultaneous hermaphrodite,
Aplysia californica. Behavioral Ecology 14:554-560.
Brusca, R.
C., and G. J. Brucsa. 1990. Invertebrates. Sinauer
Associates, Inc, Sunderland, Mass.
Calfo, A.
& R. Fenner, 2003. Reef Invertebrates: An Essential
Guide to Selection, Care and Compatability. Reading
Trees, Monroeville, PA.
Carefoot,
T.H., S.C. Pennings & J.P. Danko, 1999. A test of
novel function(s) for the ink of sea hares. Journal of
Experimental Marine Biology and Ecology 234:185-197.
Gosliner,
T. M. 1995. The Genus Thuridilla (Opisthobranchia:
Elysiidae) from the Tropical Indo-Pacific, with a Revision
of the Phylogeny and Systematics of the Elysiidae. Proceedings
of the California Academy of Science 49:1-54.
Hay, M. E.,
J. R. Pawlik, J. E. Duffy, and W. Fenical. 1989. Seaweed-herbivore-predator
interactions: Host-plant specialization reduces predation
on small herbivores. Oecologia 81:418-427.
Kriegstein,
A.R., V. Castellucci & E.R. Kandel, 1974. Metamorphosis
of Aplysia californica in laboratory culture. Proceedings
of the National Academy of Sciences of the United States
of America 71:3654-3658.
Pawlik, J.R.
1988. Larvae of the sea hare Aplysia californica
settle and metamorphose on an assortment of macroalgal
species. Marine Ecology Progress Series 51:195-199.
Pennings,
S.C., 1990. Size-related shifts in herbivory: specialization
in the sea hare Aplysia californica Cooper. Journal
of Experimental Marine Biology and Ecology 142:43-61.
Pennings
S.C. & V.J. Paul, 1993. Sequestration of dietary secondary
metabolites by 3 species of sea hares - location, specificity
and dynamics. Marine Biology 117: 535-546.
Pennings,
S.C., V.J. Paul, D.C. Dunbar, M.T. Hamann, W.A. Lumbang,
B. Novack, & R.S. Jacobs, 1999. Unpalatable compounds
in the marine gastropod Dolabella auricularia:
distribution and effect of diet. Journal of Chemical Ecology
25: 735-755.
Ruppert,
E. E., and R. D. Barnes. 1994. Invertebrate Zoology.
Saunders College Publishing, Harcourt Brace Jovanovich
Publishers, Orlando, FL.
Secord, D.,
2003. Biological control of marine invasive species: cautionary
tales and land-based lessons. Biological Invasions 5:117-131.
Sprung, J.
2001. Invertebrates: A Quick Reference Guide. Sea
Challengers, Danville, CA.
Zulijevic,
A., T. Thibaut, H. Elloukal, & A. Meinesz, 2001. Sea
slug disperses the invasive Caulerpa taxifolia.
Journal of the Marine Biological Association of the United
Kingdom 81:343-344.
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