Over the past
few years, I have had many questions dealing with keeping
sponges in reef aquaria, and thus far, my general impression
is that few aquarists have success with these odd animals.
I have gotten a couple of questions about sponges recently,
and decided it was time to write an updated article about
them. I think that there are really two primary reasons
for the unfortunate failure of most hobbyists with sponges
in their aquaria. The first reason is that most collectors
and hobbyists are ignorant of sponge biology, and do not
realize that removing most reef sponges from the water,
even for several seconds, will kill them (I will explain
this in more detail below). Second, very little is known
about sponges even within the scientific community, and
their physical tolerances and husbandry requirements remain
as much a mystery to marine biologists as they are to reefkeepers.
For example,
there is an ongoing debate among sponge biologists concerning
the factors controlling sponge distribution in the wild.
Some researchers contend that the many sponge species found
primarily or exclusively in reef areas are excluded from
mangrove habitats by physical tolerances and/or seastar
predation (e.g., Wulff 1995). Other researchers have shown
that transplanted sponges from the mangrove habitat are
consumed by predatory fishes on the reef within hours of
being moved, and suggest that, because sponges moved in
cages seem to survive perfectly well, predation pressure
by reef fishes must limit the range of these sponges to
mangrove habitats (e.g., Chanas and Pawlik 1999; Dunlap
and Pawlik 1996; Pawlik 1999).
Given that marine
biologists studying these animals cannot currently agree
on an unambiguous answer to explain how and why these animals
live where they do, it is not surprising that reefkeepers
have highly variable success at keeping sponges in captivity.
In fact, given our ignorance of sponge biology, it is surprising
how successful many people are keeping these animals! They
are remarkably hardy and adaptive, if healthy, and many
not only survive in reef tanks, but can even grow well and
reproduce. Of course, some species are substantially easier
to keep than others, but most species of sponges are likely
to survive in a well established and maintained reef aquarium
if we understand their needs. I cannot give specific details
for many taxa in this article, and will simply try to explain
how I select and introduce sponges into my own reef tanks.
First, always
select a sponge that has a uniform consistency. By consistency,
I mean that there are no dead, dying or discolored sections
of the body. You should not see any `fuzzy' regions, or
clear spots anywhere on the sponge. I did not say select
a sponge with uniform color because many healthy sponges
may display different color patterns on different body regions;
if you are unfamiliar with selecting sponges, however, it
is best to avoid ones that have variable colors, because
you may not be able to differentiate an unhealthy sponge
from one that is simply mottled. Second, make sure that
the sponge goes into a tank with the same relative environment
as the one from which it is collected. Sponges from protected
environments have different requirements than those collected
from a fore-reef habitat. For an extreme example, if you
see that a sponge is growing well in a protected and darkened
corner of your dealer's tank behind the live rock, do not
stick it into the middle of your high-flow reef tank under
500W halides and expect it to do as well. If neither you
nor your retailer have any idea of the habitat from which
the sponge was collected, you are better off not buying
the animal, because chances are low that it will survive
the transition into your tank. Finally, make sure that the
sponge never leaves the water when you are moving it. Although
there are many species of intertidal sponges which are stranded
in the air each time the tide goes out, reef sponges are
not among them. Although there are many intertidal sponges
out there, I have yet to see many in the reef trade, and
the chances are high that any sponge you see for sale in
the hobby is an obligate reef sponge that will not tolerate
being out of water for any length of time.
Personally,
these days I use a drip acclimation method for most of my
animals in order to minimize the stress of transition into
my aquarium. After floating my new arrivals in the tank
for a few minutes to equalize the temperature, I move them
to a large bucket and prop the bag up while I drip water
from my tank into the bag at about 2 drops per second. In
the case of a sponge, when the bucket is close to full,
I use a Ziplock bag to seal the animal with a small volume
of water (done completely underwater without any air in
the bag at all), and transfer the animal along with a minimal
amount of seawater into my tank, making sure the bag is
completely underwater before releasing the animal and placing
it where I think it will do best. Particularly hardy reef
sponges which are well suited for the novice and nervous
include Callyspongia vaginalis (Lavender tube sponge,
typically with Parazoanthus throughout the body wall),
Chondrilla nucula (Chicken-liver sponge), Cliona
delitrix (Red boring sponge), and Cinachyra kuekenthali
(Orange ball sponge).
Click
on images for a larger view
Photos
by Julian Sprung
Sponge biology
OK, this is
probably an aside for most people, but I always try to explain
the biology of the animals I am writing about in each article,
and I think it is important for people to know something
about the biology in order to give their animals the highest
chance of survival in captivity. A sponge is the common
name for members of the phylum Porifera, which has about
5,500 currently described species (Brusca and Brusca 2003).
Some sponges can grow to more than 6ft in height, and can
make up a substantial portion of the total biomass in some
habitats. Tropical reef habitats house the richest diversity
of sponge species, but sponges can account for up to 75%
of the total animal biomass on the Antarctic sea floor (Brusca
and Brusca 2003).
Traditionally
there were four classes of sponges defined primarily on
the basis of the skeletal elements, although this has recently
been reduced to three, and some debate remains about the
validity of even these three classes (Vacelet 1985). The
first group, Class Calcarea, is entirely marine, and produces
spicules of calcium carbonate which are laid down entirely
as calcite. Although these sponges are not particularly
common or obvious in the wild, they are interesting to reefkeepers
because they are one of the most common to be found in reef
tanks. They generally occur between pieces of live rock
and in sumps or overflows in virtually every tank seeded
with live rock. There are several common species, all small
(usually about the size of a rice grain) and often with
a very fine, funnel-like extension on one end (e.g., Leucilla,
Leucandra, Scypha = Sycon, Clathrina,
etc.).
The second class,
Hexactinellida -- better known as the glass sponges -- is
also entirely marine. These sponges produce spicules made
of silica, and although beautiful, are almost entirely deep-water
species unsuitable for aquaria. The only specimen of this
class anyone reading this article is likely to have ever
seen is Euplectella aspergillum, the Venus's flower
basket. This sponge has become popular as a collectors'
item, but was traditionally given as a wedding gift in some
Asian cultures because there are symbiotic shrimp which
colonize the sponge as larvae, and then become trapped within
as they grow. These shrimp (Spongicola) form mated
male-female pairs, and the 'lovers imprisoned within the
sponge,' I am told, is considered a good luck gift for the
betrothed as a symbol of the lifetime bond between the two
partners.
The final class
is the Demospongiae (for readers following the incorrect
taxonomy presented in texts, such as Moe (1992) or Haywood
& Wells (1989), this is the class which largely absorbed
the Sclerospongiae, although some were discovered to be
Calcarea, as well - see Brusca & Brusca 2003). Demosponges
are the animals everyone thinks of when you hear the word
"sponge." They typically have siliceous spicules, and often
supplement or replace the silica-based skeleton with a collagenous
network referred to as 'spongin' (this is the material of
which your authentic bath sponge is composed). The Demosponges
are found in marine, brackish and freshwater, and at all
depths. This classification becomes more complicated and
confusing, however, by the introduction of an archaic system
of classification by 'body type.' There are three basic
body types among the sponges: asconoid, synconoid and leuconoid
(in that order) levels of organizational complexity. Rather
than getting into all sorts of technical details about these
definitions, let me just say that they have no basis for
classification (they simply refer to how the body is designed
and how water travels through the sponge), and all three
classes have sponges with all three levels of complexity.
If you really care what the differences are, go to the library
and take out a good invertebrate zoology textbook like Ruppert
& Barnes (1994) or Brusca & Brusca (2003).
There are two
basic attributes that are shared by all sponges: their water
current channels (aquiferous system) and the totipotent
nature of sponge cells (ability to revert to an immature
state and become a new cell type. This is a very unusual
characteristic in animal cells - for an extreme example,
if we had totipotent cells, a cell from our tongue could
become an undifferentiated cell and travel through our bloodstream
to replace a damaged eye or brain cell). In fact, some sponges
are so good at this that they can reform after being mashed
up, squeezed through a cheesecloth mesh, and poured into
a beaker of seawater. People have even done this experiment
with two different kinds of sponges, and have them sort
themselves out of the mix to reform two distinct little
sponges from the puree.
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Sponge feeding
and nutrition
The aquiferous
system is just as amazing: an individual Leucandria
10 cm long and about the diameter of a pencil pumps 22.5
liters (about 5.5 gallons) of water through it's body every
day. The impressive sieving capacity of even a relatively
small sponge is what has led some to champion their use
as natural filters for reef aquaria (Tyree 2003). There
is no doubt that sponges can filter a surprising amount
of water through their bodies on a daily basis. This pumping
capacity is even more amazing when you realize that the
cells responsible for moving this water (choanocytes)
are about the size of our white blood cells. Aggregations
of several hundred of these cells form chambers, and these
choanocyte chambers may be as dense as 18,000 per cubic
millimeter in complex sponges. Each cell has a tiny hair
(flagellum) surrounded by a collar made of other
even smaller hairs (microvilli). The flagellum waves
back and forth from base to tip, pushing water ahead of
them as they do. Each cell beats at it's own pace, and pulls
water from very tiny openings (ostia) all over the
surface of the sponge (the largest of which are about 1/10th
of a millimeter) into the sponge, along the cell body, through
the collar which captures food particles from 0.1-1.5 μm
(that's less than 1/600th of a millimeter -- about the size
of a bacterium), and pushes the water away from itself towards
a common exhaust system (the oscula). As water moves
along the cell body, oxygen diffuses into the cell, while
carbon dioxide and other wastes diffuse out of the cell
into the 'exhaled' water. Some free cells (ameobocytes)
cruise around through these water channels and ingest small
algal cells, protozoans, detritus and other organic particles
in the range of 2-5 μm. Other freely moving cells (archeocytes)
take these captured particles and complete the digestion
of them before passing nutrients along to the rest of the
body. Dissolved organic matter (DOM) is extremely important
to the nutrition of many sponges; for example, studies on
three species of Jamaican sponges showed that 80% of organic
matter taken up by sponges was below the resolvability of
microscopy, while the other 20% was comparised primarily
of bacteria and dinoflagellates (H.M. Reiswig, unpublished
data, from (Reiswig 1975).
Click
on images for a larger view
Photos
by Julian Sprung
That is not
to say that all sponges require the same types of tiny particles,
bacteria or dissolved organic matter to thrive. Some sponges
are almost entirely dependent on photosynthetic symbionts
to produce their nutritional requirements. For example,
Wilkinson (1983) showed that six of the ten most common
sponges species on the Great Barrier Reef (GBR) are primary
producers rather than consumers. In fact, these sponges
are more similar to plants in terms of their requirements
in the aquarium than animals; they actually produce three
times more oxygen through photosynthesis of their symbionts
than they use in respiration! For species such as these,
the availability of light for their symbionts will be of
much greater importance to survival in the aquarium than
the presence of any given particulate food. Other sponges
are actually predatory on large prey items, having dietary
requirements more along the lines of fish than a typical
sponge. The amazing sponge Asbestopluma is capable
of capturing, swallowing and digesting mysid shrimps (Vacelet
and Boury-Esnault 1995). In fact, one could argue that the
diversity of feeding modes among the sponges is greater
than that of any other group brought into the aquarium trade:
some are completely autotrophic (self-supporting
through photosynthesis), some filter only a narrow range
of specific-sized particles from the water column (such
as bacteria or phytoplankton), some subsist almost entirely
on dissolved organic matter absorbed from the aquarium water,
and some capture and ingest live prey. Surprising habitat
specificity was demonstrated by a study that examined the
effects of transplanting sponges to different conditions
of light and current on natural reefs (Wilkinson and Vacelet
1979). For species with obligate (always present)
symbionts (e.g., Verongia aerophoba) growth was enhanced
by high light levels, whereas growth of species that always
lack symbionts (e.g., Chondrosia reniformis) was
often inhibited by strong lighting (Wilkinson and Vacelet
1979). Species which had facultative symbionts (may
or may not have symbionts present), such as Chondrilla
nucula and Petrosia ficiformis did not appear
to be affected by the light regime, and did equally well
in bright and subdued lighting (Wilkinson and Vacelet 1979).
The majority
of sponges brought into the aquarium trade, however, are
going to fall into the category of small-particle suspension
feeders. These animals actively pump water through their
body and sieve out the tasty particles of appropriate size,
and/or absorb dissolved organic material from the water
passing through their body. They get a hand in transporting
water through their bodies by oceanic water currents around
them and something called the Bernoulli Principle.
Basically, when water or air flows over a smooth surface,
and then hits something that is raised, it creates suction
at the raised area. If you look closely at a living sponge,
typically you see a more-or-less flat surface with a few
raised holes in it -- these are the oscula (exhaust
system). As water flows across the surface of the sponge,
the lift generated by flowing over the raised holes leads
to suction pulling water through the aquiferous system and
giving the choanocytes (water-pumping cells mentioned
above) a helping hand. However, the sponge builds the bumps
on its surface to specific sizes and diameters under certain
flow regimes, and changing the amount or direction of flow
over those bumps can lead to them not really working so
that wastes and oxygen cannot be efficiently exchanged -
or worse yet, water being forced back into the holes.
Flow and lighting requirements
In general,
most colorful reef sponges that grow out in the open tend
to do best under conditions of relatively high flow. In
the study of Wilkinson and Vancelet (1979) discussed above,
growth and survival of all species tested was greatly reduced
among sponges grown in low flow relative to high flow areas.
This finding may presumably result from the sponges needing
to spend much more energy on pumping water through their
bodies when they get less help from the Bernoulli principle.
To support this presumption, the researchers found that
sponges tended to change their shape and size depending
on where they were living; among the sponges that survived
the transplant, sponge morphology differed dramatically
between the individuals of each species grown under different
light and flow regimes (Wilkinson and Vacelet 1979). This
morphological specialization to specific environmental conditions
may be part of the reason that few hobbyists have a lot
of success with sponges. The problem of a lack of knowledge
about the biology of sponges is compounded by the fact that
we have no idea of the conditions under which the sponge
was initially collected. That alone will reduce the likelihood
that people will have wide-scale success with these animals,
but if the animal was mishandled during collection (especially
if it was exposed to air for any length of time), the chances
of success essentially slide from poor to none...
The reason that
removing sponges from the water typically proves lethal
to these animals has to do with these mechanisms of water
transport. If you remove the sponge from the water, airlocks
often form in the channels of the aquiferous system, and
with only a flagellum to move water, there is no way to
force that air out of their body. The choanocytes soon die,
and that leads to a general necrosis in the area, typically
proving fatal. This may sound ridiculous to you, but consider
yourself as a choanocyte. I give you a jump-rope to move
water over yourself and gather food and exchange wastes
(this sounds silly, but it's basically to scale). You're
sitting there whipping your skipping-rope back and forth
to push water through the pipeline in which you live, when
the water flow is cut off and your pipe drains. Your pipe
is at an odd angle, such that when the water is turned back
on, you're trapped in a bubble. Think you can swing that
skipping-rope back-and-forth hard enough to push the air
out of your pipe? It essentially works the same way for
the sponge. That trapped air causes those cells in the area
to die, and as they decompose, they produce gas which makes
the problem worse, and the sponge starts to decay right
before your eyes… The best way to deal with a dying
area in your sponge is to cut that portion of the animal
away and discard it. Although this sounds a little extreme,
it will greatly increase the chance of survival for your
animal if you can cut away the sickly region and leave only
healthy tissue behind.
However, assuming
that the sponge was collected and shipped properly, the
fact that we know nothing about the habitat or conditions
under which it was originally collected does not doom the
sponge. Contrary to popular belief, sponges are capable
of moving, and if they are unhappy, they can slowly (on
the order of 0.5 cm per day) reorganize themselves, change
the shape and size of their oscula to match changed flow
conditions, or even slide across the bottom to find a place
they prefer to live. It takes a great deal of energy for
a sponge to be able to move, and the animal simply cannot
afford that energy if it is not healthy in the first place.
That they can move and thrive assumes, of course, that they
are completely healthy and water conditions are otherwise
ideal for them (which is often not the case when the animals
are imported for the hobby). The ability of a sponge to
tune its body shape to the new conditions (flow, lighting,
food availability, etc.) under which it find itself in our
aquaria requires that the animal be in excellent condition
when first introduced, and the tank has an acceptable habitat
for the sponge once it gets settled into its new home. Without
both of these conditions, you are sure to fail with most
animals that could be added to a reef tank!
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Reproduction
of sponges
One of the best
ways to get started with a sponge in your reef is to find
a local hobbyist who has something that is doing well in
their tank, and get a cutting to try on your own. This way,
you'll know exactly what kind of conditions the sponge was
thriving under when you get it, and can try to place the
new cutting in the location in your own tank that most closely
matches the conditions under which it was previously growing.
All sponges appear to be capable of sexual reproduction
and typically also exhibit one or more forms of asexual
reproduction. Sponges are hermaphroditic, but typically
produce eggs and sperm at different times. In terms of methods
of reproduction, "sponges probably win the prize for variety"
(Brusca and Brusca 2003). Common methods of asexual reproduction
include regeneration from fragments, budding, and possibly
asexual production of larvae (although this possibility
still remains contentious). Once larvae are formed (whether
this would occur by sexual or asexual production), they
are usually released through the excurrent water flow, or
may also rupture out of the body wall. Sponge larvae are
typically free swimming, all are non-feeding, and after
a short period of swimming or grubbing about on the sea
floor, these larvae attach to the substrate and metamorphose
into tiny sponges. Given this mode of reproduction, sponges
are among those reef invertebrates likely to be successfully
reared in captivity if we can induce them to reproduce predictably.
However, even
if we could get sponges to spawn reliably in captivity,
growth rates are highly variable among different species
of sponges. In general, tropical reef Demosponges probably
live on average from 20 to 100 years (Brusca and Brusca
2003), and for the slower-growing species, it would take
a considerable amount of time for a sponge recruit to grow
to a size at which they could be sold or traded. Some sponges,
like Callispongia vaginalis (lavender tube sponge)
grow so quickly one can notice differences within a week.
One sponge, Terpios from Guam, grows an average of
2.3 cm per month! Others, like Xestospongia muta
(tub or barrel sponge) grow so slowly that no difference
can be seen in the sponge from one year to the next; these
sponges obviously grow, however, since some of them are
large enough for an adult SCUBA diver to climb into and
hide. Depending on the species of sponge, and the average
growth rate of those animals, the likelihood of being able
to spawn them in captivity for trade is highly variable.
A corollary of this variable growth rate is the impact that
hobby collection has on the natural population. Collection
of a common, fast-growing species will tend to have much
less impact on natural populations than harvest of a very
slow-growing species that takes decades to reach reproductive
maturity. In addition, research with caged sponges suggests
that the faster-growing species tend to have increased survival
and growth rates after transplantation to new sites (e.g.,
Pawlik 1998; Wulff 1997). These results together with the
potential ecological impact of hobby collecting argue for
avoiding the slow-growing sponge species for our tanks;
obviously we would all prefer to select species that will
thrive in our tanks and have a minimal impact on the natural
reefs rather than support trade in animals that have a low
chance of survival or a dramatic impact on natural reefs.
Sponge coloration
and Defensive Chemistry
Sponges are
highly variable in color, and almost every color possible
is found within the group, ranging from white to black,
with many brilliant shades of red, orange, yellow and even
blue in between. The pigments responsible for the color
of the sponges appear to be derived from a number of sources,
including de novo synthesis, translocation of pigments
from food particles and symbiotic bacteria and/or algae.
Some texts (Tyree 2003) have attributed the bright colors
of many sponges as a warning to potential predators, and
suggest that drab colored sponges are therefore safer for
the reef aquarium than brilliantly colored ones. Tyree has
an entire discussion of which sponges are members of the
"cryptofauna" and the importance of choosing drab sponges
from this functional group. Haywood and Wells (1989) even
go so far as to suggest color may provide an indicator of
preferred depth, with dull sponges collected from deep sites
and colorful sponges collected from shallow ones. Put in
the simplest terms, there is no scientific evidence to support
either claim.
Many colorful
sponges are undefended by antipredatory chemicals (e.g.,
Callispongia vaginalis), while many dull species
are heavily defended (e.g., Neofibularia nolitangere
- the "touch-me-not" sponge, which causes severe contact
dermatitis in most humans), and vice-versa (Pawlik et al.
1995). Likewise, there is no pattern of greater defenses
in sponges from different habitats or geographical locations
(such as the tropics versus temperate oceans) for either
physical or chemical defenses (Becerro et al. 2003; Burns
et al. 2003; Burns and Ilan 2003). Researchers have argued
that ubiquitous chemical defenses are an effective defense
against many scavengers, generalist predators and perhaps
even other invertebrates seeking to settle and grow on the
sponge, and are therefore a critical component of the life
history of all living sponges (Becerro et al. 2003). However,
despite the presence of these potent chemical defenses,
some sea slugs, polychaete worms, sea turtles and fishes
have managed to find a way around the nasty toxins produced
by many of the tropical sponges, and not only eat them,
but some even specialize on sponge diets (Pawlik 1999).
Furthermore, research has shown that fish attack transplanted
sponges on natural reefs without any regard for their color
(Dunlap and Pawlik 1996). Of 35,301 bites recorded during
this study, 50.8% of bites were taken by angelfish, 34.8%
by parrotfish, and 13.7% by trunkfish and filefish. In these
paired transplants of similarly-colored sponges from reef
and mangrove habitats, fishes preferentially ate the mangrove
sponges of all colors while avoiding similarly-colored sponges
found naturally on the reef placed directly beside them
in the transplant array (Dunlap and Pawlik 1996). Likewise,
there is no evidence to support the claim that dull sponges
come from deeper depths than brightly colored ones (Pawlik
et al. 1995). For exampled, I have collected the beautiful
scarlet sponge Cliona delitrix and the more variable
Aplysina lacunosa - ranging from bright yellow to
pink to lavender to rusty red - at depths of 180 ft. At
that depth, everything pretty much looks black without the
aid of a dive light.
Click
on images for a larger view
Photos by Scott Michaels
This discussion
highlights an interesting point that I believe was the reason
for these erroneous claims: many chemically defended sponges
are simply unsuitable for reef tanks because their antipredatory
chemistry adversely affects not only predators, but potentially
also tankmates and even reefkeepers as well. For example,
the fire sponge, Tedania ignis has such potent defensive
chemicals that after simply putting my arm into the tank
in which this sponge was kept my arm turned red and appeared
(and felt) badly sun burned wherever it had been in contact
with the tank water -- even though this was aflow-through
system (i.e., we pump water in from the ocean on one
side of the tank, and out back into the ocean on the other)!
Few sponges have this potent an effect (hence the common
name), and it may be that I am more sensitive to these animals
than the average, but it is worth noting that some sponge
species (e.g., T. ignis and N. nolitangere)
can elicit very painful reactions if handled by people.
Other potentially undesirable sponges include species like
Siphonodictyon which use a type of 'chemical warfare'
to prevent crowding from scleractinians by exuding a toxic
mucus from their oscula which kills coral polyps on contact.
Obviously, this sponge should not be high on the list of
potential aquarium species for anyone who wants to maintain
corals in their aquarium. Another example, the boring sponge
Cliona, although not often attacking live corals,
do often hollow entire pieces of live rock as they grow.
Over time, this sponge can hollow the entire coral head
to the point that the live tissue forms only a weak crust
surrounding the sponge which could collapse with any pressure,
and colonies of this stage are particularly vulnerable.
Again, this is not a great choice for a reef aquarium. The
sponge Terpios, mentioned for it's extremely fast
growth above, produces some toxins which appear to kill
algae, clams, hydrocorals, and even molluscs prior to contact,
allowing the sponge to overgrow potential competitors for
space on the reef (Brusca and Brusca 2003).
Sponges are,
in fact, the most chemically rich group of animals discovered
to date (Pawlik 1999), and some predict that the majority
of new pharmaceuticals discovered over the next decade or
so will be isolated from marine sponges. Halichondria
moorei, for example has long been used by New Zealand
natives to aid healing. Recent chemical analysis of the
sponge discovered that nearly 10% of the sponge weight is
composed of the potent anti-inflammatory drug potassium
fluorosilicate. Even though the Maori didn't know what the
compound itself was, they quickly learned that the use of
this sponge had amazing ability to reduce painful swelling
on injuries. These chemicals turn out to be incredibly important
to the sponges, and our long-held views that the glass spicules
laced throughout the matrix of the sponge prevented predation
(like miniature quills of a porcupine) is simply wrong.
Pawlik and colleagues have now clearly demonstrated that
fishes will readily accept artificial foods with higher-than-natural
concentrations of spicules incorporated into it, but will
reject the same food when the chemicals from the sponge
are mixed into it (Chanas and Pawlik 1995; Chanas and Pawlik
1996; Pawlik 1993; Pawlik 1998). Furthermore, there can
be extreme variation from individual-to-individual, even
within the same species of sponge collected from the same
reef (Swearingen and Pawlik 1998). It is difficult to make
sweeing generalizations about the habitat requirements of
sponges within each species, and there is simply no way
to make such generalizations about habitat requirements
or defensive capacity of sponges based on their color!
Pawlik (1998;
1999) provides a good review of what is known about sponge
defenses and the factors limiting their distribution in
the wild. I would direct interested readers to these reviews
for more information regarding this subject.
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Sponge symbioses
The final point
I want to discuss in this article is the remarkable suite
of symbioses common among the sponges. Reefkeepers in general
are all familiar with the association of zooxanthellae and
corals, but the same appears to be true of sponges. Most
marine sponges have symbiotic bacteria (primarily Pseudomonas
and Aeromonas), and in some Verongid sponges, bacteria
can account for about 40% of the body weight on average
(Brusca and Brusca 2003). However, it is not just any bacteria
that thrive within the body of these sponges. In fact, there
are a variety of new antimicrobial compounds that have been
isolated from some of these same sponge species, which suggests
that at least some sponges have a highly specific association
with the bacteria found within them (Newbold et al. 1999).
Sponges are also the only animals known to commonly maintain
symbioses with cyanobacteria, and recent work suggests that
both bacterial and/or cyanobacterial symbionts are present
in the majority of sponges (Brusca & Brusca 2003). In
general, bacterial symbionts are located deep within the
sponge, whereas cyanobacteria are typically restricted to
living close to the surface where light is readily available.
Some sponges have symbiotic dinoflagellates (zoochlorellae),
and others maintain symbioses with red algae, filamentous
green algae and diatoms.
In addition
to the various associations with micro-organisms that inhabit
sponge tissues, there are important interactions among many
species of sponges as well. For example, Wulff (1997) showed
that both growth rate and survival of three common Caribbean
reef sponges (Iotrochota birotulata, Amphimedon
rubens, and Aplysina fulva) were greatly enhanced
when these sponges were grown in direct contact with one
another! Wulff set up several experiments including sponges
of the same size and genotype that were grown (1) in close
association with conspecific vs. heterospecific sponges,
(2) alone vs. in close association with conspecific sponges,
and (3) alone on the primary substratum vs. attached to
an intact branch of a conspecific or heterospecific sponge.
Wulff (1997) demonstrated that these sponges do better when
adhering tightly to sponges of other species that differ
from them in chemistry, tissue density, and skeletal construction.
Although the mechanisms by which growth rate is enhanced
by adhering to a heterospecific sponge are unknown, Wulff
(1997) showed that sponges growing alone tended to succumb
to a variety of hazards (including predation by angelfishes
and trunkfishes, predation by starfish, smothering by sediment,
breakage by storm waves, pulverization by storm waves, toppling
by storm waves, fragment mortality, and pathogens), whereas
those growing on the surface of another species more often
survived. Little is known about such effects in other species
or among sponges in captive reefs, but these results suggest
that some sponges may survive better in captivity when maintained
together than in isolation.
On many healthy
reefs, sponges are second only to corals in overall biomass,
and some have argued that the success of both groups is
a function of their ability to benefit from these symbiotic
relationships. Many sponges also have numerous small commensals
living within their bodies. For example, a single specimen
of Spheciospongia vesparium in Florida was found
to contain over 16,000 pistol shrimps (Alpheiidae). Another
study counted over 100 different species in a 15x15 cm piece
of Geodia mesotriaena from the Gulf of California.
Such studies are the rule rather than the exception, and
most sponges play host to a myriad of other species living
within the cavities of their bodies. Obviously, the addition
of a sponge to your tank is likely to involve considerably
more than just the sponge itself.
Given the complexity
of the associations among sponges and their symbionts and
the general lack of concern for or knowledge about their
specific biology in the hobby, it is not surprising that
results have been highly mixed in keeping these animals
in reef aquaria. However, it is becoming ever more common
for retailers and hobbyists alike to be careful during the
transport and acclimation of these animals to new homes,
and with increasing knowledge should come increasing rates
of success. Hopefully, with a bit more fore-thought and
knowledge about these amazing animals our success rate with
sponges will continue to increase in years to come.
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