Aquarium Invertebrates: A Look at the Sponges
There are approximately 8,000 species of sponges, and they can be found living in many different marine environments from shallow water reefs down to the depths of the abyssal plains and trenches. They can range from a full size of less than one centimeter up to well over one meter, and they come in many beautiful colors, too. They also come in many shapes, having thin or thick branched forms, short and fat to tall and thin vase-shaped forms, round to fan-shaped clumps, or growing as encrustations over various surfaces.
Unfortunately, as widespread, diverse, and attractive as they may be, their survival rate in aquariums in generally poor, though. Yes, there are some successes, but many sponges do not survive long-term in captive systems. There are obvious reasons for this at times, and not so obvious reasons at other times, so I'll give you some information about sponge biology, why they often die, and what you can do to increase your chances of success.
Sponges are the simplest of the multi-cellular animals, lacking any sorts of tissues or organs, etc. Instead, they are collections of a few types of cells living together in an organized mass. They stay attached to the bottom for their adult life, and with the exception of a few that can slowly move by re-arranging the placement of some cells, they stay in one place.
They're also filter feeders that strain tiny food particles from the water, using specialized types of cells called choanocytes or collar cells, which line their interior surfaces. These cells have a mesh-like collar at one end that encircles a tiny whip-like structure called a flagellum, and when thousands of them simultaneously wiggle these whips it creates a current of water. This current moves water (and food particles carried in it) from the outside into the sponge's body through numerous tiny openings, called ostia, then through lots of very tiny canals in the body, and then into a larger internal chamber called an atrium. From there it passes out of a relatively large opening, called an osculum, back to the environment.
What you should understand from this is that through the activity of numerous cells, water moves into, through, and out of a sponge via many canals, and that some of the canals are very small. When you look at a sponge, the big oscula where the water exits are typically obvious, and in looking a little closer you can sometimes pick out the numerous, but much smaller, ostia where water goes in, too. However, it's the much, much smaller system of collar cell-lined canals that run between the two that are most important right now. Important because during the passage of water through the sponge, those same collar cells that created currents also capture food particles with their collars. And, because all of the food particles a sponge eats are captured by these single cells, the particles obviously have to be extremely small in size. Bacteria and other microplankton are the largest things a typical sponge can eat in quantity, with a large portion of their diets often being much smaller carbon-rich bits of dead plankton and other specks of organic material. Typically this stuff is smaller than 0.5 micrometers in size, and may make up 80% of a sponge's diet.
There are also some other sponge cells, called archeocytes or amoebocytes, that can grab particles that are bigger. But, while these food bits are larger than what the collar cells eat, they are still quite small, typically being only a few micrometers in size.
Here's a great video on water flow and feeding in the phylum Porifera (sponges):
On top of all this cellular-level feeding, many sponges also acquire nutrients from internal populations of symbiotic microorganisms. Photosynthetic algae similar to that found in reef corals are present in some, while others contain large quantities bacteria and/or cyanobacteria, which can make more food than they need for themselves when conditions are optimal. So, they give off their surpluses and the host sponge takes them. Sponges can generally absorb some nutrients directly from seawater, as well.
A few sponges can eat larger foods, and some can live entirely off the excesses produced by their symbionts and/or what they can take directly from seawater, but these sorts of sponges are not the typical sorts you'll find at a store. Instead, you're usually going to find those that must eat tiny food particles and can't live off the generosity of their symbionts, or by simply sitting in nutrient-rich waters.
Why Sponges Die
Now that you have a little understanding of how a sponge works, we'll look at some of the causes of death, which should make sense. These are not in any particular order or ranking, and they aren't the only things that can lead to the death of a sponge, but they are the most common.
Exposure to air
Sponges are full of tiny canals, so if you remove a sponge from water it will drain out to some degree and parts of it will be filled with air. There are a few sorts of intertidal sponges that have adapted to being temporarily exposed to air during low tides, but most sponges do not tolerate an exposure to air very well at all. The problem is that the water runs out, air goes into the canals, and then when the sponge is re-submerged, some of the air gets trapped inside the tiny canals and other areas. It gets trapped as little bubbles, and the bubbles won't come out.
Cells surrounding the bubbles may then die, and their deaths can lead to the deaths of their neighbors, and so on, and so on. It typically doesn't take much for an entire sponge to die, and how long it was out of water often has little to do with it. If it comes out of the water for even one second, the end result will often be the same - death.
Most aquariums do not contain enough food particles of small enough size to keep a sponge alive. So, they starve to death. Even top-quality plankton-in-a-bottle products that you may add yourself may not help much (or any) in most cases, as what most sponges need is too small.
Remember that collar cells feed on particles that are typically smaller than 0.5 micrometers? Well, the phytoplankton in one of the most popular brands of these products ranges from 2 to 12 micrometers. Thus, even the smallest of it is larger than the largest thing an average collar cell can eat. Other similar products may also be fine for many other organisms, but in general they have even larger particles and/or tend to form clumps, and may simply be unsuitable for trying to keep sponges alive. If you intend to try using any such things, it is very important to find out the size of the particles they contain.
Some sponges can thrive in areas of low current, such as under or behind rocks. But, for many, a stronger current is essential. Currents obviously bring food and oxygenated water to a sponge, and also remove wastes, but there's more to it than that.
As water flows over many sponges that are properly oriented, it typically creates a draft over a sponge's oscula (where water exits). This, in turn, pulls water through the rest of the sponge and helps the collar cells. They work hard, but i such cases the shape of the sponge's body, and the shape, size, and position of the oscula, and the nature of the surrounding currents also influence how much water can move through the canals. Thus, each type of sponge may be adapted to a certain type of current, and individuals typically grow in an orientation and form that makes best use of it.
So, if a sponge is randomly placed anywhere in an aquarium, it may not be positioned correctly to make use of the prevailing currents. This leaves the collar cells to do all the work, and that is often too much. This is especially so in aquariums where the food supply is very low already. As you might guess, placing a sponge in an area of strong current won't necessarily help if it is oriented incorrectly. In fact, you may end up forcing water the wrong way through a sponge, which would be very bad, too. Therefore, even if plenty of appropriately-sized food is provided, a sponge may still starve to death if it is positioned incorrectly.
Overgrowth by algae
Sponges that spend their lives in brightly-lit waters have developed chemical defenses to keep various sorts of algae from overgrowing and smothering them. However, many sponges come from dimmer waters where algae is less common or even absent, and have not developed any such defense. They don't have to.
So, when these defenseless sponges are placed in a brightly-lit reef aquarium, trouble can come quickly. It is not uncommon for them to be overgrown by algae, which can block the ostia, and cut the flow of water into the sponge. Thus, being overgrown can obviously lead to the death of the sponge.
Yes, there are lots of fishes and other animals that will feed on sponges. Most hobbyists will only purchase a sponge for a reef aquarium stocked with reef-safe fishes and such, but there are those that mistakenly try sponges in non-reef aquariums due to their bright colors. Regardless of the circumstances, it is imperative that you check the diet of what you have before you add a sponge. A variety of angelfishes, triggerfishes, and filefishes with eat them, as will numerous sorts of nudibranchs, cowries, sea stars, and other invertebrates.
Shopping and Aquarium Care
In general, only aquariums that are set up with a deep sand bed full of life and/or a refugium, etc. can consistently keep many sponges alive. These methods of reef-keeping produce a constant supply of natural foods of varying size, some of which are small enough for sponges. Thus, it seems that at least for now, finding a way to let your own aquarium make natural food for sponges is the best way to go. Really, it's the only way to go unless you just like taking chances, with low odds for success. Non-reef aquarium owners and bare-bottom reefers beware.
Of course, there are exceptions, and it is not uncommon for a few small sponges to occasionally arise from nowhere on the live rock in mature systems that lack deep sand beds/refugia. However, these are typically small specimens of a few specialized species. I've even seen a few large sponges do fine in bare-bottom tanks, too. This is likely because the specimens happen to have a large population of algal/bacterial symbionts, and/or for whatever reason the system they were in carried a lot of bacteria and/or organic matter, etc. Once again though, these are exceptions to the general rule and you should not assume that you will have the same luck.
All of this being considered, if you still want one, it is imperative that you buy the healthiest specimen that you can get. A thorough inspection is important, as any potential purchase should be looked at closely. Look for any discolored areas, and especially any areas that appear milky, like they are filled with gray or whitish mucous. This is a common sign of decay. Likewise, if you can find an air bubble of any size anywhere on or within a sponge, it is likely a sign that the specimen has been exposed to air at some point between collection and being put up for sale, or it is a gas by-product of decay. It will also pay to ask if a sponge has been kept submerged, even if it looks great, as it may have been exposed shortly before your visit and the signs haven't showed up yet. Likewise, you can also ask about how it was shipped. You're paying for it - so ask questions. Any specimens with signs of physical damage should be avoided, too.
If you can find a suitable specimen, you'll obviously want to make sure that it stays submerged all the way to its new home. Make sure the store employee submerges the bag, then fills it with water and lets all the air out, then adds the specimen, and then closes it up while the whole thing is still submerged. I don't even want a little air trapped in the top of the bag - water and sponge only. It's true that corals can be exposed to air with no problems, but sponges are not corals!
Any new sponge must be slowly acclimated to your aquarium (or preferably a quarantine tank). You can very carefully put it in a container with the water it came in and then drip water from the tank into the container, or you can put the whole bag in the tank, open it up and then scoop some tank water into it every few minutes. If you use a drip, you'll need to put the whole container in the tank when you are done to transfer it without exposing it, or conversely, you can dunk the whole bag when you are done if you don't use a drip. Dripping is slower and better, but it gives you more chances to expose the sponge, too.
If you can find out what sort of environment the specimen came from, try to put it in the same sort of conditions in your aquarium. Of course, assuming that its specific origin will be unknown, I suggest that you place the specimen in an area with moderate intensity lighting (not directly under bright lights), and in such a position that it is bathed by a moderate to strong and turbulent current. You should try to position it so that water flows around the body and over the oscula, if you can see them.
Some specimens may have a round or oddly-shaped body with numerous oscula all over, or may even have none that you can see, but these may be signs that the sponge is adapted to currents that come from different directions. This is why a current that is turbulent in nature and not always straight from one direction may be much better for many sponges. You'll have to take a look at what you get and try to figure out what is best, given this information. Conversely, if it has only one (or a few) large osculum you MUST NOT position the specimen so that currents blast directly into it. This would likely push water the wrong way through the sponge's body, as mentioned earlier.
In addition, avoid placing the specimen in direct contact with, or even very close to other specimens, regardless of what they are. Many corals can produce defensive and/or offensive chemicals, and sponges can, too. They can make and exude various sorts of compounds, and some of them may have the potential to injure other specimens. Correct placement may not be easy to figure out, but do your best.
Once you have it in place, keep a close watch on it. If the current is altogether unacceptable, many sorts of sponges will typically close down, as the body will contract and the oscula will close. If this happens and the specimen stays this way for a few days, you should go ahead and try another position with different conditions. Conversely, if the placement/current is great the specimen will likely expand and the oscula will open wide.
For situations in between, you'll need to be patient. Many sponges can and will change their shape to some degree, and can also change the size and orientation of their oscula at times. So, if currents aren't just right, but aren't too bad, a specimen may slowly change itself and adapt to the new environment. As I mentioned earlier, some may even crawl a bit and attempt to relocate themselves to a better position without your help.
Once a specimen is situated, you need to continue to watch it for a while. Over the next several days it is particularly important to look for decay and/or bubbles that may result from an unknown exposure to air. If you see something, but it is only a localized problem, get a razor blade and carefully cut it out. The rest may survive if all other conditions are optimal.
You'll also need to watch for algal growth for a couple of weeks, at the least. If the sponge can't defend itself against algal overgrowth, you'll need to relocate it again, but in an area with lower illumination. If it can't keep the algae away, then it was likely living in relatively dimly-lit waters in the first place. Thus, relocating it to a darker area of your aquarium should not be a problem.
Now, with all that covered, I'll give you just a bit more advice. First, you should start small. When starvation is such an issue, you'll be much better off to start with a very small specimen and see if it grows. If there is sufficient food, a small specimen will get bigger, whereas a large specimen will die if there isn't enough. There's absolutely no good reason to buy a big, beautiful sponge if you aren't sure that your system can provide enough food to keep a much smaller specimen alive and well.
Secondly, do more homework. If you have a certain sponge in mind, if at all possible try to find out as much as you can about that particular type. What environment it comes from, whether it is symbiotic or not, if it is highly toxic (some are), how big it gets, etc. The more you know in the beginning, the greater your chances for success.
- Rupert, E. E., R. S. Fox, and R. D. Barnes. 2004. Invertebrate Zoology: A Functional Evolutionary Approach: 7th ed. Brooks, Cole, Thomson, Belmont CA. 963pp.
- Bergquist, P.R. 2001. Porifera. Encyclopedia of Life Sciences. John Wiley & Sons, Ltd. URL: http://www.els.net/WileyCDA/ElsArticle/refId-a0001582.html