Q&A - Sponges
by Rob Toonen
I am just about to order some new live sand from an online vendor, and would like to add a couple of sponges to that order. I have a 55 reef, with lots of live rock, a Royal Gramma, Bangaii Cardinal, Six-line wrasse, and a small Snowflake Eel. There are also a few corals in the tank. I would like to try several different sponges and/or a gorgonian as well. So far all I can find out about rules for keeping sponges seems to be: 1) don't let them hit air, 2) keep them out of direct light, and 3) feed them plenty of phytoplankton. Is that really all there is to keeping sponges? Is there any reason that I should be aware of not to get a sponge?
Well, there are a number of things I want to address in your question. I’ll stick to the issues dealing specifically with sponges, however. First of all, let me say that I only agree completely with one of the three recommendations you have located for sponges so far. Sponges are traditionally considered very sensitive and short-lived animals in the reef trade, but that is likely due to the lack of information about their biology and care, and what information is easily available is unfortunately often misinformation. One of the most common causes of sponge deterioration, I suspect, is that people do not realize that removing the animal from water, even for a second, is likely to doom them to decline and decay for the aquarist at home. Although there are actually a number of exceptions to the rule that a sponge should never be exposed to air, it is always a safe bet for you as a consumer to insist that your sponge never be removed from the water, and to make sure that it cannot contact air either in the bag (it ought to be filled completely without any air) or during transfer to your aquarium (the animal should be bagged and released under the waters surface). That is the only one of the rules you have found that I do not feel needs to be explained with caveats before following it.
The second rule that you have found, "keep them out of direct light" is simply wrong for many sponge species. Some sponges are actually more dependant on light than are the corals, and by keeping them out of strong light, you would be dooming them to slowly wasting away. Let me back up a step here. The first thing I should emphasize is that there is no "general" recommendations possible for any group as diverse as sponges. Sponges are among the diverse of all multicellular animals on the planet in terms of their nutritional mode. What I mean by that is that sponges range from entirely predatory (supported only by digestion of particles captured by suspension feeding) to entirely autotrophic (supported entirely by photosynthetic output of a host of symbionts ranging from bacteria to true algae) and nearly every combination in between these two extremes. I have said a number of times during talks that asking about an orange sponge is akin to asking about the care of an orange fish – not very useful, and surprising as it may seem, a question to the effect of “what are the requirements for an orange fish?” is actually easier to answer than one for an orange sponge! People seem to think that because sponges look so simple and similar to us that they are simplistic or alike in their requirements. Unfortunately, this just isn’t true. Sponges are at least as variable as corals on the whole in their requirements and habitats on modern reefs, and in fact, while there has never been a cnidarian found to produce 100% of its energy requirements via photosynthesis, some species of sponge are actually primary producers – more similar plants than to animals in terms of their physiological requirements! That makes it very hard to make generalizations about the care and requirements of sponges without some idea of the species in question.
Having said that, perhaps it is obvious that I disagree with the blanket recommendation to keep all sponges out of direct light. Sponges species that are primary producers, and rely almost entirely on the photosynthetic output of symbionts for their energetic requirements will obviously not thrive unless well lit. The advice to shade sponges is usually provided in order to minimize the growth of algae on the surface of the sponge. Algal growth on the sponge not only smothers the sponge because of reduced water exchange, it also decreases the amount of food a sponge can collect via either photosynthetic output of symbionts or filtering of the water. This ends up being a double-whammy for the sponge and is a serious condition that needs to be avoided. However, that does not mean that a healthy sponge cannot thrive in direct light in a well-maintained reef tank. In fact, for some species that rely heavily on photosynthetic output of symbionts (such as the beautiful blue and purple sponges Haliclona, Halichondria, and Callyspongia), placing the sponge in shadow may be delivering a death sentence through slow starvation. Most of the sponges available for sale in the hobby grow in open water, primarily on reef walls and ledges, and are frequently tolerant of a wide range of light intensity in the wild. The popular finger, tree and vase sponges found in the hobby (such as Axinellid, Amphimedon, Agelas, Aplysina, Ptilochaulis, Niphates and Iotrochota) tend to be most common on walls or ledges in open water areas that often support large colonies of gorgonians as well. If you saw my article on gorgonian care (Feb. 2000), then you have some idea of the general water flow conditions required by many popular reef sponges as well (I’ll come back to this below).
The bottom line is simply that algae should not grow on any healthy sponge kept under proper conditions of light and water flow, and if algal filming is a problem for sponges in your aquarium, I would suggest that you first try to isolate the cause of the problem rather than treating the symptom of algal growth by placing your animal in the shade. That cause may be nutrient loading in the tank on the whole, it may be the declining health of a sponge mistreated during shipping or acclimation, or it may be a lack of water flow, but whatever the cause, shading out the algae growing on the sponge is unlikely to lead to it suddenly thriving in the tank; instead, consider vacuuming or blowing the algae off the sponge and trying to address issues of flow to see if the animal can be revived.
The last topic that you mention is feeding. Again, there is no way to make a recommendation that will cover every species of sponge, but before we can discuss feeding I first have to explain how sponges work. First, sponges are amazingly efficient pumping machines; for example, an single Leucandria about the size of a pencil pumps 22.5 liters (or roughly 5.5 gallons) of water through it's body every day. This volume is even more amazing when you realize that the cells responsible for pumping this water (called 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 (or flagellum) surrounded by a collar made of other even smaller hairs (called microvilli). The flagellum waves back and forth from base to tip, pushing water ahead of it as it sways. The flagellum of each cell beats at it's own pace, and pushes water away from itself that creates a slow drawing water into very tiny openings (called ostia) all over the surface of the sponge (the largest of which are usually 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 micrometers (that's about the size of an average bacterium), and pushes the water away from itself towards a common exhaust system (the oscula – these are the large holes you can see in the surface of the sponge). As water moves through the sponge, oxygen diffuses into the cell, while carbon dioxide and other wastes diffuse out of the cell into the ‘exhaled’ water destined for the oscula. Some free cells (called ameobocytes) cruise around through these water channels and ingest small algal cells, protozoans, detritus and other organic particles in the range of 2-5 microns (about the size of the phytoplankton Nannochloropsis). Other freely moving cells (known as archeocytes) then take these captured particles and complete the digestion of them before passing nutrients along to the rest of the sponge body.
Sponges also take advantage of something known as the Bernoulli Principle to aid them in moving water through their bodies. The oscula (exhalent holes) are typically raised into little volcanoes that generate lift as water flows over them. This lift generated by water flowing over the raised bumps on the surface of the sponge act to create suction that aids to pull water out of the sponge, and helps the animal to conserve energy. Because of this use of current to generate water flow through the sponge, a simple change in position or flow regime around a sponge can result in the animal no longer being able to filter water or possibly even having water forced through the body in the wrong direction! That is why flow rates and relative position of the sponge is so important. Unfortunately, factors such as lighting and water flow are rarely considered when discussing the shipment and introduction of a sponge to a new aquarium. Even though sponges are capable of very slow movement (and I mean very slow), you should try your best to recreate the position and general flow regime in which a sponge is thriving if you want to introduce one successfully to your tank.
OK, so that is how sponges actually accomplish the water pumping that allows them to feed, but I only briefly mentioned what they are actually feeding on above. Well, it turns out that despite the fact that sponges are effective filters for removing tiny particles from the water, dissolved organic matter (DOM) turns out to be extremely important to the nutrition of the few species of sponges that have been studied. For example, studies on three species of Jamaican sponges showed that 80% of their energy came from sources too small to be seen under a microscope (the researcher attributed this portion of the diet to DOM), while the other 20% was comprised primarily of bacteria and tiny phytoplankton. Most estimates of the relative amount of dissolved organics and bacteria in a reef tank are ten or more times higher than that of a natural reef, so there is little reason to expect nutrient limitation for sponges reliant on these food sources in any well-established reef tank. However, the use of ozone, UV, heavy skimming and the like will certainly reduce the availability of such food for sponges in a reef tank, and people obsessed with ultra-pure water conditions may find difficulty in maintaining sponges for that reason. As explained above, the majority of the particles captured by a sponge will be between 0.1_1.5 microns in length. That means that although many sponges can and do capture some small phytoplankton (such as Nannochloropsis), they are simply unable to capture large phytoplankton (such as Tetraselmis, Isochrysis, Spirulina, Chaetoceros, Pavlova or the like), so simply feeding phytoplankton is not a guarantee that the sponge will be able to consume what you are adding to feed it. Unless you’re adding Nannochloropsis, chances are that the sponges will be unable to ingest the phytoplankton food you’re adding.
One more thing that I probably should mention is that although most people think of urchins as herbivores, they are not. Most are omnivorous, and some are actually predatory in nature. Some species are more prone to eating sessile invertebrates (such as soft corals and sponges) than others, of course, but it is safe to say that many of the urchins available in the reef hobby will eat the tissue of some sponges if they are hungry. Numerous studies have shown that urchins naturally eat a wide variety of sponges, and if you happen to be fond of urchins and have a bunch of them in your tanks, you may find it difficult to keep sponges lacking strong defensive chemistry for any period of time...