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Aquarium.Net June 1997 Sponges, out from under the counter...Ronald L. Shimek

Ron Shimek's artilce on Reef Sponges, Aquarium Net has numerous articles written by the leading authors for the advanced aquarist

Sponges, out from under the counter...

by Ronald L. Shimek

Sponges are probably the simplest animals that most people ever encounter, and can be quite common in both marine and fresh-water communities. Under appropriate conditions they can make good additions to both marine and freshwater aquaria.

Nonetheless, their attributes and care are generally poorly known by aquarists. I rather like sponges; aesthetically, they are colorful and often have pleasing morphology. As a biologist,

The red sponge at the bottom is overgrowing and killing the grayish coral. Photographed in the Caribbean.

I find them fascinating because they can do so much with so little. So, I thought I would discuss some aspects of their biology briefly and then discuss how some sponge may be kept by those aquarists who would like to add some diversity and color to their captive ecosystems.

Organisms can be grouped by their "level" of organization. The level any organism belongs to can be assessed by determining the predominant type of organization found in the animal. Most animals are made of groups of cells. These cells can be organized into tissues. Tissues, in turn, can be organized into organs. Finally, organs can function as organ systems. Humans, mollusks, arthropods, and annelids can be considered to be organized at the organ-system grade of structure. Cnidarian animals like corals or sea anemones have few organs, but most of their cells are arranged in tissues, and are considered to be at the "tissue" or "tissue-organ" grade of structure. Sponges, however, are so primitively organized that their cells are not even made into tissues. Sponges are at the cellular grade of structure. In effect, sponges consist of cells and a rather large amount of extra-cellular material wound around some open spaces that function as water canals or channels (Kozloff 1990; Ruppert and Barnes 1994).

Sponge cells

The basic sponge cell looks rather like a wayward Amoeba . These cells have the name of archaeocyte, but calling them amoeboid cells works just as well. For those who either have blessedly forgotten any basic biology courses they have taken or who have never taken any,

The Northeastern Pacific sponge Stylissa stipitata , being eating by the nudibranch Discodoris heathi . The sponge is about 60 cm high. Note the trail where the snail has eaten its way up the sponge. It took the nudibranch about 5 days to eat this sponge.

an Amoeba is an organism consisting of only one cell. It lives rather like the blob in the old horror picture of the same name. Amoebae move by sending out lobelike extensions of their bodies and pulling themselves along behind those extensions. To eat, they engulf small prey with their body surface and they digest the prey internally in the cellular goo that constitutes the body. To reproduce, amoebae simply split into two smaller amoebae and these go on their merry way searching for more stuff to engulf.

Well, sponge amoeboid cells work pretty much like amoebae, except that in large groups, they rather loosely collaborate to make sponges. In most animals, the constituent cells are tightly bound to one another with special cell-to-cell junctions (called "tight junctions" or desmosomes). These junctions hold the animals together and mean that once a cell has reached maturity, it generally is tightly fastened to its neighbors (with the exception of cells like blood cells, of course). Sponges lack these tight junctions and this property sets them apart from all other animals. Because of this, and some other peculiarities of structure, most authorities have gone so far as to put sponges in a separate subkingdom of the animal kingdom (Kozloff 1990; Ruppert and Barnes 1994).

Sponge cells are so loosely bound to one another that it is actually possible to shake a sponge apart. One exercise that I have had some of my classes do was to use a blender to homogenize a small piece of marine sponge with some sea water. After a few minutes, the concoction typically looks like a gray milk-shake. This is poured through a coarse filter (a piece of nylon stocking works fine) to remove the big chunks, and a small amount is put into a beaker and some clean sea water is added. The beaker is covered and put aside and not disturbed for about a week.

Upon examination after a week or so, many little sponges will be found all over the sides and bottom of the beaker. The blender's action is to cause the sponge cells to separate into individual cells by physically severely beating them. When they are put into the beaker, these cells seek out other cells and reaggregate into viable little sponges. If this exercise is attempted with any other animals, the blender simply destroys the cells as they cannot separate from one-another.

As I have noted, the basic sponge cell is an amoeboid cell called an archaeocyte. Many other cell types have been named in sponges (Kozloff, 1990; Ruppert and Barnes, 1994), however, these are just amoeboid cells given different names due to their position and function. For example, if one of the amoeboid cells is seen secreting a piece of spicule or some protein used in the sponge matrix, that cell can be called a "sclerocyte" which is the name given to all cells secreting skeletal material. At some other time that same cell could be given a different name, depending on where it was or what it was doing at the time. All sponge cells except eggs and sperm can change into amoeboid cells, and crawl away to take up some other function in the animal. What determines which cell does what at any given time is unknown.

Sponge Morphology

The basic sponge is REALLY a simple animal. It consists of a cylinder lined inside and out by cells. In between the outside layer of cells and the inside layer of cells is the area of the matrix (also called mesoglea, a name which means "middle jelly"). The cylinder wall is perforated by many tiny pores connecting the water outside of the sponge cylinder with the inside cavity. These pores give sponges their scientific name of Phylum Porifera (Porifera is derived from Greek words meaning "pore bearing").

The cells lining the inside of the cylinder each have a single beating flagellum (a long hair-like cellular process - these "hairs" are REALLY tiny, by the way, about 0.000004 inches wide and about 0.00002 inches long) growing out of them, and around the base of this flagellum the cell surface is a elaborated into a ring or collar of very tiny projections called microvilli (microvilli means "tiny fingers" and is a pretty good description of what these projections look like). Because of this collar, these cells are termed choanocytes or collar cells. When the flagella of the choancytes beat they force water out of the cavity of the cylinder and more water is sucked in through the pores. The large opening of a sponge where water exits is called the osculum, and the tiny pores over the surface are called ostia (the singular is ostium) (Reisweig 1971; Kozloff 1990; Ruppert and Barnes 1994).

For many links to further information on sponges, go to these sites:

http://www.york.biosis.org/zrdocs/zoolinfo/grp_spon.htm

http://www.tcd.ie/People/Bernard.Picton/porifera/index.html

As this incoming water passes over the collar cells, bacteria in it are trapped by the collar processes and ingested by the cells. This is the way sponges eat. Generally, it is considered that bacteria and other similar-sized particles of organic material are the only things that sponges eat.

A large, 1 m high, Caribbean sponge, possibly Cribrochalina

When some of the collar cells have eaten sufficient food, and have become packed with storage chemicals such as fats, they may pull in their flagellum, let loose of their neighboring cells and wander off into the mesogleal matrix of the sponge. Food storage granules or vacuoles may be transmitted to other cells as the animal moves along may just be used as the fuel for the trek through the matrix. Sponges lack a circulatory system, and these wandering cells are one way materials can be moved from cell to cell.

Good illustrations of collar cells can be found by following these links:

http://www.ucmp.berkeley.edu/protista/choanos.html http://www.ucmp.berkeley.edu/porifera/pororg.html

Although most sponges eat only bacteria and the like, many sponges may contain symbiotic algae either zooxanthellae (dinoflagellates) or zoochlorellae (chlorophytes) in their bodies. As with other animals containing algal symbionts, the sponges get some nutrition from these algae (Hill 1996). There have also been some recent reports of truly carnivorous sponges that eat small crustaceans. This mode of feeding may be common in deep-sea sponges, however, much work needs to be done to confirm this (Vacelet and Boury-Esnault. 1995).

Basically, then a simple sponge is a cylindrical animal with holes through it which pumps water through itself filtering out food. The shape of the sponge is determined by the arrangement of the water canals connecting the outside to the inside and by the amount and type of the matrix material. This matrix material is largely the skeletal material of the sponge. The arthropod exoskeleton and the human endoskeleton largely function as a series of levers to transmit muscular force. Not so with the skeletons of sponges. The sponge skeleton simply gives shape and form to the animal.

Depending on the sponge, the skeleton can be made of either mineral or proteinaceous material or both. The mineral material in sponges is either calcium carbonate or silica. Calcium carbonate is found in relative few, generally small, sponges. Silica, silicon dioxide, is the mineral component of most sponges. The basic form of either of these minerals is as series of crystalline slivers called spicules. These can range from less than 0.001 m (0.04 in) to over 8 cm (3.75 inches) in length depending on the sponge. Generally the spicules are glued together with some protein, but they may be just embedded in the proteinaceous matrix. In one large group of sponges, the Class Hectaxinellida (commonly called the glass sponges), the spicules are fused to form a rigid structure.

In many sponges there is a proteinaceous component to the skeleton. Proteins may be used to bind the mineral spicules together or the proteins may be secreted in a large intertwined and interconnected mass. Some of these latter sponges lack mineral spicules altogether and their skeleton consists only of this protein mass. Bath sponges are the skeletons of these sponge animals. The animals are collected, brought ashore, killed, allowed to rot, and the tissue residue is cleaned off and the remaining skeletons sold. As with many fisheries, over harvesting has severely impacted the target species, and the bath sponge animals are now quite rare through parts of their original range.

Sponges reproduce both asexually and sexually, and both means are important. Asexual reproduction is facilitated by the tenuous nature of the connections between the various sponge cells. During disturbances, such as the wave turbulence generated by a storm, shallow-water sponges may be severely battered, breaking into numerous fragments. These pieces may be tossed around by currents and spread over a wide area. When the waters become calm again, any fragments in acceptable habitats are able to resume growth and can reattach to the bottom (Wulff 1991). Some years ago a researcher working on sponge ecology in a Caribbean bay found about 170 individuals of one sponge species growing in a portion the bay. Later examination using DNA analysis showed that he actually found one sponge individual growing in the bay - only in 170 places (H. Reisweig, Personal comm).

Freshwater sponges have an overwintering stage called a gemmule. Gemmules are small clumps of sponge amoeboid cells that are covered with a thick protective proteinaceous coat. The gemmules are about the size of fine grain of pepper and can be produced by the billions. Many freshwater sponges live in transitory habitats, for example, ponds that dry up in late summer. Gemmules produced by sponges in such ponds can blow in the wind like dust or be transported on waterfowl feathers from pond to pond. When they arrive in a new habitat, the archaeocytes inside can break free of the covering and start over again in a new habitat.

Blowing around like dust can spread the fresh-water sponges far and wide. The largest sponge I have personally seen, about the size of bathtub, was growing in a snow-melt catch basin not much larger than the sponge. The basin was in alpine tundra, at an elevation of over 10,000 feet above sea level on the Beartooth Plateau in Northeastern Yellowstone Park. The basin was not much larger than the sponge, and the sponge was brilliant green, presumably from some symbiotic algae living in its tissues.

Sponges can reproduce sexually as well. They are hermaphroditic, but act only as one gender at a time, producing either the eggs or sperm. These gametes are produced by amoeboid cells that have moved into the mesogleal areas where they develop into either sperm or eggs. As this process reduces the genetic material in each cell by half, it is irreversible. Once the sperm and eggs have matured, and presumably in response to some environmental factor, the animals can spawn. The males release sperm into the water, often in large plumes. Females can suck in water containing the sperm. Often their choanocytes will actually ingest the sperm, retaining the sperm nucleus in a cavity in their cell body. Then that choanocyte will change into an amoeboid cell and move to where an egg is found in the mesoglea. It will pass the sperm remnant to the egg, resulting in fertilization. Development to a larva occurs, and several different types of larvae have been described in sponges. In any case, the larvae typically leave the sponge, swim to a new locality where they metamorphose into a small sponge.

More information on sponges may be obtained by following this link:

http://www.oit.itd.umich.edu/bio/Porifera/shtml

Sponge Ecology

Sponge structure is reasonably simple; however, as with many animals that are structurally simple, their ecological interactions may be relatively complex. Sponges can be viewed simply as pump-filter modules living on material in the water surrounding them. As adults they don't move (except by fragmentation) and are essentially glued to the substrate they are on. From the aspect of many other organisms in the environment, sponges are essentially either a substrate to utilize or a meal waiting to be encountered. Sponges have effectively no behavior - it is hard to have any behavior when you have no nerves (let alone a nervous system) and no muscles to move with. Consequently, if they are to survive they must have some other means to combat predators and to successfully compete for space to live.

Sponges typically compete for space and deter predators by chemical means (Porter and Targett 1988). They can't bite, sting, or flee from predators - but they surely can poison them. Many sponges are chemical factories producing some of the most poisonous materials found in nature. Depending on the sponge, they can release the chemicals actively in growing regions to poison corals and other animals that would tend to overgrow the sponges. Additionally, the chemicals may be stored in their cells and released only when a predator bites into a sponge. In some sponges as well, some of the spicules appear to be "designed" to assist the chemical action by puncturing or lacerating any predator foolish enough to try to eat them.

The presence of such potent poisons in a sponge implies that the sponge is responding to predation. After all if there are no predators, why have the anti-predator chemicals? Potent chemicals typically confer some protection against most predators, however, if some predators have some enzymes that allow them to detoxify the chemical, then they have an advantage over other predators in the exploitation of that sponge. This puts the sponge at a disadvantage, and if some of the sponges have yet more potent chemicals, then they get an additional advantage. I think the positive feedback loops here are obvious, and basically this results in the co-evolution of a predator and prey each "evolutionarily racing" to win over the other (Proksch 1994).

The sponge is under selective pressure to produce ever more potent poisons, and the predator is under selective pressure to develop some better means to detoxify the poison. The final effect of such a race is that many sponges are absolutely immune to the predation of most predators - but that there are a few predators that can eat and, in fact, are obligately bound to eat only those sponges. By the time a group of species has gotten to this stage, the prey animals are spending a lot of their metabolic energy producing and containing these poisons, and the predatory animals have many specific enzymes adapted to detoxifying them. The predator can't go eat something else as it also has a finite limit on its detoxifying production. Both species are near their limits, as both have only finite metabolic capacities either for poison production or for detoxifying enzyme production.

Sponges and the Aquarist...

Well, what does this all mean to the aquarist? The first point to remember is that the sponge morphological plan puts one significant constraint on those who would like to keep them. Consider that a sponge is effectively a collection of very fine water channels held together by some stiff goo and slivers of either glass or calcite. Those fine water channels are really tiny, and if they get exposed to air, small air bubbles can be trapped in them. It is effectively impossible to get the air out. The bubbles block the channels. As the animals feed by pulling water through the channels, blocked channels result in a loss of feeding currents, and this results in the starvation of those cells dependent upon those currents. Furthermore, with the exception of some very few sponges that live in the intertidal zones, these animals are not adapted to the presence of air in contact with their cells. Air next to these cells kills them. So, in addition to the death of the cells downstream of the bubbles, those cells adjacent to the bubbles die. Finally, decomposition byproducts of those deaths result in more gases and more deaths. The moral here is clear. Sponges should never, ever, be exposed to air. With few exceptions even short exposures to air will kill them.

Secondly, sponges need to feed. Although this might seem obvious, it is an amazing discovery to some folks... Large sponges can filter the bacteria out of an amazing amount of water; in some cases several thousand liters per day. They are very effective at removing bacteria; sponge beds have even been designed and used to detoxify sewage outfalls. Most aquaria have moderate amounts of bacteria in the water, but not enough to support that kind of filtration. Large sponges will likely die in most aquaria unless those sponges have zooxanthellae or some other supplemental food source. For most sponges, it is probably impossible to provide much in the way of supplemental food. There is little we hobbyists can add to the water that is either of the appropriate size or composition. Consequently, my advice would be to stick with small sponges and start sparingly. If the animals do well, perhaps add others. Small sponges often are found growing on live rock and these may be a better way to start with than a specific purchase of a larger animal. Sponges will also compete in our systems with other bacteriovores. These other animals would include tunicates, some clams, and many SPS corals. There will be a finite limitation to the number of these animals that any one system can have. So assess the array of organisms in the system before purchasing additional animals.

Additionally, sponges generally need to have silica in the water, and they need a rather significant amount of it. Normal sea water is saturated with dissolved silica and the sponges have no problem in nature. However, many aquarists use RO/DI water and specifically treat the water to remove silica. This removal assists in the control of diatom blooms, but will also significantly limit the number and types of sponges that can be grown.

Sponges, and indeed, all aquarium animals, are not passive blobs sitting on rocks. They are aggressive and capable of damaging many other organisms - including in some cases, the aquarist (Toonen 1996). Many sponges are quite capable of killing and overgrowing nearby organisms such corals, soft corals, tube worms, and bryozoans (Porter and Targett 1988). Other sponges will chemically bore into the coral skeleton and dissolve it from underneath the living coral veneer on top. Before a sponge is purchased, it will probably be of benefit to examine some pertinent references to determine some of these particular aspects of the sponge's biology. In many cases, close examination of photographs in many of the standard aquarium references can provide clues as to the important aspects of the biology of the sponge. For example if you see a photograph where the sponge is growing next to a live coral, and you can see coral skeleton shapes under a thin layer of sponge, then this sponge can kill and over grow that coral. This might be a sponge to either avoid or to place in some area of a tank where it will not encounter a coral.

There is a final aspect to the predator-prey interactions that has to do with sponges, but only indirectly. That is the attempts to maintain their specific predators in a system without them. Probably the most commonly imported invertebrates that prey on sponges are nudibranchs in the group called the "doridacea." These nudibranchs are characterized by a relatively smooth back, and a tuft of gills surrounding the anus on the dorsal midline. Dorids typically eat either sponges or bryozoans, and typically each nudibranch is obligately bound to a particular species or small group of species of prey (Bloom 1976; Beeman and Williams 1980). Importing such animals for aquaria, without providing their food is simply barbaric. The animals will simply starve to death in the aquarium. Occasionally, they will survive for a while on the sponges found in the aquarium. The nudibranchs, however, generally can eat sponge flesh faster than the sponges can produce it, and they will soon devour all appropriate sponge and then they will starve to death.

In the those tanks with adequate bacterial supplies, and enough silica in the water, sponges can be an attractive addition to the array of animals. In these sorts of tanks the sponges provide a natural appearance to the artificial reef. They require minimal care; if the system is right for them they will grow, generally relatively slowly, although they can grown rather rapidly, if conditions are suitable.

References Cited:

Beeman, R. D. and G. C. Williams. 1980. Opisthobranchia and Pulmonata. pp. 309 - 354. In: Morris, R. H., D. P. Abbott and E. C. Haderlie (Eds). Intertidal invertebrates of California. Stanford University Press. Stanford. 690 pp.

Bloom, S. A. 1976. Morphological correlation between dorid nudibranch predators and sponge prey. The Veliger. 18:289-301.

Hill, M. S. 1996. Symbiotic zooxanthellae enhance boring and growth rates of the tropical sponge Anthosigmella varians forma varians. Marine Biology. 125: 649-654.

Kozloff, E. N. 1990. Invertebrates. Saunders College Publishing. Philadelphia. 866 pp.

Porter, J. W. and N. M. Targett. 1988. Allelochemical interactions between sponges and corals. Biological Bulletin. 175: 230-239.

Proksch, P. 1994. Defensive Role for secondary metabolites from marine sponges and sponge-feeding nudibranchs. Toxicon. 32: 639-655.

Reisweig, H. W. 1971. Particle feeding in natural populations of three marine demosponges. Biological Bulletin. 141:568-591.

Ruppert, E. E. and R. D. Barnes. 1994. Invertebrate Zoology. Saunders College Publishing. Philadelphia. 1056 pp.

Toonen, R. J. 1996. A reefkeeper's guide to introductory invertebrate zoology. Part I. Sponges. Aquarium.Net Cybermagazine. November 1996.

Vacelet, J. and N. Boury-Esnault. 1995. Carnivorous sponges. Nature. 373: 333-335.

Wulff, J. L. 1991. Asexual fragmentation, genotype success and population dynamics of erect branching sponges. Journal of Experimental Marine Biology and Ecology 149: 227-248.

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Last modified 2006-11-18 20:15
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