Sponges (phylum Porifera) are considered to be among the simplest living organisms. Almost all of the 5000 species are marine, and all operate as filter feeders, collecting small particles of food from the sea water flowing through their bodies.
Begin by coloring the cutaway diagram in the upper right corner, which shows the circulation of water through a generalized sponge. Color the body wall and the ostia. The size of the ostia has been exaggerated in the drawing; they are actually microscopic in a living sponge. Next, color the filter chambers, the atrium, and the osculum. Color the arrows which indicate water flow.
The body wall of the sponge is perforated by many small pores or ostia (singular ostium), through which water enters the sponge. In simple sponges the water flows into the atrium and finally out a larger opening called the osculum. In more complex sponges with folded body walls, shown here, the water flows through the ostia into small filter chambers, and finally out the osculum. Sponges rely on this flow of water for feeding, gas exchange, excretion, and often reproduction.
Color the enlargement of the single filter chamber in the circle. Next color the enlargement of the collar cell, noting how the water flows through the collar. Finally, color the enlargement of the spicules.
The heart of the sponge’s water system is a unique cell called a collar cell or choanocyte. In simple sponges, the collar cells line the inside of the atrium, and in more advanced sponges, they are found lining the filter chambers. Each collar cell possesses a single whip-like flagellum that beats in a rhythmic fashion. The independent rhythmic beating of many flagella creates a positive pressure within the atrium of the sponge, forcing water out the large osculum and pulling water in through the ostia. As the water moves through the sponge, it passes through the fencelike collar of the collar cells. The collar consists of bundles of fused cilia with spaces in between. Small particulate matter becomes trapped on the collar where it can be engulfed by the body of the collar cell or picked off by motile amoeboid cells and then digested in food vacuoles. Amoeboid cells may also engulf larger particles that get caught at the entrance to a filter chamber or an ostium on the sponge’s surface.
Sponges vary in size from tiny lumpish forms to massive structures. Small skeletal elements called spicules are embedded in the body wall of the sponge and support its structure. In most sponges, the spicules are scattered individually in the body wall, as in this illustration. In the glass sponge, however, and some others, the spicules are organized into an elaborate, latticework skeleton. Instead of, or in addition to the spicules (siliceous or calcareous), some sponges have fibers of a protein called spongin.
The morphology of sponges varies from very simple tubular forms, to those with a filter chamber system (as shown), to complex systems involving more infolding of the body wall and the proliferation of smaller and more numerous filter chambers. The increased number of filter chambers allows more water to be filtered through the sponge; a 10 cm3 (0.6 in3) sponge is capable of filtering 20 liters (5.3 gal) of water in 24 hours.
Color the four different types of sponges. The body of each sponge can be colored the same color as the diagram above or the natural color, as given in the text.
The form of sponges is influenced greatly by the available space, type of substratum, and the strength of the water movement. Most sponges are attached to hard substrata in relatively shallow water.
The purple encrusting sponge is found low in the rocky intertidal zone, often in large patches. This sponge may grow to 2.5 cm (1 in) thick, and its oscula are quite large. The encrusting sponge does not grow tall in areas of heavy wave action that quickly tear and destroy it; in quiet waters the oscula are raised on elevated volcano-shaped projections of the body wall.
Not all encrusting sponges attach to inanimate substrata. The smooth pink pecten sponge is found on the shells of scallops – mutually beneficial relationship. In exchange for the substratum, the sponge covers the scallop with its porous, yielding body, offering some protection from sea star predators.
In the quiet water of subtidal habitats, such as the coral reef, large sponge forms flourish. the azure blue tubular sponge grows very tall.
The boring sponge burrows into the shells of abalones, oysters, and other molluscs. This yellow sponge lives in the tunnels it chemically etches out of the shell. Its tunneling can be extensive, severely weakening the shell. Some species of boring sponge attack corals and are responsible for much erosion of coral reefs.