on the sponge, but whose presence doesn't benefit the sponge. Others appear to be ectoparasites, intercepting and eating foods brought to the sponge by its filtering currents. Still others may benefit the sponge in some manner. Lacking structures for attacking other animals, sponges might seem to be at a disadvantage in the rough and tumble competitive world that constitutes a coral reef's benthic environment. This is decidedly not the case, however. Sponges are powerhouses of chemical synthesis, and many produce highly toxic chemicals. These may serve to make them unpalatable to predators, but similar (or the same) chemicals also may be liberated from the sponge to kill any nearby animals. On a natural reef, where water exchanges are continuous, these chemicals generally act over only very short distances, from a few millimeters to a centimeter or two. In an aquarium, the water movement is not sufficient to flush the chemicals from the system. In such enclosed environments, highly toxic sponges may well have the potential either to kill potential competitors, such as corals, or to stress them so severely that they become diseased and die. Unfortunately, while it is clear that many sponges may be able to do this, there is no practical way to determine if such chemical releases are occurring and what their affect is; however, it is beyond neither belief nor reality that sponges are responsible for many of the mysterious deaths that plague aquarists. Calcarea S ponges with skeletal spicules made of calcium carbonate form the