Many colonies take on a green colour because they contain algae. The algae are trapped in the pockets or pores that go all through the sponge. To keep the sponges from collapsing, sponge animals produce rod-like 'spicules' or pillars of silica. Silica is the main substance that makes up sand or glass. The hard spicules, along with the unpleasant smell of wastes given off by the sponge, help protect the sponge from predators.
Some small animals are able to attack the soft, jelly-like cells inside the sponge.
These are the main enemies of the sponges. Sponges die when the water becomes too cold in the fall or winter. They leave small round structures called 'gemmules'. The gemmules drop to the bottom of the pond when the dead sponge rots. These gemmules are hard and tough.