Gemmules are small buds in sponges which are used to reproduce asexually and allow populations to survive adverse conditions. They are resistant to freezing, drying and lack of oxygen, conditions that would kill adult sponges. They allow the sponge population to recover quickly when conditions improve.
Some marine species of sponges actually produce gemmules as a normal form of reproduction, in addition to their use in adverse conditions. Freshwater species of sponges in particular create them to survive winter conditions, and the gemmules are almost totally inactive until conditions improve. In both cases, gemmules are small masses of cells surrounded by protective membranes and supplied with reserve food granules. The protective membranes are often reinforced by spicules, small hard structures also found in the bodies of many adult sponges.
Gemmules are not the only asexual reproduction used by sponges. Some sponges use root-like extensions, called stolons, to spread to new areas. Individual sponges sometimes are fragmented as well, and each part forms a new sponge. Most sponges also reproduce sexually and produce both male and female reproductive cells, called gametes. Male gametes are released into the water current, and only fertilize the female gametes of another sponge when they happen to encounter it and are captured by their filter-feeding mechanisms.