Wikipedia comments about sponge gemmules:
Gemmules are internal buds found in sponges that are the result of asexual reproduction, and are a response to a hostile environment.They are resistant to desiccation (drying out), freezing, and anoxia (lack of oxygen) and can lie around for long periods of time. These are similar to a bacterium's endospore. Gemmules are made up of amoebocytes surrounded by a layer of spicules and can survive conditions that would kill adult sponges. When the environment becomes less hostile, the gemmule resumes growing.
- Gemmule Wikipedia. Accessed 23 May 2010.