Acinula

An acinula (plural: acinulae) is a type of resting cell or spore formed by certain freshwater sponges belonging to the family Spongillidae. These structures are resistant to desiccation, freezing, and other harsh environmental conditions, allowing the sponge to survive periods of unfavorable conditions and regenerate when conditions become more suitable. Acinulae are spherical or ovoid in shape and are composed of a hardened outer layer surrounding a mass of undifferentiated cells, called archaeocytes. These archaeocytes are totipotent and can differentiate into all the cell types required to rebuild the sponge body. The formation of acinulae is a form of asexual reproduction. They are typically released from the parent sponge when it begins to deteriorate due to environmental stress. When conditions improve, the acinula hatches, and the archaeocytes migrate out and begin to differentiate, forming a new sponge. The morphology and structure of acinulae can be important characteristics in the taxonomic identification of some sponge species.