Muscle tissue is a long-term form of energy storage for protein in the Creatures series. It was introduced in Creatures 2.
In Creatures 3 and Docking Station, excess amino acid is converted into muscle tissue by the liver anabolic organ, and muscle tissue is converted back into amino acid by the liver catabolic organ. When muscle tissue is low, the floating loci organ creates protease. Muscle toxin breaks down muscle tissue.
Creatures are generally born with an initial concentration of muscle tissue.