The stickleback fish, *Gasterosteus aculeatus*, has two distinct forms, the saltwater form and the freshwater form. The larger, freshwater form is thought to have evolved from the smaller, saltwater form. Both forms have armour plating on each side of the body. The plates are made of bone and contain a high proportion of calcium. The ectodysplasin gene, *EDA*, codes for a protein involved in the development of armour plates. The *EDA* gene has two alleles, low armour and high armour. Three main morphs of armour plating have been described. Complete morph armour plating: • is found mainly in the saltwater form • has many plates from head to tail to cover most of the body • provides defence against large, predatory fish • limits the growth of the fish. Partial morph armour plating: • is found mainly in the freshwater form • has a reduced number of plates to cover only part of the body. Low morph armour plating: • is found mainly in the freshwater form • has very few, undeveloped plates and no body cover.