A strain is a variety of guppies that show the same characteristics. When two guppies of the same strain breed, most of the offsprings will look almost identical. This is because their genes are nearly identical. Selective breeding has produced hundreds or more strains of guppies.
When a mutation occurs in a guppy, the guppy can then be bred. Likely, some of the offsprings with also carry the mutation. Then, selectively breed the mutants into a new strain of guppies. Go to the breeding section for more details on selective breeding.
Grass guppies have fine black dots on their caudal and dorsal fin against a solid background. They also usually have a black streak across the sides of their bodies. The one pictured is a blue grass. They exist in blue, red, or purple.
Snakeskin guppies have a chainlike pattern on 90% of their body and fins. They are most common in a yellowish green, although they can also be red, blue, white, and purple.
Lace guppies have a finer chainlike pattern than snakeskin guppies. They can be red, blue, purple, yellow, white, and green.
Moscow guppies usually have a pure dark colour. They are know as the "charmeleon guppy" because their colour changes shades depending on their mood. They can be any colour and they can be combined with other strains, resulting in strains like the snakeskin moscow guppy.