Scuds AKA Gammarus or Caledonian shrimp are small freshwater shrimp amphipods that can grow to about the size of a grain of rice.
Scuds are an excellent source of live food in aquariums and will rapidly colonize the gravel, keeping it clean by eating uneaten food as well as fish wastes. They are omnivorous and will eat almost anything. Scuds also colonize the filter pads and keep them clean while breeding in them, this makes the filters last well over twice as long.
They are extremely resistant to changes in environment. Outdoors they can survive Minnesota winters as long as they have unfrozen water beneath the ice, and indoors have no problems with my 86* F crayfish aquariums.
They are super fast breeders and will rapidly colonize, When scuds mate the male carries the female on his back as they are swimming. The female carries about 50 fertilized eggs in her egg pouch and these are orange in color and show through her semi-transparent body. The young shrimp hatch within the egg pouch and emerge as fully developed young shrimp but MICROSCOPIC in size.