cultured algae daily, and have their water changed every-other day. This ensures no pathogens or foreign organisms enter the system and compete with or eat the larvae, and their water quality stays pristine to encourage growth. This is the most fragile stage of an oyster's life history. After about two weeks an oyster will be ready to set. They will develop a small, round discoloration called an eyespot despite not being used for seeing. Their muscular foot will be visible under a microscope. At this point, the larvae can be put in a system with a variety of cultch options. The best cultch is usually full or ground up oyster shell because oysters are naturally attracted to other oyster shell to ensure their future reproductive success. After the larvae settle, they are considered "spat." Three methods of cultivation are commonly used. In each case oysters are cultivated to the size of "spat," the point at which they attach themselves to a substrate. The substrate is known as a "cultch" (also spelled "cutch" or "culch"). The loose spat may be allowed to mature further to form "seed" oysters with small shells. In either case (spat or seed stage), they are then set out to mature. The maturation technique is where the cultivation method choice is made. In one method the spat or seed oysters are distributed over existing oyster beds and left to mature naturally. Such oysters will then be collected using the methods for fishing wild oysters, such as dredging. In the second method the spat or seed may be put in racks, bags, or cages (or they may be glued in threes to vertical ropes) which are held above the bottom. Oysters cultivated in this manner may be harvested by lifting the bags or racks to the surface and removing mature oysters, or simply retrieving the larger oysters when the enclosure is exposed at low tide. The latter method may avoid losses to some predators, but is more expensive. In the third method the spat or seed are placed in a cultch within an artificial maturation tank. The maturation tank may be fed with water that has been especially prepared for the purpose of accelerating the growth rate of the oysters. In particular the temperature and salinity of the water may be altered somewhat from nearby ocean water. The carbonate minerals calcite and aragonite in the water may help