The order of Gobiifromes contains 210 genera and 1,700 species of fish and are some of the smallest known vertebrates. Gobies are primarily marine fish, although they occasionally wander into adjacent freshwater areas. Their pelvic fins are united and form a sucker, which is used to adhere to objects such as plants and rocks. There are some species of sandy substrate dwelling goby which form a symbiotic relationship with shrimp. These shrimp are blind and unable to defend themselves. They will however, dig burrows in the soft sandy substrate in which both organisms may live. In return, the goby alerts the shrimp of incoming danger. There are also species of reef dwelling gobies which exhibit bidirectional sex change. Generally these fishes are protogynus and transition from female to male. Scientist have induced the opposite transition to occur by removing a female in the presence of males. The largest male present will then transition to a female. Likewise when two males were placed in the same area, the largest transitions to female. This is supposedly to have a matching pair of male and female. However it is unlikely this would occur in the wild, as the gobies would be more likely to migrate to a new area as this is more energy efficient.