Option 1: Snails are not effective in controlling mosquitoes. They are usually found in freshwater habitats and feed on algae and decaying organic matter.

Option 2: Crabs do not play a significant role in controlling mosquitoes. They are often found in saltwater habitats and feed on a variety of organisms such as small fish, insects, and detritus.

Option 3: Dogfish, which is a type of shark, does not contribute to mosquito control. They are marine creatures and have no direct impact on mosquito populations.

Option 4: The correct answer is Gambusia fish. They are commonly known as mosquito fish and are often introduced into ponds and wells to control mosquito larvae. Gambusia fish are voracious predators and feed on mosquito larvae, thereby reducing mosquito populations in the water bodies where they are released.

In summary, option 4, Gambusia fish, is the correct choice for controlling mosquitoes in ponds and wells.