Marine hermit crabs, like the one pictured below, must remain submerged in synthetic or natural saltwater at all times. Their aquatic gills can only breathe while under water. However, they can still be handled and caressed by experienced human hands that enter the saltwalter. If you do this, however, make sure that all aquarium electrical equipment has been unplugged and that there are no other creatures within the tank that can bite or sting.