 Many marine animals that float near the surface are highly transparent , giving them almost perfect camouflage . However , transparency is difficult for bodies made of materials that have different refractive indices from seawater . Some marine animals such as jellyfish have gelatinous bodies , composed mainly of water ; their thick <unk> is acellular and highly transparent . This conveniently makes them buoyant , but it also makes them large for their muscle mass , so they cannot swim fast , making this form of camouflage a costly trade @-@ off with mobility . Gelatinous planktonic animals are between 50 and 90 percent transparent . A transparency of 50 percent is enough to make an animal invisible to a predator such as cod at a depth of 650 metres ( 2 @,@ 130 ft ) ; better transparency is required for invisibility in shallower water , where the light is brighter and predators can see better . For example , a cod can see prey that are 98 percent transparent in optimal lighting in shallow water . Therefore , sufficient transparency for camouflage is more easily achieved in deeper waters . 
