The Animals That Are Almost Invisible.
Transparency is common, even dominant, in animals of the open sea, especially those that live in relatively shallow waters. It is found in plankton of many species, as well as larger animals such as jellyfish, salps (floating tunicates), and comb jellies. 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 mesogloea 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. Gelatinous planktonic animals are between 50 and 90 per cent transparent. A transparency of 50 per cent 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 per cent transparent in optimal lighting in shallow water. Therefore, transparency is most effective in deeper waters. Credit: Wikipedia.
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