Armed with some amazing senses and skills, animals can communicate in many different conditions, some of them extreme and challenging. On this page we have picked out some fascinating examples from the environment featured in the programme Water Worlds.
Light, sound and electricity travel differently in water than air, so marine animals have specialised ways to communicate.
Light travels well only in very clear water. Animals that live in the clear waters around coral reefs use an abundance of startling visual signals, many of them beyond our perception. Most coral reef fish have receptors for up to six different kinds of light and cuttlefish even have receptors for polarised light. Humans only have three, so animals living on coral reefs see a very different world to the one we see.
Coral reefs often have 'cleaning' stations, where a cleaner organism removes ectoparasites from a 'client'. Cleaners often share a similar and rather unusual colour - blue, with a long wavelength component that we cannot see. Amazingly, creatures as diverse as fish and shrimps, from both the Pacific and the Carribean, have evolved the same 'blue' uniform, so potential clients over the whole ocean can understand the cleaner's trade.
Even in the clearest water, light only penetrates to 1000m, but the average depth of the ocean is 4000m, so many animals have to make their own light. 'Dragon fish' use a red spotlight to hunt for prey and to communicate. As dragons are the only animals in the sea who can see or produce red light, this gives them a 'secret signalling' channel.
Bioluminescent algae use light for a different purpose. When fish try to eat them, they produce flashes of light. This acts like a burgler alarm attracting in the predators of the fish who are trying to eat them, often deterring the fish from grazing.
The cookie cutter shark has a more sinister use for light. Viewed from beneath, the soft glow from the shark's many light-emitting cells blends in with dim light filtering from the sky and disguises the predator's outline. But there is a dark patch on the predator's chin, with no bioluminescent cells. Against the glow, the dark chin patch looks like just the sort of little fish a predator such as a tuna is hunting. The big fish darts up for the kill, only to be bitten itself by the cookie-cutter shark.
Sound is also commonly used in the oceans. Dolphins have a 'signature whistle' which they use like a name. One dolphin will produce a distinctive set of whistles, only to have them immediately copied by another dolphin a few hundred metres away. They then produce other whistles, which are probably for exchanging information once contact has been established.
