Many marine animals and other organisms generate light to help them survive at night or in the deep.
Light Generation
- Luciferin produces light when it is oxidised. The enzyme luciferase adds the oxygen, and a cold light is emitted. The typical colour of this light is blue-green, but pigmented layers over the source can change this (to greens, yellow or occasionally reds). Have a look at ‘Jellyfish GFP’ for additional information.
- Photophores are special organs developed by a number of animals. These can gather the light produced (sometimes by the organism’s own cells, and sometimes by symbiotic bacteria) and direct and focus it – depending on the use to which it will be put.
Producing Light for Protection Causing confusion in predators, or improving camouflage in the dark are strategies used by many marine creatures.
- Many Dinoflagellates emit light when they are disturbed. Their usual predators are copepods (which do not hunt visually), and it is thought that the light attracts fish who then eat the copepods.
- Confusing the enemy is always a good tactic, and many of the larger creatures produce spots of light to break up their outline. Presumable visual predators find it difficult to know exactly where to attack! (Look at the image of an Antarctic Krill at night – below.)
- Shedding shiny bits, or leaving luminous clouds behind is a slightly different strategy. Squids are masters of this tactic, and some can squirt out clouds of glowing ‘ink’ when attacked in the dark. The predator attacks the light and gives the squid a chance to jet out of trouble.
- Countershading (being dark above – to make it difficult to be seen from above against the murky depths, and light below – to make hide from below against the bright upper waters) is all very well during the daytime, when most visual hunters are active. Unfortunately some predatory hunt from below at night, using the very dim light to see the silhouettes of their prey. Countershading is of little use in these circumstances, but photophores (directed downwards and generating exactly the same amount of light as is falling on the creature’s upper surface) can provide effective camouflage.
Producing Light for Hunting Deep Sea Anglerfish often have luminous wiggly lures. These hang in front of the gulping mouth, and bring unsuspecting creatures within range.
Sometimes light beams are directed forward to illuminate prey, occasionally with red light that is not seen by most deep-sea animals.
Producing Light for Communication
Some squid can advertise their species, sex and even current mood by varying the patterns and colours of the lights that they produce. This is obviously very useful at night, or at depths where there is little light (but it obviously exposes them to visual predators, so must be used with caution).
