Although many living creatures have skins or coats that absorb and reflect light in a manner that provides camouflage or attracts attention, there are some that possess the impressive additional facility of being able to emit light. The phenomenon of bioluminescence is not known among mammals, birds or reptiles, but occurs sporadically amongst insects, fish, and marine invertebrates. The majority of bioluminescent organisms occur in the sea and they are relatively common at depths below 500 metres, where daylight is effectively absent.
The emission of light by living creatures is often (though not always) based on the oxidation of an organic material, a luciferin, in the presence of an enzyme (a biological catalyst) known as a lu-ciferase. There are a number of luciferins found among the various bioluminescent species, and each one requires a specific luciferase to generate light. This conversion of chemical energy to light is extremely efficient; virtually no heat is produced in the process. The names of these chemicals are derived from the Latin for 'light bearer'. (The name 'Lucifer' has been applied both to a fallen angel and to matches manufactured in Britain about a century ago and immortalized in a line of a First World War song.)
A sophisticated exploitation of bioluminescence occurs among insects. There are more than a thousand species of fireflies in the family Lampyridae within the order Coleoptera. Fireflies are soft-bodied beetles up to 25 mm long with luminescent panels on the rear abdominal segments, supplied with oxygen via an abdominal air-tube or trachea. Although in some species the larvae emit light, generally it is behaviour of the adult insects that is most intriguing. The species Lampyris noctiluca found in chalky areas of the British Isles and elsewhere in Europe has the misleading common name of 'glow-worm'. Figure 7.10 (colour plate) shows a wingless adult female, which emits light from its last three segments in order to attract a flying male. In some types of firefly, the nervous system is able to control the chemical reaction with the luciferin so that the insect emits regular flashes. In one common North American firefly species, Photinus pyralis, the males flash for about 0.3 second at intervals of 5.5 seconds as they fly around, advertising themselves and seeking responses from potential mates. The emission is strongest around a wavelength of 567 nm, which is in the middle of the visible spectrum and easily observed by the human eye. The females generally are not airborne and respond by flashing an invitation about two seconds after detecting and assessing a passing male. The timing of the flashes varies from species to species and males are normally able to identify responses from females of the same type. In another North American species, Photinus consimilis, the females are more responsive to males that have faster-than-average flashing patterns. The best strategy for males with slower flashing patterns is to accompany a fast flasher and wait for females to respond to him.
Fireflies normally do most of their feeding in the larval stage, and those that continue in the adult form usually have a vegetarian diet. Nevertheless, in North America there is a genus of fireflies in which the females exploit bioluminescence to attract meals as well as mates. Females of the genus Photuris adapt the characteristics of their flashes to mimic females of the genus Photi-nus, which enables them to entice Photinus males to approach and become food instead of fathers.
Another adaptation of the basic flashing cycle is found in an Asian species, in which the males assemble in a bush and synchronize their flashes to create an impressive spectacle. But the response time is insufficiently fast for matching the individual light pulses when very large numbers of fireflies are assembled in hedges many metres long. The result is a sequence of bursts of light that start at one end of the hedge and finish at the other.
In New Zealand there is a glow-worm Arachnocampa luminosa that lives in damp dark caves. It does not belong to the firefly family and displays quite different behaviour. The adult form is known as a fungus gnat and displays no bioluminescence. It is the larval form that emits a bluish green light as a means of attracting small insects, which are trapped by sticky threads. The brightness increases with the time elapsed since the last meal, but there are no abrupt changes in light output. After growing to a critical size, the larva is transformed into a pupa, from which emerges the adult gnat. The gnat has no mouth and devotes its brief life to mating and (if female) to egg production.
The sea absorbs light quite rapidly, particularly at the red end of the spectrum. As the depth increases photosynthesis becomes progressively more difficult, and virtually ceases about 120 metres below the surface. Creatures that live below this depth are usually part of a food chain starting near the surface. An unaided human eye cannot detect sunlight 500 metres below the surface, because it has not been optimized for low intensity blue light. At this depth many fish species have large eyes directed upward rather than forward, suggesting that they are looking at the faint blue background above. A dark patch moving across the background may reveal the next meal. To avoid being eaten, the hatchet fish Argyropelecus gigas attempts to conceal itself from predators below by generating a matching bluish light from photophores underneath its body.
At a depth of a kilometre the sun makes no impression at all, so more than half the species of fish at this depth possess a means of generating light. Many of them maintain a colony of bioluminescent bacteria for this purpose. Anglerfish, such as the female melanocetus johnsoni, have a modified dorsal spine with the tip inhabited by light-producing bacteria. The tip is held above the mouth but can be moved independently and the light can be switched on and off. This mobile flashing light acts as a lure; anything that comes to investigate is liable to be eaten. A male of this species is much smaller than a female. He attaches himself permanently to a female, acquiring nutriments by merging blood circulations, becoming little more than a sperm-producing appendage.
Other deep-sea fishes maintain bacterial colonies on their head or side. The bacteria themselves emit the light continuously, but the host fish can control the light output with folds of mobile dark skin or by a mechanism similar to that described earlier in the section on squid colour changes. At the London Aquarium, close to Westminster Bridge, you can see blue flashes coming from the faces of flashlight fish Photolepharon palperbratus. Most light emitted by deep-sea fish is blue, and the eye response has evolved to match. It therefore appears a good survival strategy to be coloured red rather than blue. Some predators have moved on to the next stage in the piscine arms race by acquiring the ability to generate and see light at longer wavelengths. (Technology has achieved an analogous shift to longer wavelengths only during the last few decades, providing soldiers and security staff with infrared-sensitive equipment for observations at night.)
The black dragonfish Malacosteus niger has separate organs for emitting red and blue light. The red light sources are located just beneath the eyes. There, a fluorescent material absorbs blue light and uses the acquired energy to create red light, which then passes through a filter to select the longer wavelengths. The method by which its eyes detect red light is intriguing and quite different from that of human vision. The retinal photosensitiv-ity for long wavelengths is enhanced by the presence of a compound with a molecular structure somewhat similar to chlorophyll, which absorbs both red and blue light.
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