Light is an important ecological factor that influences organisms significantly. Light is the primary source of energy intake in plants (primary production based on photosynthesis), it warms up air, bottoms, and water and allows direct absorption of heat by animals. Light facilitates the orientation in the environment and the recognition of food and mates.
Numerous adaptations in behaviour and body structure can be derived from light: many plants have a large light-absorbing surface (leaves), eyes and light-sensitive organs are widely spread in the animal kingdom, optical signals are common in plants and animals including marked body structures and colours and eye spots, courtship and ritualised displays, visually controlled predation, or sun compass navigation can be found in many invertebrates and vertebrates.
Light from the sun
Light from our main energy source, the sun, does not arrive at equal intensity and continuity in all environments on earth. There are daily and seasonal variations in light availability that influence organisms in their activity rhythm. Organisms are, dependent on locality and habitat structure, quite differentially exposed to light. The strongest light exposure is experienced by organisms in desert areas close to the equator and those living on high mountains.
In contrast to this, animals living below the canopy of tropical forests experience considerably less light. Animals of the polar zones live several months per year in darkness or dawn conditions. But all this is surpassed by habitats that are not reached by sunlight such as the deep sea, caves, and the interstitial of sediments. In these habitats photosynthesis-based primary production is lacking. Animals living in the deep sea, for example, are confronted with extreme foraging conditions and depend mainly on food input from other habitats such as the rain of organic material deriving from detritus, dead plants or animal bodies, faeces, of accumulated aggregates known as “marine snow”.
Light is rapidly absorbed in the sea and at 1 m depth only 60% is left. Long waves (red colour spectrum) get lost more rapidly than short waves (blue green colour spectrum). The limit of the euphotic zone, where photosynthesis still exists, marks the beginning of the dysphotic zone with a remaining light level of less than 1 %. In clear waters this level is at about 200 m depth. The greatest depth at which atmospheric light can be still perceived is at about 1000 m and animals with particularly large eyes may sense day-night or seasonal light changes even down to 1200 m. Below this level the aphotic zone starts that however may not be completely dark due to bioluminescence or chemically based light emission by black smokers of hydrothermal vents.
Light and diurnal movement
Light may influence deep-sea organisms as exemplified by the diurnal vertical migration of mesopelagic plankton (planktonic organisms inhabiting waters between 200-1000m) and nekton (organisms swimming actively in the water). Animals living at greater depths frequently show small or completely reduced eyes, loss of pigmentation, well developed mechano- or chemoreceptors and locomotion organs for slow energy-saving forward movement. But there are also animals with well-developed eyes and a generally high diversity of forms in the deep sea.
Read about some of the other physical challenges in the deep sea
By Franz Uiblein, a MAR-ECO scientist