Neuromag May 2017 - Page 21

Lorrianne Clarke Vision in the Deep Sea: Studies in evolutionary biology Written by Prof. Dr. Hans-Joachim Wagner The deep sea is by far the largest habitat on earth and yet our knowledge about its inhabitants is ru- dimentary at best, and is progressing only slowly. Historic concepts about the deep sea as a lifeless desert have long been proven wrong, just as the common concept about the continual darkness of the abyss. Sunlight plays a minor role between 500 and 1,000m of depth, and is no longer detectable below 1,000m. However, an alternative kind of light is present in the deep sea. Bioluminescence is the major source of light at depths below 200m and is found in most forms of metazoan marine life and across all taxa. Bio- luminescence is light that is typically generated by the organism itself, via light emitters known as luciferins (conserved genetically) and enzymes called luciferases (genetically diverse). Bioluminescence appears to be so powerful that it is thought to have evolved independently more than 40 times. Residual sunlight and biolumi- nescence at depths between 500 and 1,000m, also called the mesopelagic habitat, create a visual environment that is markedly different from our world; instead of an evenly illumi- nated scenery, it consists mainly of point light-sources in varying spatial and temporal patterns, vaguely remi- niscent of fireworks, that are visible at distances up to about 10m. Observations in the “wild” from sub- mersibles, and from specimens re- covered alive from catches in the laboratory have shown remarkably diverse patterns of bioluminescence. This begs questions about the role of bioluminescence. So far, the biologi- cal significance of these often highly elaborate light displays is largely a matter of speculation but the prob- able uses range from camouflage by counterillumination of the ventral side (hatchetfish Argyropelecus, Fig. 1, right), disturbance of predators by release of luminous clouds, commu- nication with and/or identification of sexual mates, luminous lures to cap- ture prey (anglerfish), and illumination of potential prey by “headlight photo- phores” (some lanternfishes). In gen- eral, the wavelengths emitted by the photophores are a bluish green (about 480nm), which closely matches the colour of the downwelling sunlight at mesopelagic depths. In very few cases, however, dragonfish carry light organs under their eyes that emit far red light in addition to the ordinary bluish pho- tophores elsewhere on their bodies. This red light gives them a “private” communication channel and makes them invisible to other animals. Adaptations of visual systems: Eye designs Judging by the relative volume of the optic tectum, vision plays a major part in the behaviour of mesopelagic animals. Given that residual sunlight plays no major role in the mesopelagic habitat, increased visual sensitivity is of utmost importance. Evolution ap- pears to have used two main mecha- nisms that serve this aim: (1) Increasing the pupil size to allow for more light to enter the eye. (2) Optimising photoreceptor sensitiv- ity via several strategies: (i) The vast majority of deep- sea fish use only rods, the more light-sensitive of the two class- es of photoreceptor. (ii) The area of photoreceptive membrane is maximised either by increasing outer segment length to over 100µm or stack- ing shorter rod inner and outer segments to form multiple banks, giving them an advan- tage over humans by at least a factor of two. (iii) Deep-sea fish rhodopsin’s absorption maximum is tuned to the bluish-green colour wavelength of the downwelling sunlight and bioluminescence, and (iv) Furthermore, the visual pig- ment density is unusually high. May 2017| NEUROMAG | 21