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 |
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