NATURE
WildAustralia
JULY. AUGUST 2019ILLUSTRATION: PAVEL RIHA, UNIVERSITY OF SOUTH BOHEMIA, CESKE BUDEJOVICEJOHN PICKRELL
is a former AUSTRALIAN GEOGRAPHIC editor.
Follow him on Twitter: @john_pickrell38 Australian Geographic
V
ERY LITTLE
surface light
penetrates
ocean depths. Even at
just 100m down in
clear tropical water,
only 1 per cent of the
visible light spectrum
penetrates and most of it is blue.
Because of this, it used to be thought
that deep-sea fish species, living
below about 1500m, are colourblind.
But the Sun is not the only source
of light in ocean waters. Recent
research suggests many sources of
bioluminescence – including jelly-
fish, anglerfish and microorganisms
- could explain why some deep-sea
fish species seem to have surprisingly
excellent colour vision.
The eyes of vertebrates (fishes,
amphibians, mammals, birds and
reptiles) have two kinds of cells –
called cones and rods – containing
light-sensitive pigments (photopig-
ments) known as opsins.
Cone cells have up to four types
of opsins that detect different colours,
typically in bright conditions. Rod
cells are much more sensitive to light
and dark contrast and can detect very
low light levels. However, about
99 per cent of vertebrates, including
humans, have rod opsins that come in
only one type and can’t be used to
detect different wavelengths of light
and therefore colour.
Humans have cone cells that detect
wavelengths of red, green and blue
light, allowing us to see in colour
during the day. Our rod cells allow us
to distinguish shapes and contrasting
patterns in very dark conditions.
Now an international team that
included University of Queensland
(UQ) scientists has carried out a new
DNA analysis of genes that produce
rod opsins in 101 different deep-sea
fish species and they’ve made a very
surprising discovery.
They found a number of different
types of rod opsins in 13 of those
species, such as the tube-eye
(Stylephorus chordatus) and lantern
fishes (Benthosema sp.). And they
found a staggering 38 rod opsins in
one species, the silver spinyfin
(Diretmus argenteus), making it the
vertebrate with the greatest number
of photopigments yet detected.
The rod opsins in that species
“cover the range of [wavelengths of ]
the residual daylight, as well as the
bioluminescence spectrum present in
the deep sea”, the authors explained in
the journal Science in May. What this
suggests is that fish in an environment
where bioluminescence produced by
other creatures is the major sourceof light have evolved special adapta-
tions in their eyes to be able to
discriminate between light coming
from different animals.
“There are many colours of biolu-
minescence down there, and it mainly
appears in f lashes,” Dr Fabio Cortesi,
a lead author of the study and a
researcher at UQ’s Queensland Brain
Institute, told reporters, adding that
“[deep-sea fish species] might have
evolved specific behaviours hard-
wired to different colours. If you want
to survive down there you need to
quickly decide if you want to avoid
being eaten or eat what you see.”
Because deep-sea fishes can’t
survive at surface pressures and
temperatures, it’s almost impossible
to test the findings behaviourally.
However, analyses of the complete
sets of genes – the genomes – of
these fish species have been cleverly
used in this case to make inferences
about the systems of vision they have
at depth.
Intriguingly, the species found to
have a range of rod photopigments in
their eyes are not all close relatives,
and some may be on evolutionary
lineages that split more than
100 million years ago. This means
that this unusual system of vision has
evolved separately in the deep sea on
a number of occasions.
Before this new study, most
students would have learnt in
school biology classes that cones are
responsible for colour vision and rods
are responsible for detecting bright-
ness or contrast in dim conditions.
This surprising finding has revealed
this might not always be the case.A fish-eye perspective
Strange visual systems not seen in any other animals have evolved in deep-sea fishes.
These deep-sea fi shes are now known
to have enhanced vision: (top to bottom)
the silver spinyfi n, lanternfi sh and a tube-eye.