FISH 133 Spring 2019

This combination of pressure and cold has
strange effects on animals’ bodies.
All animal cells are surrounded by fatty
membranes, which must stay liquid to transmit
nerve signals and shuttle materials in and out of
cells. But under these conditions, they would
solidify. So deep-sea animals must adapt their
membranes to keep them liquid. They do this
by having lots of unsaturated fats – the group
of chemicals that includes vegetable oil - in
their membranes. These remain liquid at low
temperatures and keep the membranes loose.
It’s not just cell membranes. Pressure also has a
crippling effect on proteins, the huge molecules
that do much of the work in our cells, such as
breaking down food for energy. To function,
proteins must be free to change their size and
shape, for instance becoming larger.
This is difficult under pressure. To enable this
deep-sea animals collect small organic molecules
called piezolytes in their cells. These piezolytes
bind tightly to water molecules, which gives the
proteins more space and stops water being
forced into the proteins’ interiors and distorting
them. The deeper an animal lives, the more
piezolytes they tend to have in their cells.
One piezolyte, TMAO, gives fish their ‘fishy’ smell.
TMAO increases with depth, so deep-sea fish
taste fishier than shallow fish. But there’s a limit
to this. As animals take in more piezolytes, their
cells become saltier. Around 8,200m down, it
has been calculated, the cells would be as salty
as the surrounding water. Any more piezolytes
and seawater would rush into their cells,
bursting them.
World’s deepest fish
In line with this, an expedition in 2014 discovered
the world’s deepest fish living 8,145m down.
The new record-holder is the Mariana snailfish
(Pseudoliparis swirei), which lives at depths of
up to about 8,000 metres (26,200 feet) along the
Mariana Trench near Guam. The snailfish has a
bulbous head, partly transparent body and
no scales.
Snailfish are found at many different depths in
marine waters around the world. In deep water,
they cluster together in groups and feed on tiny
crustaceans and shrimp using suction from their
mouths to gulp prey. However, little is known
about how these fish can live under such intense
water pressure.
Exploring the deep
A handful of researchers have explored the
Mariana Trench, but few comprehensive surveys
of the trench and its inhabitants have been
completed because of its depth and location.
To catch specimens of the snailfish, traps with
cameras on were dropped to the bottom of the
trench. It can take four hours for a trap to sink to
the bottom.
After waiting an additional 12 to 24 hours, the
researchers sent an acoustic signal to the trap,
which then released weights and rose to the
surface with the help of flotation. That allowed
scientists to catch fish specimens and take video
footage of life at the bottom of the ocean.
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