18 April 2020 | New Scientist | 41T
HE alligator lay motionless on a flat
expanse of mud and stared into the
camera with a toothy grin. Two metres
long, its skin covered in dark green scales, it
wasn’t in the usual location for a dead reptile.
A day earlier, the carcass had been loaded
into a wire cage and lowered over the side of the
ship I was aboard. For an hour, it had travelled
down to the sea floor, 2 kilometres below. There
it was met by a deep-diving submersible with
live-feed cameras. Controlled by pilots on the
ship, a robotic arm had reached into the cage,
picked up the reptile and placed it on the seabed.
This was to be the alligator’s final resting place,
at the bottom of the Gulf of Mexico.
The next day, I joined the scientists and
crew of the ship around the monitor screens
to watch a grisly scene unfolding. Despite its
tough hide, the alligator was already being
eaten by a horde of scavenging giant isopods –
think pink woodlice the size of rugby balls.
This was February 2019 and the first time
an alligator had been left by scientists in the
deep sea, so we didn’t know what to expect.
But no one aboard the ship had anticipated
just how quickly it would be found and eaten.
This reptile’s demise would show how entire
ecosystems spring up on the carcasses that fall
from the surface in this strangest of deep-sea
habitats. It would also shed light on a mystery
dating back to the era of the dinosaurs.
More than a century ago, naturalists
dispelled the myth that the deep oceans are
a lifeless void. Even today, exploration of the
largest habitat on the planet is redefining the
possibilities of life on Earth with discoveries
of bizarre new species and even whole new >
ecosystems in this vast realm that covers 71 per
cent of the planet’s surface. There is a snailfish
that lives 8 kilometres down in the Mariana
trench, which in 2014 broke the record for the
deepest living fish known, and a hydrothermal
vent system in the Gulf of California with
towering white carbonate chimneys
surrounded by beds of red-tipped tubeworms.
“Even after 10 years working in this field, I’m
still amazed there are organisms down here,
under these extreme conditions,” says
MacKenzie Gerringer, a deep-sea biologist at
the State University of New York at Geneseo.
Below 200 metres, the only sunlight that
pierces through the water is dim and blue,
not strong enough to power photosynthesis.
Below 1000 metres, there is no sunlight at all.
It means that in open, deep water, no new
food is made and animals rely on a shower of
organic particles made from clumps of dead
plankton and their faeces, known as marine
snow, sinking from the surface.
A lot of that marine snow gets eaten on
the way down. Shrimp sift the water with
combs on their legs. Swimming snails, called
pteropods, build snow-catching mucous webs.
Vampire squid reel out a long filament to
collect falling flakes, then pack them into
snowballs to swallow. At most, about 2 per cent
of the food produced at the surface reaches
2000 metres. With such meagre supplies,
any larger consignments of organic matter
that arrive in the deep, like a dead alligator,
are likely to be welcomed with open jaws.
The alligator drop was carried out by the
Louisiana Universities Marine Consortium
(LUMCON). The experiment was a first, but“ We gathered
around the
monitor screens
to watch a grisly
scene unfolding”
DAVIDSHAL
E/NAT
URE^ PL
Whiplash squid in
the deep sea dine on
meagre scraps that
arrive from the surface