In the deep sea, there is one group of organisms that has become specialised in unexpected
ways in order to survive. With some growing 9 feet tall, and others hanging ghost-like
in the water column, worms of the deep are among the most diverse and widespread inhabitants
of this dark and otherworldly realm.
Many of these worms are known as polychaetes - meaning ‘many bristles’, due to the
fact that each body segment has a pair of fleshy protrusions called parapodia that are
covered in bristles, called chaetae. Found in nearly every corner of the deep, polychaetes
rule the sea, and play a vital role in the deep sea ecosystem.
Here, at the towering, rocky structures of hydrothermal vents, we find one particular
polychaete worm that has adapted to rely entirely on the vents. This is a Pompeii worm. An extremophile,
and the most heat-tolerant animal on Earth, that is found only at hydrothermal vents in
the Pacific Ocean. They form heat-resistant tube colonies attached to the vents, within
which they reside. But what’s most unique about this worm, is the fact that their tail
end rests within the superheated vent fluid at 80º C (176º F), while their feather-like
heads stick out of their tubes into water that is far cooler. The advantage of doing
so is that it allows Pompeii worms to form a symbiotic relationship with bacteria. The
hair that lines their body is actually a thermal blanket of colonies of filamentous bacteria.
The bacteria is kept alive as the Pompeii worm secretes an enzyme-rich sugar mucus from
its back to feed them. In return, the bacteria act as insulation, and as detoxifiers of the
vent fluid in the worm’s tube, converting the lethal chemicals into energy that the
worm requires in order to survive.
But unlike many creatures that dwell at deep sea vents, polychaete worms are abundant also
in the open expanse of the deep, where they instead survive by feeding on plankton and
marine snow that drifts down from above. Some secrete a giant mucus net around themselves
in order to do so. Here in the midwater zone, far from both the surface and the sea floor,
polychaete’s adaptations appear even more unique. Flota and swima worms both move by
undulating their bodies and beating their paddle-like appendages. Far stranger is tomopteris
- the dancing bristle worm - which propels itself with its large parapodia.
A common feature you might have noticed is that many of the worms that inhabit the open
ocean are transparent. This is because there is no concealment in the midwater zone, and
so some worms like the green bomber worm have adapted to use bioluminescence as a way to
startle and distract predators. When threatened, they explode their light-producing organs
called photophores, dropping a kind of bioluminescent bomb.
Inevitably, when these drifting worms die, or release their large mucus nets, the organic
material sinks to the sea floor. But there are polychaetes there, too, feeding on this
nutrient-rich supply. Every now and then, they are treated to a feast. When the carcass
of marine mammals sink to the ocean floor, a whale-fall community rapidly gathers that
can last for 50 years. But for the majority of this time period, it is not the large scavengers
that make up this diverse community. Once the flesh has been stripped, it is the polychaetes
that remain, possessing the ability to get at the otherwise inaccessible nutrients locked
away within the bones. Specifically, this is the job of the bone-eating osedax worms,
or zombie worms. With their deep, green roots they bore into the bones of the whale carcass
to reach the lipids within, on which they rely for sustenance. In doing this task, osedax
worms carry out the important role of cycling nutrients back into the deep sea ecosystem.
There you have it. The rulers of the deep, the polychaete worms, capable of colonising
every corner of this harsh and barren landscape. Found burrowing into the mud, clinging to
rocky seamounts, colonising vents and eating dead whales, these worms play a fundamental
role in the deep sea ecosystem as detrivores, symbionts, and a source of nutrients
for predators.