Of all the animal groups on Earth, the crustaceans are by far the most successful. Coming in
a boundless array of colours, shapes and sizes, they have diversified so much that crustaceans
are often called the insects of the oceans, a title reflecting their remarkable diversity
and abundance. But nowhere are they more weird and wonderful than in the deep sea, where
crustaceans have adapted in countless ways in order to survive, and play key roles in
the ecosystem. Let’s take a closer look at these incredible invertebrates.
As crustaceans are a subphylum of arthropods, meaning ‘jointed feet’, they display many-jointed
legs, as found on crabs and lobsters. Generally, crustaceans also consist of a head, thorax
and abdomen, at the end of which is a tail piece called a telson. But what sets them
apart from other arthropods, is two pairs of sensory antennae and at least three pairs
of mouthparts. From mantis shrimps to copepods and the giant Isopod, all crustaceans share
these common features, giving them an evolutionary advantage over other animals.
The antennae are used for making sense of their surroundings, as well as locating food
and sensing danger. But in many species, we often find more unusual adaptations. Decapods,
like crabs, have 10 walking legs, making them well-suited to a benthic lifestyle down on
the sea-floor, where they scavenge for food. Contrastingly, Euphausid shrimps like the
infamous Antarctic Krill have elongated bodies and paddle-like legs that help them swim through
the open ocean, far from the sea floor. Under each eye, they possess light-producing photophores
that they use to communicate via the phenomenon of bioluminescence.
To survive in the expanse of the open ocean, krill have had to become filter-feeders, consuming
plankton that drift by on the currents. In repeatedly moving their appendages, water
is drawn past the mouthparts which are covered in fine hairs, or setae. These form a highly
effective food trap.
The wonder of Krill doesn’t end there, for these tiny creatures also serve to demonstrate
the important role that crustaceans play in the marine ecosystem. With their oil-rich
bodies, and tendency to gather in huge swarms so large that they can be seen from space,
Antarctic krill are high energy food able to sustain the enormous bulk of baleen whales.
The largest animals on the planet, relying on one of the smallest.
This shows that crustaceans are vital in the marine food web, but in more ways than just
as a source of prey, as we’re about to discover.
There are no insects in the ocean, but crustaceans rule in their place and display an amazing
variety of sizes and morphologies. Their role is very similar to that of insects on land.
In addition to being a source of food, they occupy the role of detrivores. The deep ocean’s
clean-up crew. Most important are the amphipods, which you might have found leaping like fleas
around washed-up seaweed on the shore. Even here, they are doing their job of cleaning
up the stranded debris. And amphipods are found in every part of the ocean, from beach
hoppers on the shore, to 35,797 feet (10,911 meters) down in the furthest depths of the
Mariana Trench. Here, the hadal amphipod takes to the unlit stage, even possessing enzymes
that are able to digest wood. They can and do consume just about anything that falls
to the seafloor, filling an important ecological function by recycling nutrients from even
hard-to-digest material back into the environment.
Without them, the seabed would vanish beneath a deluge of dead plant and animal material.
These crustaceans can consume detritus at a rate of between 60% and 100% of their body
weight each day, for they are highly adapted to this task. Their legs have pincers with
a movable half, and a sharp edge used for tearing off pieces of food. In fact, bait
that is sunk to the sea floor often comes back stripped to the bone by amphipods, in
just a matter of hours.
Amphipods are important for another reason. Researchers have noticed that their size increases
dramatically with depth, from a length of 8 mm in the shallows, to over 34 centimetres
long in the hadal zone of the ocean. This remarkable size is achieved by the species
Alicella gigantea, a slow-moving scavenger which has been filmed clumsily stumbling into
bait, latching on, and gorging itself for hours. It exhibits the phenomenon of abyssal
gigantism. But although it is far larger than many of its cousins, the super-giant is not
so odd when we consider it has evolved to fulfil a niche that, in shallower water, is
occupied by other large bottom dwellers like crabs, one the most diversified crustaceans
in the world.
Amphipods are not the only crustaceans to exhibit gigantism. A relative of the land-dwelling
woodlice, the giant isopod, can grow to 76 cm or 2.5 feet long. It, too, feeds on sunken
detritus, clinging on with sharp claws and living at depths of 2140 metres (7020 feet).
Overall, crustaceans play a number of key roles in the deep sea ecosystem. From amphipods
and isopods cycling nutrients from decaying material, to copepods parasitising larger
fish, and krill providing nutrients for some of the largest creatures on the planet, they
are a diverse and ecologically vital group of organisms, capable of adapting in numerous
ways to survive in every corner of the expansive deep sea.