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.