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FIND OUT MORE. Arthropods 107 • Invertebrates 102 • Oceans 40–41

Crustaceans

These invertebrates include crabs, barnacles,. KRILL, and woodlice. They are

sometimes called the insects of the sea, because they are the most numerous ocean

arthropods. All crustaceans have hard skin, gills, and two pairs of antennae.

ARE KRILL UNDER THREAT?

Krill are extremely numerous and there is no threat of

their dying out. However, they are being taken from

the ocean in increasing amounts by fishermen.

As krill disappear, the animals that feed on

them suffer. Krill trawling in the Antarctic

has had a major effect on penguin numbers.

WHERE DO CRUSTACEANS LIVE?

Most crustaceans live in the ocean – although some

species live in fresh water, and woodlice and a few

crabs live on land. Prawns and shrimp swim freely in

open water. Barnacles

live attached to rocks,

harbour walls, or the

sides of ships.

WHAT DO CRUSTACEANS EAT?

Many crustaceans are scavengers, feeding on scraps

and dead creatures. Crabs, shrimp, and prawns search

for food mainly at night and hide in crevices by day.

Some crabs and lobsters are active predators, seizing

prey in their powerful claws. Barnacles filter tiny

creatures from the water using their hairy legs.

Woodlice munch on plant remains.

ARE BABY CRUSTACEANS LIKE THE ADULTS?

Most crustaceans hatch from eggs into nauplius larvae,

which do not resemble their adult form at all. These

tiny creatures float near the ocean surface, where

they feed and grow. They moult their hard skin

several times before becoming adults.

4 HERMIT CRABS

These creatures wear empty mollusc shells for

protection from predators. When they outgrow

one shell, they simply move into another.

4 KRILL NUMBERS

Krill sometimes occur in such huge

numbers near the ocean’s

surface that their rosy colour

appears to turn the water red.

HUMPBACK WHALE 3

Baleen whales sieve up to 2 tonnes of krill from

the water in a single feeding session, using

fringed baleen plates in their mouths.

Legs
drawn inside
shell at the first
sign of danger

Crustaceans make up the second largest class in the animal kingdom.

They include about 58,000 species, split into eight sub-classes:

• Lobsters, crabs, prawns, shrimp, and woodlice • Barnacles
  • Seed shrimp • Fish lice • Copepods • Branchiopods
  • Cephalocarids • Mystacocarids

CRUSTACEAN CLASSIFICATION

Antenna

helps feel the way

in murky water

Baleen plates

keep krill in but

let water out

1 GOOSE BARNACLES

These crustaceans extend their

slender legs to trap tiny floating

creatures for food. As adults,

they live fixed to one spot.

2 NAUPLIUS LARVAE

Most young crustaceans float

freely near the ocean’s surface.

Creatures that live in this

way are known as

plankton. The word

plankton means

“wanderer”.

WHAT DO KRILL EAT?

Krill feed on plankton, such as

animal larvae (young) that float

with the ocean currents. They, in

turn, are eaten by everything from

penguins to whales. Some whales

migrate thousands of kilometres from warmer

waters just to feed on seasonal swarms of krill.

KRILL

Krill are small, pinkish, shrimp-like

creatures found in the oceans in huge

numbers. They form an important part of

the marine food chain and are the main

diet of many larger sea creatures.

CRAB

LARVAE

CRAB

SHRIMP

COPEPOD

crustaceans