Crustaceans

Crustaceans are a big group of mostly water-dwelling invertebrates with hard outer shells and jointed legs. Famous examples include crabs, lobsters, prawns and shrimps, crayfish, barnacles, krill and the humble woodlouse (the only crustacean that lives mostly on land). There are around 67,000 known species, ranging from microscopic copepods to the Japanese spider crab with its 4-metre leg span. Crustaceans are essential parts of the marine food web and a major source of food for humans.

  • Known speciesapprox. 67,000Almost all live in water
  • Largest crabJapanese spider crabUp to 4 m leg span
  • Largest lobsterAmerican lobsterUp to 1.1 m, 20+ kg
  • SmallestStygotantulus stockiapprox. 0.1 mm long, lives on copepods
  • Most abundant speciesAntarctic krillapprox. 500 million tonnes total biomass
  • Only land crustacean groupWoodliceapprox. 3,500 species

What makes a crustacean?

  • Hard exoskeleton: an outer shell made of chitin, often strengthened with calcium carbonate.
  • Jointed legs: at least five pairs in most species (more in many).
  • Two pairs of antennae: more than any other arthropod group.
  • Body split into segments, often grouped into a head, thorax (sometimes fused as a cephalothorax) and abdomen.
  • Gills: most breathe with gills, even ones that live on land.
  • Mostly aquatic: live in seas, rivers, ponds or wet habitats. Only woodlice are truly terrestrial.

The main groups

  • Decapods ("ten-footed"): crabs, lobsters, shrimps, prawns, crayfish. The biggest crustacean order, around 15,000 species.
  • Krill: small shrimp-like creatures that swim in vast swarms in the oceans. The base of many marine food webs.
  • Copepods: tiny, mostly microscopic crustaceans. Some of the most abundant animals on Earth.
  • Barnacles: actually crustaceans, despite their stationary lifestyle and shell. They live their adult life glued head-down to rocks, ship hulls and whales, kicking their legs out of the top to catch food.
  • Isopods (woodlice and their relatives): the only group with major land-living species.
  • Amphipods (sand-hoppers and freshwater shrimps): mostly small swimming crustaceans.

Why crustaceans shed their shells

Because their exoskeleton is rigid, crustaceans cannot grow continuously the way mammals or birds do. Instead, they grow by moulting: shedding the entire old exoskeleton, then expanding rapidly before the new soft shell hardens. A lobster might moult 25 times in its first 5 years of life, slowing down as it gets older.

The hours after moulting are dangerous: the crustacean is soft and helpless until the new shell hardens. Many hide for a few days. Some species are eaten as "soft-shell crab" or "soft-shell lobster" precisely because they are caught in this brief window.

Krill: the foundation of the ocean

Krill are small shrimp-like crustaceans that drift in huge swarms in the oceans, especially in the Antarctic. Each krill is only a few centimetres long, but they exist in unbelievable numbers. The total biomass of Antarctic krill alone is estimated at approx. 500 million tonnes, possibly more than any other single species on Earth.

Krill are the base of the southern ocean food chain. They are eaten by everything from tiny fish to giant blue whales. A blue whale can eat 4 tonnes of krill in a single day, scooping up swarms of them in its enormous mouth. Without krill, the entire ecosystem of the Antarctic Ocean would collapse.

Fact The Japanese spider crab is the largest known arthropod. From the tip of one leg to the tip of the opposite leg, a full-grown male can measure nearly 4 metres. They live on the seabed off the coast of Japan, mostly between 150 and 300 metres deep. Despite their size and intimidating appearance, they are gentle scavengers, eating dead fish and algae on the sea floor.

Woodlice: the only land crustaceans

Most crustaceans need to live in water because their gills only work when wet. The exception is the woodlouse family, which has evolved to live on land. Even so, woodlice still have gills and need a moist environment; they hide under stones, logs and leaves to avoid drying out. Pet woodlice (yes, some people keep them) need a damp habitat to survive.

Woodlice have around 3,500 species worldwide, including the familiar UK garden woodlouse (Oniscus asellus), which is grey and around 1 cm long. The largest land crustacean of all is the coconut crab, a kind of hermit crab from Pacific islands, which can weigh over 4 kg and is strong enough to crack coconuts with its claws.

Did you know? Crustaceans are the closest living relatives of insects. Recent DNA evidence shows that insects actually evolved from within the crustaceans, around 470 million years ago. So insects are technically a kind of crustacean that left the water and learned to fly. The crustaceans and insects together are now sometimes called the Pancrustacea.
Deeper dive: how lobsters can live for over 100 years

Lobsters do not seem to age the way most animals do. Many of the usual signs of biological ageing (gradual decline in strength, slower reproduction, organ failure) just do not happen in lobsters. A lobster's fertility, appetite and metabolism remain almost the same throughout its life, and a lobster gets stronger and bigger every year that it survives. The oldest known American lobster was estimated at over 140 years old.

The biology behind this is fascinating. Lobster cells produce an enzyme called telomerase throughout life. Telomerase repairs the tiny caps on the end of chromosomes (called telomeres) that wear away every time a cell divides. In most animals, telomerase is switched off in most cells after birth, so the telomeres slowly shorten and the cells eventually stop dividing. This is one of the main causes of ageing. Lobsters keep their telomerase active forever, so their cells can keep dividing without limit.

Lobsters do still die, just from external causes. Eventually they grow so large that moulting becomes deadly: the energy needed to shed and rebuild a giant exoskeleton becomes too much, and the lobster can die during the moult. Disease, infection or predation also takes most lobsters long before they reach extreme ages. But in principle, a perfectly protected lobster might live forever, in the sense that no internal clock seems to be ticking down. Studying lobster biology may eventually help us understand the chemistry of ageing in humans.

For other invertebrate groups, see insects, arachnids and molluscs.