Crab vs Lobster: Differences in Body, Habitat, Size and Lifespan

Key differences at a glance

Crabs and lobsters are both decapods, an order of crustaceans whose name means ten-footed. Both have a hard outer skeleton, stalked eyes, two pairs of antennae and five pairs of legs, and both must molt to grow. The differences lie mainly in body shape and lifestyle.

A typical crab, such as the blue crab, is built short and wide, with a flat shell and legs spread to the sides. A typical lobster, such as the American lobster, is built long and narrow, with a big tail for swimming backward. These two body plans lead to different ways of moving, different habitats and very different lifespans.

Body plan: the tucked tail vs the long tail

Every decapod’s body has two main parts: a front section covered by a shell called the carapace, and a segmented abdomen behind it. In lobsters, the abdomen is the large tail. It is packed with muscle, carries small paired swimming limbs called swimmerets underneath, and ends in a fan of flat plates.

In crabs, the abdomen has become small and thin and is folded forward under the body, where it fits into a groove. Crabbers call it the apron. In many crabs its shape tells the sexes apart: narrow in males and broad in mature females, which use it to carry their eggs.

The crab’s carapace is usually wider than it is long, protecting the gills and organs beneath. A lobster’s carapace is a long cylinder, and its eyes, antennae and claws point forward.

Legs, claws and how they move

Crabs usually walk sideways. Their leg joints bend mainly outward and inward, and their legs are packed closely along the sides of the body, so stepping to the side is fastest, as explained in why do crabs walk sideways? Swimming crabs, including the blue crab, have their last pair of legs flattened into paddles.

Lobsters walk forward or backward on the seabed. When threatened, they curl the tail sharply under the body, shooting backward through the water. Speeds of about 5 m/s (11 mph) have been reported for this tail-flip escape, far faster than they can walk.

Claws differ too. True crabs have one pair of claws on the first legs. Clawed lobsters have small pincers on their second and third pairs of legs as well as two big claws on the first pair, often a heavy crusher and a slimmer cutter. Spiny lobsters, by contrast, have no big claws at all and defend themselves with spines and long, stiff antennae.

Where crabs and lobsters live

Crabs live almost everywhere water is found, and some even on land. True crabs range from the deep sea to rocky shores, mangroves, estuaries and coral reefs. About 1,300 species live in fresh water, and land crabs such as the Christmas Island red crab return to the sea only to release their eggs.

Lobsters are strictly marine. Clawed lobsters live on rocky, sandy or muddy seabeds from shallow coasts to the edge of the continental shelf and beyond, often hiding in crevices or burrows by day. Their closest freshwater relatives are crayfish, which look like small lobsters but live in rivers, lakes and streams.

Which is bigger, a crab or a lobster?

The answer depends on how you measure. The heaviest crustacean ever recorded is an American lobster caught off Nova Scotia in 1977, which weighed about 20.1 kg (44.4 lb); records like this are explored in how big do lobsters get? The Japanese spider crab has the longest legs of any crustacean, with a span of up to about 3.7 m (12 ft), and can weigh up to about 19 kg (42 lb).

At the small end, crabs win easily: pea crabs, which live inside the shells of clams and mussels, are only a few millimeters across.

Lifespan, molting and growth

Both animals grow by molting, shedding the entire shell and expanding before the new one hardens. They differ in how long they keep doing it. Many crabs stop molting after a final molt when they reach adulthood. A female blue crab, for example, molts for the last time when she matures and usually lives only 1–3 years in total.

Clawed lobsters keep molting and growing throughout their lives, more and more slowly as they age. American lobsters are thought to live 50 years or more, and NOAA Fisheries says some may reach about 100. That steady growth and long life have fueled the myth of the immortal lobster, examined in do lobsters live forever?

Look-alikes that blur the line

Not every animal called a crab or a lobster fits neatly into these groups.

• King crabs look like crabs but are more closely related to hermit crabs. Their small last pair of legs is hidden inside the shell, so they seem to have eight legs.
• Porcelain crabs are small, flat crab-like animals that are actually close relatives of squat lobsters.
• Squat lobsters have a lobster-like front end, but their tail is partly folded under the body, halfway between a crab and a lobster. More than 900 species are known.
• Spiny and slipper lobsters are called lobsters but are not closely related to clawed lobsters.
• Horseshoe crabs are not crustaceans at all, as explained in is a horseshoe crab a crab?

The crab-like body shape has evolved independently at least five times among decapods, a process biologists call carcinization. A broad, flat shell with a tucked-in tail seems to work so well on the seabed that several unrelated lineages arrived at it separately. Lobsters, on the other hand, keep the long-tailed body plan that crabs and their relatives started from.