BBC Wildlife Magazine

What’s the difference between crabs and lobsters?

Aseafood platter is not only a culinary adventure; it’s a zoological one, too. Staple ingredient­s include vertebrate­s (fish), molluscs (oysters, clams, scallops, snails, squid, octopus) and crustacean­s (prawns, shrimps, crabs, lobsters). In some parts of the world, echinoderm­s (urchins, sea cucumbers), cnidarians ( jellyfish, sea anemones) and sipunculid­s (peanut worms) might feature, too. It is surely among the most biodiverse of all meals.

It’s the crustacean­s that interest us here, of course. This vast, ancient group of arthropods includes more than just seafood delicacies. Woodlice, barnacles, water fleas and sandhopper­s also count amongst its members. A defining characteri­stic of the group is that their limbs develop from versatile, two-pronged structures. A seafood platter contains no shortage of such ‘biramous’ appendages. A bit of rooting around at the base of a lobster’s leg, for example, reveals the second prong, which projects under the protective carapace and functions as a gill. Or there are the doublepadd­led

“Evolution seems to like the crab body shape”

oars, called pleopods, that hang beneath a prawn’s abdomen.

The pleopods are also obvious in shrimp and lobsters, but they are harder to find in crabs. This is the result of a significan­t difference between crabs and lobsters, both of which belong to a branch of the crustacean family tree called the decapods (a name derived from their 10 walking legs, the front pair of which often terminates in claws or pincers).

Decapods come in two basic body shapes. There are the longer, narrower ones, which we call shrimp, prawns or lobsters, and the shorter, wider ones, which we call crabs. Most decapods are of the long, narrow variety and much of their length is accounted for by an elongated muscular abdomen, called a pleon. This bears the pleopods and also powers a flick of the tail that generates a burst of speed in a backwards direction to escape danger.

In crabs, the pleon is greatly reduced in size and folded beneath the body. It can be seen as a triangular flap on a crab’s underside. The pleopods are still there, but you have to lift the flap to see them. In females, they hold the eggs and larvae securely under the pleon, which has been converted into a brood-pouch. Meanwhile, a crab’s famous sideways gait makes sense for an animal that is wider than it is long.

Evolution seems to favour the crab body shape. While most short-wide decapods are Brachyuran­s, or ‘true crabs’, other distantly-related groups have also converged independen­tly on this body plan – porcelain, king and hermit crabs and squat lobsters, to name just a few. A running joke in zoological circles is that, given enough time, all life on Earth is destined to become a crab.