Shrimps, crabs and lobsters belong to a group of arthropods that lives underwater. They share the common feature of walking on ten legs. They number about 15,000 species in all.
Although some people use the words shrimp and prawn interchangeably, those are actually two different things, classified differently scientifically. Prawns have gills under their tails which are branched filaments. Shrimps, crabs and lobsters have gills which look like a stack of thin plates. Prawns let their eggs drift away on the current, while the others hold them under their tails until hatching.
Shrimp have long narrow bodies, with pairs of claws in front. They can walk on their ten legs, or swim backwards and forwards.
Lobsters are like giant shrimp. Lobsters usually walk rather than swim, but when they swim, they only move backwards, not forwards. Many lobsters have large claws, but the spiny lobsters have very tiny claws.
Crabs have wider, rounder and thicker bodies than shrimp. The sections of their bodies that correspond to the tail in shrimps and lobsters are very shortened. Various groups of crabs appear to have no tail at all, while some have a very short tail tucked underneath their bodies.