There are two main groups of lobsters which are all marine arthropods - Clawed lobsters (30 species) and Spiny or Rock Lobsters (45 species). The best known Clawed lobsters are the American and European lobsters - these are called "true" lobsters. Marine spiny lobsters, some of which are also called crayfish in Australia, New Zealand and South Africa, do not have large claws, their first pair of legs are only slightly larger than the other four pairs.
Lobsters are found in all the world's oceans. They live on rocky, sandy, or muddy bottoms from the shoreline to beyond the edge of the continental shelf. They generally alone in crevices or in burrows under rocks.
Lobsters are omnivores coming out at night to eat fish, molluscs, other crustaceans, worms, and some plant life. They also are scavengers when necessary. Their predators include bottom dwelling fish, such as cod, flounder, and eels, crabs, and seals.
Most lobsters are 25 to 50 centimetres in length, Adult crayfish are usually 5 to 17.5 centimetres long. The largest lobster caught weighed over 20 kilograms They move by slowly walking on the sea floor. When avoiding predators they swim backwards quickly by curling and uncurling their abdomen.
Lobsters must moult to grow, during the moulting several species change color. Lobsters have 10 walking legs; the front two acting as claws with one often being much larger than the other. Their antennae and tiny hairs that cover the entire body are sensitive to touch.Short bristles cover the insides of the pincers on the walking legs, these are taste receptors. Their head and thorax are fused into one section called the cephalothorax.
Lobsters, like snails and spiders, have blue blood due to the presence of the blood pigment haemocyanin, which contains copper rather than iron which is in haemoglobin. Lobsters possess a green organ, the hepatopancreas, which acts as both liver and pancreas.