Homarus americanus is the largest species of lobster and can reach a length of up to 1.1 m and a weight of 20 kg. However, the size of a lobster which is commonly caught is approximately 25 cm in length and weighs about 0.5 kg.
A lobster's body is divided into twenty-one segments: six segments from the head region, eight segments compose the thorax (mid-section), and seven segments make up the abdomen (often called the tail). Commonly thought of as being red, the body is really blackish-green or brownish-green. The red color results when a lobster is boiled and is a result of pigments in the shell breaking down.
The eyes are on the first segment of the head and are stalked. They can only detect motion in dim light. The second segment of the head has anntenules with delicate hairs that have more than 400 types of chemoreceptors. The lobsters can detect other species, potential mates, prey and predators with the receptors.
Being in the Order Decapoda (meaning "ten feet"), the lobster has ten legs. Five pairs of jointed legs extend from the thorax region. The first pair of these legs extends towards the head and has claws (chela) on the end. One claw is usually larger than the other and has thick teeth which are used to crush objects. The other claw usually is smaller and has sharp teeth used for cutting.
Lobsters go through exceptional growth during their lifetime. When they first hatch, a lobster weighs less than one tenth of a gram. By the time they are full adults, they can reach a weight of up to 10 kilograms. This growth is an increase of 100,000 times. Lobsters achieve this growth by going through periods called molts. When a lobster is ready to molt, its body absorbs the mineral salts that had hardened its shell, drawing the salts further into its skin. When the shell softens, the lobster is able to break it and slide out. The lobster takes in more water and thus swells in size. The new shell is already covering its body but takes a few days to harden. During this period the lobster stays in seclusion to avoid predators. Each time a lobster molts its body can grow 10-15% in size. Newly hatched lobsters molt for the first time within the first week, and three more times within the first month. (Author unknown, 2001; Author unknown, 2003; Gulf of Maine Aquarium, 1999; Romanowsky, 2000a; Whale, July 1993)