Its closest relative is the European lobster Homarus gammarus, which can be distinguished by its coloration and the lack of spines on the underside of the rostrum.

[13] Together with Sagmariasus verreauxi, it is also the longest decapod crustacean in the world;[2] an average adult is about 230 mm (9 in) long and weighs 680 to 910 g (1.5 to 2 lb).

[15] The two species can be distinguished by several characteristics:[16] The antennae measure about 51 mm (2 in) long and split into Y-shaped structures with pointed tips.

[20][6] On rare occasions these colors are distorted due to genetic mutations or conditions creating a spectacle for those who catch them.

Social media influence making reporting and sharing more accessible to a drop in predator populations have been suggested as possible reasons.

When the eggs hatch, the female releases them by waving her tail in the water, setting batches of larvae free.

These molts take 10–20 days, during which the planktonic larvae are vulnerable to predation; only 1 in 1,000 is thought to survive to the juvenile stage.

[41] To reach the fourth stage – the post-larva – the larva undergoes metamorphosis, and subsequently shows a much greater resemblance to the adult lobster,[43] is around 13 mm (1⁄2 in) long,[41] and swims with its pleopods.

[44] The American lobster thrives in cold, shallow waters where there are many rocks and other places to hide from predators.

It is dominated by mollusks (especially mussels, clams and snails), echinoderms and polychaetes, although a wide range of other prey items may be eaten, including other crustaceans (such as crabs), brittle stars, cnidarians and small fish.

Gaffkaemia or red-tail is an extremely virulent infectious disease of lobsters caused by the bacterium Aerococcus viridans.

The "red tail" common name refers to a dark orange discoloration of the ventral abdomen of affected lobsters.

The same sign is also seen in other diseases of lobsters and appears to be a nonspecific stress response, possibly relating to the antioxidant and immunostimulatory properties of the astaxanthin molecule.

[48][50] Paramoebiasis is an infectious disease of lobsters caused by infection with the sarcomastigophoran (amoeba) Neoparamoeba pemaquidensis.

Infection occurs throughout the tissues, causing granuloma-like lesions, especially within the ventral nerve cord, the interstices of the hepatopancreas and the antennal gland.

Paramoebiasis is strongly suspected to play a prominent role in the rapid die-off of American lobsters in Long Island Sound that occurred in the summer of 1999.

The most generally attributed factor is an increased duration of warmer temperatures in the bottom of the Long Island Sound.

[2] The American lobster was given its current scientific name of Homarus americanus by Henri Milne-Edwards in his 1837 work Histoire naturelle des Crustacés ("Natural History of the Crustacea").

Hard-shells (lobsters that are several months past their last molt) can survive out of water for up to four or five days if kept refrigerated.

[61] Lobsters have a greenish or brownish organ called the tomalley, which, like the liver and pancreas in a human, filters out toxins from the body.

[62] Some diners consider it a delicacy, but others avoid it because they consider it a toxin source; dislike eating innards; or are put off by its texture and appearance, that of a grainy greenish paste.

Most lobsters come from the northeastern coast of North America, with the Atlantic Provinces of Canada and the U.S. state of Maine being the largest producers.

[64][65] Massachusetts offers scuba divers lobster licenses for a fee, and they are only available to state residents.

There is also a legal maximum size of 5 in (130 mm) in Maine, meant to ensure the survival of a healthy breeding stock of adult males, but in parts of some states, such as Massachusetts, there is none.

This notch remains for two molts of the lobster exoskeleton, providing harvest protection and continued breeding availability for up to five years.

[72] American lobster tends to have a stable stock in colder northern waters, but gradually decreases in abundance moving southward.