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The American Lobster

P1066
by Eleanor Ely

The American lobster, though indisputably tasty, has an otherwise rather unsavory reputation.  Lobsters have been called cannibals, aggressive fighters, and scavengers (eaters of dead flesh and refuse).

This reputation is only partly deserved.  Cannibalism—especially the eating of soft, helpless lobsters that have just molted (shed their old shells)—is common among lobsters kept in the crowded conditions of an aquarium, but appears to be uncommon in nature.  American lobsters are aggressive, and will fight for the possession of rocky cave shelters.  As for diet, there is no evidence that lobsters are primarily scavengers.  They eat mainly live food: fish, small crustaceans, and mollusks.

Taxonomy. Taxonomists place lobsters in the phyllum Arthropoda, along with shrimp, crabs, barnacles, and insects.  The arthropods (from the Latin word arthro, meaning jointed, and the Greek poda, foot) are a group of invertebrates with jointed appendages and a hard outer shell (exoskeleton).

Lobsters, crabs, and shrimp constitute the order Decapoda (Greek deca, ten).   As the name implies, all decapods have five pairs of legs.  Lobsters and shrimp have similar body structures, possessing large muscular tails that they use to propel themselves backwards through the water away from danger.  Crabs, in contrast, have a very small tail that they keep tucked under their bodies.

There are two commercially important families of edible lobsters.  The Nephropidae, which include the American lobster Homarus americanus, have one pair of large claws and are found in the northern Atlantic Ocean.  The Palinuridae, or spiny lobsters (also called rock lobsters), lack the large claws and have spines all over their bodies.  They live in subtropical and tropical oceans.

Structure. Lobsters crawl rather than swim.  In Homarus, the first of the five pairs of walking legs is modified to form the large crusher and ripper claws.  Underneath the abdomen (stomach) of Homarus are six pairs of swimmerets (pleopods); the last pair is enlarged to form the tail fan.  The shieldlike shell covering the main portion of the lobster's body is called the carapace.

Lobsters have compound eyes that are carried on movable eye stalks.  Each eye is made up of approximately 14,000 individual units.  Lobsters can detect movement, and they may be able to perceive images.  It is not known whether lobsters can detect colors as can other decapods.

Pigment.  Live American lobsters are usually olive green or greenish brown, though dusky orange and even bright blue lobsters are sometimes found. Diet, heredity, and exposure to light all affect a lobster's color.  The major pigment in a lobster's shell, astaxanthin, is actually bright red in its free state; but in the lobster's shell astaxanthin is chemically bound to proteins that change it to a greenish or bluish color.  When lobsters are cooked, heat breaks down these bonds, freeing the astaxanthin so that it reverts to its normal red color.

Molting. A lobster's hard outer shell does not grow.  Homarus can only increase its size by molting periodically.  In preparation for molting, the lobster lays down a new, soft shell underneath its old shell.  Just prior to shedding the old shell, the lobster seeks out a protected shelter—a rocky cave or crevice—because a newly molted lobster is soft and helpless, unable to move.  Then the lobster rolls over on its side, bends into a V Shape, shrinks its extremities (especially the large claws) by drawing fluids from them, and withdraws from its shell.  Over a period of several hours after molting, the lobster swells to a larger size and the shell begins to harden.

Autotonomy. Lobsters are able to detach and discard legs or claws by a process called autotonomy.  This "self-amputation" can help a lobster to escape a predator's grasp.  A lobster may also detach a claw if it is unable to withdraw from its old shell during molting.  Autotomized limbs can be regenerated.   After the next molt they appear fully formed but smaller than usual, and after sever molts they are full sized.

Reproduction.  Homarus can mate only when the female is soft after molting.  At this time the female releases a pheromone (chemical sex attractant).  The male stops his usual aggressive behavior towards the female, and the mating pair begin a courtship dance with their claws held closed.  Then the male inserts his first pair of pleopods into the female's seminal receptacle and deposits the spermatophores (sperm packets).

Sperm is stored in the female's seminal receptacle until the female spawns (produces eggs), which may be up to 15 months after mating.  When the eggs are released from the female's oviducts, they flow past the seminal receptacle where they are fertilized by the stored sperm.  Then they are cemented to the swimmerets, where they remain for 10 to 11 months before the larvae hatch.  A female carrying eggs is said to be "in berry," and indeed the eggs covering her abdomen do resemble the segments of a raspberry.

Life cycle. Lobster larvae molt four times over a period of 10 to 20 days, depending on water temperature.  All four larval stages swim swim near the surface of the water, along with the innumerable other tiny plants and animals known as plankton.  Here the lobster are extremely vulnerable to predation by fish and birds; it is estimated that only about one-tenth of one percent survive the larval period.

When it reaches the fifth stage, the young lobster is finally able to sink to the ocean bottom where it will spend the rest of its life, and where it can hide from predators (such as codfish, dogfish, catfish, and skate) in the relative safety of rocky crevices.

In the first year of its life, the young lobster molts about 10 times and reaches a length of one to one and a half inches.  As lobsters grow older, they molt less and less frequently; a large lobster molts only once every few years.  It takes about six years for a lobster to reach a weight of one pound.

Fishery. Humans are by far the most important predators that lobsters face.  Homarus americanus is intensely fished all along the northeast Atlantic Coast of Canada and the United States as far south as New England, and particularly in Maine.

Due to increased demand, overfishing, and pollution, the demand for lobsters exceeds the supply, making lobsters an expensive luxury.  Aquaculture (farming) of lobsters is technically possible, but with current methods it costs more to raise lobsters than to catch them—so cheaper lobster is unlikely in the near future.

Homarus americanus has two large claws: the crusher claw is the larger of the two, while the ripper claw has a sharper point.  The lobster shown above has its crusher claw on the right side, but the crusher is found on the left just as frequently.

To determine a lobster's sex, look at the first pair of swimmerets (the pair closest to the head).  These are hard and bonelike in the male, but soft and featherlike in the female.

The newly hatched larva (first larval stage) is about one third of an inch long and looks very different from an adult lobster.  It is transparent, and has huge eyes and a long spiny projection from its head.  Second- and third- stage larvae look much like the first stage, though larger.

The fourth larval stage, about one half inch long, resembles the adult in form.   It still swims near the surface where it is vulnerable to predators.

Taxonomy of Homarus americanus

phylum Arthropoda
superclass Crustacea
class Malacostraca
order Decapoda
family Nephropidae
genus Homarus
species americanus

June 1988.

For further reading:

Cobb, J. Stanley. 1976. The American Lobster: the Biology of Homarus americanus. Rhode Island Sea Grant, Narragansett, RI.

Bliss, Dorothy E. 1982. Shrimp, Lobsters, and Crabs. New Century Publishers, Piscataway, NJ.

Bayer, Robert and Bayer, Juanita. 1987. Lobsters Inside-Out: A Guide to the Maine Lobster.  Maine Sea Grant, Orono, ME. (for young readers. )

Somers, Barbara. 2005. Lobster Shell Disease fact sheet. Rhode Island Sea Grant, Narragansett, RI.

 

Return to Rhode Island Sea Grant Fact Sheets

Posted 6/3/98