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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, their 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.
June
1988.
by
Eleanor Ely
http://seagrant.gso.uri.edu/factsheets/fslobster.html
American Lobster. Directory and information guide on lobsters.
Lobster-i.com. |
