Follow a female blue crab throughout her life.
Zoeal stage
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To understand the blue crab life cycle, we will follow a female
blue crab from birth to reproduction. The blue crab starts
her life as a larva, an early-life stage that looks completely
different than her adult form. She will spend 31-49 days
going through seven larval stages called zoea. In each stage
she is similar in appearance, but is slightly larger than in the
last stage. Even this early in life, crabs have a hard outer
shell (exoskeleton). In order to grow and change stages, the larva
must molt, which means shed or cast off its shell. During
molting, the exoskeleton splits, and the soft-bodied larva backs
out of the hard shell. The animal remains soft for a short while,
and swells up by absorbing water. Then, minerals from the
seawater (especially calcium) harden the outer covering, forming a
new exoskeleton. When the larva loses the extra water, it
shrinks and leaves space within the exoskeleton for growth.
Megalopal stage |
During this part of her life, the crab floats in the open water
offshore where salinity is relatively
high. She probably feeds on microscopic algae and other small
larvae (plural form of larva). After the last zoeal stage,
the crab enters a megalops stage, which lasts 6-20 days. This
is the first step toward obtaining the typical crab form-the body
becomes wider with legs protruding from the sides, but with the abdomen still stretched
out behind.
Juvenile blue crab
approximately
4mm (1 1/2 inches) wide
|
The megalopa takes advantage of tidal currents to move into
estuaries where salinity is lower, food is abundant, and shelter is
easy to find. There, she molts to a true crab form, but is
only 2 mm wide (about twice the width of a paper clip wire).
As a juvenile, she is omnivorous, meaning she will eat both animal
and vegetable substances, such as fish, shellfish, and aquatic
plants. She also must avoid predators such as spotted
sea trout, red drum, black drum, sheepshead, and other crabs.
She continues to molt, growing larger each time until she reaches
adult size (about 130-139 mm or 5¼ - 5½ in.) after 18-20
molts. The amount of growth with each molt varies depending
on water salinity, temperature, and other environmental factors.
She should reach harvestable size (127 mm or 5 in.) within one
year.
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Molting blue crab
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During her adult life, the female blue crab remains in the estuary, although usually
in higher salinity water than males. She eats fish, crustaceans, worms, and
mollusks, and may be
preyed upon by large fish, birds, and mammals (including
humans). Her molting rate increases during warmer months,
although water temperatures greater than 30oC
(86oF) appear to inhibit molting. During the
cooler winter months her activity slows, although in the warmer
Florida waters she will not need to slow down as much as blue crabs
in more northern areas, who bury in the mud to wait for spring.
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Mating blue crabs--the male is
on top craddling the female
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Sometime between March and December, when temperatures exceed
22oC (72oF), the female crab moves into the
upper waters of the estuary where male crabs are
concentrated. Most female blue crabs reach a terminal molt,
after which they no longer grow. This molt coincides with the
onset of sexual maturity when mating
occurs. Evidence suggests that some females molt a second
time after becoming mature, allowing them to produce more batches
of offspring. Because
of the hard exoskeleton, mating must occur directly after a molt,
while the female is still soft. To ensure he will be there
when she is ready, a male will usually cradle a pre-molt female in
his legs. He also protects her during the vulnerable period
after she molts, until her shell becomes hard again. After
mating, the female moves offshore into higher salinity water while
the male remains in the estuary for the rest of his life.
Along the west coast of Florida, female crabs also migrate
northward toward the Apalachee Bay region.
Overgeous (egg-bearing)
female blue crab |
The female can retain sperm for a year or more before extruding
eggs. This allows crabs mating in fall or winter to wait
until warmer weather to hatch their eggs. Eggs are fertilized
as they pass out of the crab's body and are deposited under the
apron. The apron is actually the curled-under abdomen, and
has small appendages to which the
eggs attach. Egg masses have an average of two million eggs, and
can have up to eight million eggs. At first the egg mass
appears orange due to the high amount of yolk in each egg, then
turns brown as yolk is consumed and eyes develop. After one
to two weeks the eggs hatch into zoea larvae.
Thus the cycle of life is complete. Only one out of every
one million (0.0001%) eggs survives to become an adult.
Predators, adverse environmental conditions, and disease all take
their toll on the millions of larvae that hatch from one
female. Yet some do survive, enough to renew the population
and start a new generation of blue crabs.
References:
Hines, A., P. R. Jivoff, P. J. Bushmann, J. Montfrans, S. A.
Reed, D. J. Wolcott, and T. G. Wolcot. 2003. Evidence for sperm
limitation in the blue crab Callinectes sapidus. Bulletin
of Marine Science, 72 (2): 287-310.
Lipcius, R.N. and W. T. Stockhausen. 2002. Concurrent decline in
the spawning stock, recruitment, and larval abundance, and size of
the blue crab Callinectes sapidus in Chesapeake Bay.
Marine Ecology Progress Series 226:45-61.
Puckett B. J. and D. H. Secor. 2006. Growth and Recruitment of
Juvenile Chesapeake Bay Blue Crab. Technical Report Series No.
TS-497-05-CBL Ref. No. CBL 05-095 of the University of Maryland
Center for Environmental Science.
Steele, P. 1982. A synopsis of the biology of the blue crab
Callinectes sapidus Rathbun in Florida. Pp. 29-35.
In: H.M. Perry and W.A. Van Engle, eds. Proc. Blue Crab
Colloq., Gulf States Marine Fisheries Commission Publication No. 7.
Ocean Springs, Mississippi.
Steele, P. 1991. Population dynamics and migration of the blue
crab, Callinectes sapidus (Rathbun), in the Eastern Gulf
of Mexico. Proceedures of the Gulf and Caribbean Fisheries
Institute 40:241-244.
Steele, P. and T.M. Bert. 1994. Population ecology of the
blue crab, Callinectes sapidus Rathbun, in a subtropical
estuary: population structure, aspects of reproduction, and habitat
partitioning. Florida Marine Research Publications 54:1-24.
Turner, H. V., D. L. Wolcott, T. G. Wolcott and A. H. Hines.
2003. Post-mating behavior, intra-molt growth and onset of
migration to Chesapeake Bay spawning grounds by adult female blue
crabs, Callinectes sapidus Rathbun. Journal of
Experimental Marine Biology and Ecology 295: 107-130.
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