The female crab mates
only once upon maturity. She
receives sperm from the male and stores it for later use.
The females, upon mating, migrate to the higher salinity water of the
mouth of the Bay to spawn(lay eggs). Although
the female will mate only once she is capable of spawning several times using stored
sperm to fertilize her eggs. Each
time the female spawns she develops an external egg mass containing millions of
eggs, which will hatch in about two weeks.
Zoea
The
blue crab begins life as a microscopic larval form called the zoea.
The zoea use the tide and winds to float to the higher salinity waters of
the Atlantic feeding on smaller plankton.
The zoea remains in the higher salinity waters going through seven developmental stages before migrating back to the fresher waters of the Chesapeake Bay.
Megalopa
When the zoea reach
the post larval stage called the megalopa, it will travel back to the bay for
settlement. At this point the
megalopa will undergo a single molt and take the form of a juvenile crab.
Juvenile Crab
The juvenile crabs, now resembling a crab,
migrate into shallower, less-saline waters where they will use the submerged
aquatic vegetation(SAV) as a safe place to feed on clams, and small fish while
staying hidden from predators such as the striped bass, and larger crabs.
Today juveniles may
be at a higher risk to predation, due to dwindling SAV’s.
Juvenile
crabs will continue to grow for about 12 –18 months into a mature adult by
molting when water temperatures are warm.
Temperature
plays an important role in the life cycle of the blue crab.
Below 9-10 ºC the blue crab becomes inactive, and stops molting.
However as temperature rises above 9-10 ºC the crab begins to grow
again.
When
temperatures are too low, usually between November and April, the crab will bury
itself in the sediments and enter a dormant phase in which it will stay until
water temperatures rise again.
Molting
pictures courtesy of Smithsonian Environmental Education Center
The
blue crab, juveniles and adults, periodically shed or molt their exoskeleton.
Under the calcareous carapace and claws a new soft shell grows to replace
what will be lost during molting. After
the old exoskeleton is shed the new larger exoskeleton will slowly harden over a
period of approximately 72 hours. During
this time the soft crab is immobile and vulnerable to predators, as well as
other crabs that will cannibalize the unprotected soft-shelled crab.
The new carapace will be larger than the crab, as you can see in the last picture above, accommodating the crab with room for it’s soft inner body to grow. The soft internal structures of the crab will continue to grow to the limit that its exoskeleton can hold and once this limit is reached it will begin molting again. A crab will molt about twenty times in its three year life expectancy. The female will undergo fewer molts than the male since it will go through a final molt upon reaching maturation.
The growth rate and age distribution of the population are critical factors that must be considered by scientists and fisheries management assessing the blue crab populations ability to sustain its self. But this is a difficult task due to the fact that the crab molts. As you have already read, the blue crab loses all of its hard material during a molt, which presents it’s own problems for the crab, however scientists rely heavily on hard structures such as otoliths and scales to determine the age of organisms. Age can often be determined from these structures much in the same way a trees age is determined by counting the rings in a core sample. The blue crab doesn’t leave any of these physiological markers upon molting. Therefore, other methods must be used to predict the crabs age.
We will examine these methods in the following activities.
picture on this page courtesy of: Smithsonian environmental education center