The Life Cycle of A Blue Crab in Florida
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.
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.
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.
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.
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.
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.
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