LEWES – This summer, watermen will haul thousands of bushels of blue crabs from the depths of local bays and oceans, carry them to shore and heap them onto plates from Baltimore to Dewey Beach.
It’s hard to imagine, but the blue crab’s natural environment is not flanked by Old Bay seasoning. In the span of about three years, Maryland’s signature crustacean undergoes a complex life cycle shaped by currents, which take them from bay to ocean and back again.
During the winter, crabs burrow in the mud and hibernate. But once the water warms up, they’ll emerge and start feeding again.
Blue crabs mate throughout the summer, marking a once-in-a-lifetime event for the females but a frequent ritual for promiscuous male crabs.
A male crab, or jimmy, will shed his exoskeleton as he grows throughout his life, but the female crab, or sook, will shed only after she reaches maturity before mating. The sook reaches maturity when the triangular apron on her underside develops into a rounded one.
When crabs relinquish their protective armor during shedding, they also become more vulnerable to predators.
Without their shells, crabs are weakened and can’t fight back, explained Kelly Webb, a crab program biologist at the Maryland Department of Natural Resources. During this period, soft-shell crabs hide in the marshes and take shelter in seagrass beds.
Striped bass and sea bass prey upon soft crabs, but once crabs develop their hard shells again, most large fish back off from their hardened claws.
The dance and the hatch
Before mating, the jimmy courts the sook through a dance. The female will move below the male crab once she is ready to mate, but the male crab must wait until the female’s shell cracks before he can mate with her.
After mating, the sook’s apron becomes distended with a large, yellow sponge mass containing 1 to 3 million eggs. The pregnant females move into higher salinity water at the mouth of the bay. Crabs brood their eggs for about two weeks and will hatch once the spongy egg mass darkens from yellow into a chocolate brown color.
“Probably one or two survive,” said Roman Jesien, a science coordinator with the Maryland Coastal Bays Program. “There are a lot of things that eat them. That’s why they produce a lot, so their chance of surviving is good.”
Eggs typically hatch during the late summer months, but weather can also play a part in the hatching season, Jesien said. When temperatures are warmer, the eggs tend to hatch faster. Currents and winds can often bring warmer waters, speeding up the hatching process.
In their first stage, crabs hatch into zoea larvae. They spend about 40 days in this microscopic state and feed off of phytoplankton, a microscopic plant. Zoea larvae don’t resemble their blue crab parents, but rather a tiny, curled crawfish.
From the minute they are brought into this world, the crabs are at the mercy of the wind and currents. The blue crabs are versatile creatures that can live in various salinity levels, though, and have no problem moving from freshwater bays to the salty seas.
Larvae are flushed out of the bay and carried south by currents through estuaries. Charles Epifanio, a professor of Marine Science at the University of Delaware who has studied crabs, said outflow from estuaries affects larvae transportation and settlement.
“The years in general where we’re going to have proportionally higher survival of larvae in the nursery areas are years when you have low outflow, relatively low rainfall and, in the summer, when you have persistent southwest winds,” he said, adding northeast winds in the fall can also lead to high survival rates.
After their 40 days as zoea, the crab enters its post-larvae megalopae stage. The megalopae do not yet resemble a blue crab and are still microscopic, but can now feed on other larvae rather than just phytoplankton. Megalopae are pushed north by winds, and nor’easters bring them back into the bay. After about a week, the megalopae transforms into a juvenile crab.
The juvenile stage lasts about 12-18 months, Epifanio said.
A link in the food chain
Crabs thrive in a marshy environment with oxygenated water and plenty of seagrass for shelter, Jesien said.
Blue crabs hunt snails and small razor clams, but anyone who has seen the crustaceans clamor over raw chicken in a trap knows they are also hungry scavengers.
“They are known for breaking up dead and decaying matter,” Webb said. “They are opportunists when it comes to feeding. There isn’t much a crab won’t try their hand at.”
Like vultures of the bay, blue crabs play an important role in the food web, breaking down organic matter. Their sharp claws can tear apart dead organisms, making more accessible pieces for smaller species to eat.
A lucky blue crab who avoids bass, watermen and other predators can live about three years. But with 40 percent of the nation’s domestic crab supply coming from the Chesapeake Bay and another 5-10 percent from the Delaware Bay, it’s more likely the blue crab will end its life on a bed of lettuce than a bed of seagrass.
On Twitter @LeighGiangreco
THE TRANSFORMATION
1. Female crabs, or sooks, reach maturity once their triangular apron becomes rounded.
2. Unlike males, females do not continue growing once they reach maturity. Females shed once before they mate. After mating, female crabs move into higher salinity waters.
3. Two months after mating, the sook’s apron becomes distended with a large, yellow sponge mass containing 1 to 3 million eggs. Female crabs sit on the eggs for about two weeks before hatching in late summer.
4. Eggs hatch into microscopic zoea larvae and are flushed out of the bay, where they float freely to the ocean.
5. Zoea transform into megalopae after about 40 days. Nor’easters bring them back into the bay. After a week, megalopae turn into juvenile crabs, a phase which lasts about a year to 18 months.
6. A typical blue crab lives about three years.
