From the Bay and Back Again: The lifecycle of the blue crab
The blue crab embodies a unique life experience for aquatic animals in the Chesapeake Bay.
Unlike finfish, this invertebrate has a hard exoskeleton and must molt, or shed, its shell in order to grow. As you can imagine, this process puts the crab in a vulnerable situation as it loses its protective hard shell and becomes a soft crab—a top menu item on many predators’ lists, including ours.
Before a crab can achieve this delectable point in its life, it must survive a gauntlet of challenges from the mouth of the Chesapeake Bay out into the Atlantic Ocean and back up the bay toward Maryland.
We might say that the life cycle process begins with the terminal, or final molt, of an adult female crab. Just before this occurs, a male will cradle the soon-to-shed female and carry her to a protected area. These paired crabs or doublers are a common sight in Maryland waters in the late summer.
Once she molts, they mate while she is in the soft shell stage.
The male continues to cradle her and protect her until her new shell hardens. Even though mating has occurred, the eggs are not yet fertilized. The female stores the sperm and delays fertilization until water temperature, salinity, food availability and other environmental conditions are favorable for the survival of her offspring.
When the doublers separate, the female joins other recently matured females in the fall to migrate to the mouth of the bay, while the male continues to search for other mates. After mating, the female will not molt again. The male will continue to shed throughout his lifetime.
Crab eggs need a salinity of at least 18 parts per thousand to survive. Crab larvae need the higher ocean salinities of more than 30 parts per thousand in order to endure. When a female crab makes it to the salty lower bay, she fertilizes the eggs and develops an external sponge-like egg mass containing more than a million eggs. In fact, a female crab can produce more than one egg mass from a single mating event.
The eggs hatch into a microscopic first larval stage called zoea, which are swept out to the Atlantic Ocean by the bay’s currents. It takes about 31-49 days for the zoea to transform into megalops, the next larval stage. At this phase of development, the larvae can crawl along the bottom and feed on tiny fish larvae as they find their way back to the bay.
After six to 20 days, megalops metamorphose into the first crab stage. A tiny juvenile crab measures just one fifth of an inch from point-to-point with claws capable of preying on small fish and even other baby crabs. This tiny crab will molt often and grow rapidly, reaching maturity within 18 months, to bring the cycle full circle.
As you can imagine, the journeys up and down the Chesapeake Bay for both juvenile crabs and spawning adult females are not easy trips. The blue crab life cycle can cover the entire Chesapeake Bay, and it is finally dependent on the currents of the near-shore ocean. This complexity means the number of little crabs returning is highly variable. Other environmental factors such as water temperature, the abundance of predators and the amount of protective subaquatic vegetation also impact how many crabs survive to reproduce and keep the cycle going.
Maintaining a healthy and sustainable blue crab population means preserving a robust spawning female population. For this reason, Maryland shares a bay-wide commitment with Virginia and the Potomac River to conserve spawning females by managing the harvest and protecting vital spawning grounds.
Fortunately for us, blue crabs are a fast-maturing, prolific species with a high reproductive capacity, so the population has the ability to rebound quickly under favorable environmental conditions and remain a keystone of our summer diet.
Article by Brenda Davis—Fisheries Service blue crab program manager.
Appears in Vol. 19, No. 2 of the Maryland Natural Resource magazine, spring 2016.