Geoscience Reference
In-Depth Information
Blue crabs utilize the entire bay during their life cycle. Female crabs migrate to the mouth of the bay to seek
the right salinity for the hatching of their eggs. They may produce as many as 8 million eggs, with only one in
a million likely to survive; the rest are either eaten or succumb to negative environmental factors such as im-
proper water temperatures or salinities. Blue crabs begin life as larvae called zoea, with large eyes in a large
head and a shrimplike tail. These larvae go through seven or eight molts while they drift passively at the mouth
of the bay; occasionally they are blown out to sea and lost. After four or five weeks they metamorphose into a
second larval stage, becoming megalopa—a more crablike creature featuring crude claw arms, three pairs of
walking legs, and stalked eyes, but retaining the shrimplike tail of the zoea.
After a week, the megalopa undergoes another metamorphosis into a recognizable juvenile crab capable of
swimming and walking on the bottom of the bay. These tiny crabs now reverse their mother's migratory direc-
tion and begin working their way up the estuary toward its head. They keep close to the shore, taking shelter in
the sea grasses to avoid predators, including mature blue crabs, which will cannibalize the young. It may take
seven or eight months for them to migrate half the length of the bay, by which time it is late autumn. The cold
waters then send them to the bottom of the bay, where they dig into the mud to hibernate. In spring, the new
crabs resume their steady migration toward the head of the bay.
Crabs may live up to three years, though many are caught by the bay's traditional fishers, or “watermen,” or
succumb to natural mortality before then. Crabs regularly outgrow their hard shell, or exoskeleton, and must
shed it, a process known as molting, which may occur as many as twenty-seven times during their lifetime.
During the three-day period it takes for the shells to harden, the crabs are particularly vulnerable to predation.
Molting, however, is critical to the crab's growth and survival, as well as to successful reproduction. When fe-
males become sexually mature and are about to molt, they release chemicals into the water that attract males.
Blue crabs have a remarkably elaborate mating ritual. Males raise themselves on the tips of their walking
legs and wave their arms as an attention-getting ploy, at the same time kicking up sand with their swimming
legs. Recent research indicates that the arm waving by the males may help to waft sexually attractive
scents—pheromones—to the prospective mate. Receptive females respond favorably by rocking back and
forth, then backing up under the raised male, who grasps his partner with his claws. The pair assumes what is
known as the cradle position, with the male carrying the receptive female underneath him for one or two days
until she is ready to molt. The pair separates briefly while the female undergoes her molt; then they resume
their pas de deux. The male now gently overturns the female and inserts, face-to-face, his pleopods, containing
the sperm, into the female's genital pores. The mating may take from five to twelve hours, during which the fe-
male expands her abdomen so that it folds around and over her partner's back to hold him in place.
After copulation, the pair resumes the cradle position for forty-eight hours. This behavior has two possible
purposes: it may protect the female during the vulnerable post-molt stage, or the male may simply be protect-
ing his own posterity by preventing the female from mating with another male. Now the female begins her mi-
gration toward the mouth of the bay, while the male remains in the brackish rivers and inlets. The female fertil-
izes her eggs during her migration but may hold back a sperm packet for nearly a year for a second fertiliza-
tion, a strategy that may compensate for poor environmental conditions in any given year.
Blue crabs have been recognized as a keystone species in Chesapeake Bay, important as both predator and
prey in the bay's food web. As predators, they feed at more than one level of the food web, and therefore their
increase or decrease can have cascading effects on the rest of the ecosystem. It has been shown, for example,
that blue crabs enhance salt marsh production by feeding upon marsh periwinkles, which at high densities can
reduce salt marsh grasses by their grazing activities. The crabs' preferred prey is the hard shell clam, or qua-
hog, the most common clam in the Chesapeake, but as omnivores they also prey on a variety of small fishes,
marine worms, and other crustaceans, like shrimp, barnacles, mud crabs, and immature blue crabs, as well as
plants. Juvenile crabs are in turn preyed upon by striped bass and other fishes, including drums, eels, catfish,
cownose rays, and some sharks. As larvae they are consumed by filter feeders such as menhaden, bay an-
