Blue crabs start their lives as tiny zoea, floating in the ocean waters. While growing into the megalopa stage, the crabs drift back into the Chesapeake Bay, where they grow into juvenile crabs and, eventually, adults. The blue crab’s life cycle is also closely related to its migration patterns. Let’s take a closer look at the stages the life of the blue crab.
Female Crab Migration
Mature female crabs migrate to the mouth of the Chesapeake Bay to develop a brood of eggs. Depending on the size of the female, female blue crabs carry and release 800,000 to 8,000,000 eggs.
Eggs Develop into Larvae
When the larvae reach a certain developmental stage the eggs break open and the larvae –now called zoea–are released into the water column.
First Larval Stage: Zoea
The eggs hatch into the first larval stage called the zoea. Zoea are too small to swim and simply float in the ocean waters where they feed on phytoplankton. After four or five weeks of floating in ocean currents, a zoea develops into the second larval stage and is now called a megalopa. A zoea molts seven times before becoming a megalopa.
Second Larval Stage: Megalopa
The megalopa looks a bit like a crossover between a crab and a lobster. In this phase, the crab is still too small and too weak to swim. Carried by the currents and the tidal flow from the ocean, the megalopa drifts back into the Bay. The megalopa is a voracious eater and feeds mainly on the other zooplankton. This larval stage only lasts for about a week, at the end of which the megalopa molts into the first crab stage, a tiny juvenile crab. This happens about 45 days after the initial hatching of the eggs.
Juvenile crabs are tiny crabs that can swim or walk on the muddy bay bottom. The juvenile crabs continue to migrate into the Bay along the shallow areas and use the underwater sea grasses as a refuge. By hiding in these beds of underwater grasses they can escape from large crabs and other predators.
After about 12 to 18 months, a juvenile crab reaches maturity. When young female crabs become mature, it is time for them to mate to create the next generation of blue crabs. After mating, the mature females migrate to the mouth of the Bay carrying her brood of eggs.
Mature males don’t migrate toward the mouth of the Bay. They remain in the rivers and inlets of the Bay. During winter, they move to the deeper waters where the temperature doesn’t fluctuate as much as in the shallow areas. The crabs bury themselves into the mud and slow down their metabolism for a period of hibernation. They will hibernate throughout the winter–feeding only infrequently. When the Bay starts to fill with spring runoff and the waters begin to warm, the crabs start to become active again and move to the shallow rivers, creeks, and tidal wetlands.