Blue crabs’ molting miracle

Blue crabs molt about 30 times during their live, with young crabs sometimes molting every week.
Blue crabs molt about 30 times during their live, with young crabs sometimes molting every week.

Crustaceans can grow new claws and legs

Ecdysis — it sounds like something forbidden to do.

But really, it’s just the process of crustaceans shedding their shells.

Blue crabs essentially live in armored boxes — unstretchable boxes. So in order to grow, blue crabs, like other crabs, lobsters, crawfish and even shrimp, have to cast off (molt) their old shells.

As they feed and grow, they gradually pack their shells with meat until essentially nothing else will fit. With the need for a molt approaching, a crab will grow a new soft and flexible shell beneath the old hard one.

Immediately before shedding its shell, a pre-molt crab will usually seek any kind of shelter that it can find, as it will be soft and defenseless against predators for 12 hours after shedding.

Research done in Chesapeake Bay found that 54 percent of crabs taken from grass beds were near shedding or had just shed their shells. On open bottoms, only 24 percent of the crabs sampled were in these stages.

The shedding process is complex. As time for a molt nears, enzymes cause the upper and lower layers of the crab’s shell to separate. Some of the old shell is actually dissolved and used in the building of the new shell. The enzymes also dissolve thin sutures that hold parts of the shell together. Immediately before shedding, the crab absorbs enough water into its body to separate the old shell along the suture lines.

This crab is just beginning to back out of its old shell through the split in its back.
This crab is just beginning to back out of its old shell through the split in its back.

The longest opening in the old shell is along the rear edge of the crab. Through this opening, the crab gently backs out of its old shell, claws, legs and all. Once the crab begins emerging, the actual shedding process itself takes anywhere from 15 to 30 minutes. The newly molted crab has a wrinkled and extremely soft shell and moves very little, appearing exhausted.

After a crab has shed its old shell, it pumps its new soft-shelled body full of water, stretching its wrinkly skin smooth.

Within a few hours, the new shell has hardened enough that the top shell has a papery feel. Not surprisingly, a crab at this stage is referred to as a papershell.

Several hours later, when the shell has fully hardened, it will be as much as 35 percent larger from point to point (and at least double in volume) than the old cast-off shell.

The new shell will be bright in color, unstained and undamaged by wear and tear. The crab will feel very light in weight for its size because it contains only the meat it had in the old, much smaller shell. These crabs are called “kites” or “clear crabs,” and are obviously not very desirable for boiling.

As the crab feeds, it will gradually fill its new shell again. Feeding rates depend almost entirely on water temperature.

Blue crabs feed vigorously at water temperatures above 59 degrees Fahrenheit. Feeding slows gradually down to 50 degrees and slows dramatically after that. At 40 degrees, it essentially ceases and the animal goes dormant.

Blue crabs molt about 30 times during their lives. Young blue crabs may shed every week. Very large, old mature male crabs may shed once a year or less. Research shows, in agreement with popular thought, that blue crab molting is in synchrony with moon phases. Nearly twice as many crabs will be shedding during the period of the full moon than during the new (black) moon.

Lots of things besides growth happen when blue crabs molt. For one thing, this is when they exhibit their near-miraculous ability to grow new legs and claws to replace those lost to predators.

The new shell growing under the old shell develops what scientists call soft limb buds (and commercial crabbers call rubber legs) that will often partially protrude through the hole left in the shell by the lost leg or claw.

A regenerated claw is typically about half the size of an undamaged claw on the first molt. By the second molt, the claw is full-sized.

Another thing that happens during shedding is reproduction. Male crabs have a pair of long wire-like gonopods beneath their aprons that they use to transfer sperm to females during mating.

But they are unable to insert their fragile gonopods into the female as long as she is wearing her armored shell. The act is consummated while a female is in the soft-shell stage.

A female will mate once in her life and from the stored sperm produce more than eight broods of eggs.

Males find females who will be ready to mate when soft by following the scent of hormone-like pheromones that females release in their urine. Once a male finds such a female, he will cradle her between his legs and under his shell for several days before she sheds her shell, as well as several days after.

About Jerald Horst 959 Articles
Jerald Horst is a retired Louisiana State University professor of fisheries. He is an active writer, book author and outdoorsman.