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Blue Crab Growth & Molting


Molting (or Ecdysis)

Blue crabs are invertebrates meaning they lack a spinal column. Instead, the blue crab has a rigid exoskeleton (hard shell.) The shell grows in discrete stages interspersed by molting, but the growth of internal tissue is more continuous. In order for the crab to grow larger, it must periodically shed its smaller shell through a process known as molting (or, more accurately, ecdysis.) Early in its molting cycle the crab slowly begins to form a new soft shell underneath its existing hard shell. There are certain marks or signs that indicate how soon the crab will molt (see Molting Signs for more information.)


When a blue crab has grown sufficiently to require a larger shell, the following events occur:

Precdysis (pre-molt or “peeler” stage)

  • Molting hormones are released.
  • The hypodermis detaches from the existing hard shell. The hypodermis is a layer of cells directly beneath the shell.
  • The hypodermis produces enzymes which begin to dissolve the shell components. Much of the existing shell is recycled causing it to become thin. Inorganic salts are resorbed from the shell and stored internally.
  • A new inner soft shell slowly forms underneath the existing shell. When this new shell has fully formed, the crab will be ready to molt.

Ecdysis (molting or “busting” stage)

  • The crab stops eating and seeks shelter in order to avoid predation. During this process the crab is highly vulnerable to predators, including the two-legged variety!
  • The crab rapidly absorbs water which causes its tissues to swell and split the old shell open across the back between the lateral spines. Fracture planes in the claws split open to allow the claws to be pulled through.
  • The crab begins the slow, arduous process of backing out of its old shell, which is then discarded.
  • The newly molted crab pumps water into its tissues in order to inflate the shell to its new size. The new shell will be roughly one-third larger (33%) than the old shell. The new shell reaches its full size within six hours after molting.

Postecdysis (postmolt or “soft shell” stage)

  • The salvaged inorganic salts are rapidly redeposited to help thicken and harden the new shell. The new shell will only harden in water (the hardening process stops if the crab is removed from the water) and will take approximately two to four days to fully harden.

Over time, as the crab slowly grows inside its new shell, tissue water is replaced with protein. Once there is no more room left to grow inside this shell, the whole molting process starts over again.

Note that some people mistakenly believe that a soft crab is completely different species of crab. As you have just learned, this is not true.

Also note that many people believe that a full moon yields larger catches of soft crabs (click here for more information.) This belief is reinforced by the fact that most commercial harvesters claim to see an increase in peeler/soft crab abundance before and after a full moon.

To see a crab during ecdysis, click here.


Molting Signs

As mentioned earlier, there are certain signs that indicate the instar period (stage between molts) of the crab. Watermen have come up with some colorful terminology to describe these various stages.

  • Green crab - 14 to 50 days prior to molt, depending on crab size. Very hard shell.
  • White sign peeler - Two weeks prior to molt.
  • Pink sign peeler - One week prior to molt.
  • Red sign peeler - Two days prior to molt.
  • Rank peeler - Hours prior to molt.
  • Buster - In the process of shedding its old shell.
  • Soft shell - Immediately following molt.
  • Paper shell - 12 hours after molt. Slightly stiff shell.
  • Buckram - 24 hours after molt. Semi-stiff, crinkly-hard or leathery shell.
  • Whitey - Four or more days after molt. Bottom of shell is a lustrous white.

To learn how to recognize these signs, please see Molting Signs.


More about Molting

Females molt 18 to 20 times, not counting larval molts, in the course of a lifetime. Males are believed to go through the ordeal 21 to 23 times. With both sexes the time between molts (instar) grows progressively longer ranging from 10 to 15 days for one-inchers to 30 to 50 days for legal hard-crab catch size and beyond.

The older a crab grows, the more difficult the molt, to the point that there may be considerable natural mortality in the process. Considering this and the enormous catches by watermen, one expert estimates that a blue crab is lucky to live more than a year in the Chesapeake Bay (a long life span for a blue crab is about three years with males tending to outlive females.)

It is important to note that crabs will mate only when the mature female crab has just molted and is still a soft or buckram crab.

Jeffery Shields of the Virginia Institute of Marine Science says that it was once believed that blue crabs experienced anecdysis or a terminal molt (in other words, the crab would never molt again and would remain the same size.) This molt was considered the second molt after puberty for males and the only molt after puberty for females. While this is generally the case, most portunid crabs do continue to molt. There are some caveats that apply to blue crabs:

  • Molting takes energy; energy that is better used for reproductive output. Since it takes more energy to produce eggs, the theory is that mature female crabs don't grow as large or molt as frequently because of their reproductive energetics. The converse is that sperm production is cheap, so males don't put energy into reproduction, rather they put it into somatic growth.
  • Molting is energy dependent. Larger animals must store far more nutrients for molting than do smaller juveniles. Thus, a really big lobster only molts every 2 to 5 to 10 years. Similarly for blue crabs, the larger the crab, the more difficult to store energy for molting.
  • Molting is risky business. Larger animals may be at more risk, hence, they are not frequent in the population.


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