When blue crabs grow, their outer shell–the exoskeleton–doesn’t grow with them, so crabs must regularly shed these shells in order to increase in size. This process is known as molting.
A crab that is ready to molt is commonly called a peeler. When ready to molt, the crab “cracks” its shell open from the back and then backs out.
Crabs molt about 27 times throughout their lives. Young crabs molt very frequently–a zoea molts seven times with only a few days between each molt, but as the crab grows older the time between molts lengthens.
After a crab reaches maturity the next molt can even be a couple months away. Food availability and environmental conditions can slow the molting process.
Without its old hard shell (which, once discarded is called a “slough”), the crab is now temporarily a soft-shelled crab. The new exoskeleton is there, but it is still very soft.
After shedding its old shell, the crab first expands its new shell by pumping water into its body. After that, it takes about 72 hours (three days) for the soft shell to harden. Until the shell is hard again, the crab is very vulnerable and has to hide from predators.
The molting cycle has several distinct cycles.
In general, there is the pre-molt stage when the crab is getting ready to molt within a few days.
Then there is the actual molting activity (ecdysis), which can last up to a few hours.
Next is the post-molt stage, during which the crab is expanding it’s shell and avoiding predators.
The last stage is the intermolt stage, during which a crab is feeding and continues to increase in tissue mass underneath the exoskeleton.
The various stages of the molting cycle are indicated by external and internal signs. The picture below show the gill raker (or scaphognathite, a structure on the crab’s inside) and the swim paddle (visible from the outside), two structures that change in appearance during the molting cycle. The gill raker is a feather-like structure that brushes over the gills and distributes any moisture inside the crab over the gills to keep them moist so they can continue to receive oxygen.
The picture below shows how the gill raker (top series) and swim paddle change in appearance during the molting cycle. On the left, during the intermolt, the featherlike structures in both series contain tissue. Going towards the right, and moving forward in the molting cycle, the crab pulls this tissue into its body, out of the exoskeleton, which is visible as a dark (top series) or colored line (bottom series) inside.
Determining Age & Growth
Growth in decapod crustaceans is a stepwise, discontinuous process consisting of a succession of molts (ecdyses) separated by a period of intermolts. During the intermolt period the integument is hard and growth is suppressed; at each molt the integument is shed and rapid growth in a very short period occurs before the new integument hardens. Consequentially, growth is described in two components. One is the molt increment or increase in size occurring at a molt and the other is the molt interval (instar length) or duration between two successive molts. Small weight increases also occur through relative changes in tissue content during the intermolt period (relative increases of whole body protein compared with moisture).
The mechanism controlling ecdysis is an antagonistic interaction between a molt inhibiting hormone produced in the x-organ sinus gland located in the eyestalks, and the molting hormone ecdysone produced in the y-organ located ventral to the eyestalks. Just prior to molting, inorganic salts are withdrawn from the exoskeleton and stored in the gastroliths and elsewhere. The old exoskeleton is broken down by enzymes in the molting fluid, and a new cuticle is secreted. The crab rapidly absorbs water, ruptures the old exoskeleton along sutures between the carapace and sternum, and backs out of the old exoskeleton. This is followed by a rapid redeposition of salvaged inorganic salts to harden the new cuticle. Tissue growth is rapid; blue crabs ranging from 10-160 mm CW attain maximum expansion of their new exoskeleton within 6 hours following molting. Tissue water is replaced with protein during the next intermolt period. Blue crabs cease feeding and seek out shelter and isolation from non-molting conspecifics as ecdysis approaches. A lunar rhythm of molting has been observed.
Blue crab larvae normally develop through seven zoeal stages requiring 31-49 days and one megalopal stage persisting for 6-20 days. Growth of juveniles and adults occur during a series of molts and intermolt phases, each of which is termed a “crab” stage, according to the number of molts that have occurred since the megalops stage. “First crabs” measure about 2.5 mm CW at transformation and grow toward maturity through a series of 18-20 molts.
Intermolt periods increase with age of blue crabs, but are generally shortened by ample food availability, high dietary nutrient quality and high water temperatures. Small crabs molt frequently with molt frequency decreasing as carapace width increases. Molting frequency by size is : 5 mm CW – every 3-5 days; 10 to about 100 mm CW – every 10-15 days; larger crabs – at every 20-50 days. Molt intervals of juvenile blue crabs are also influenced by water temperature; molt intervals are three to four times as long in winter as in summer, with molting virtually ceasing below 13oC.
Mean incremental growth per molt in blue crabs has been shown to be highly variable. Mean percent growth increments varied from 11.9% for small juvenile females (10-19.9 mm CW), 33.7% for larger females (100-109.9 mm CW) 14.4% for small juvenile males (10-19.9 mm CW) and 32.9% for larger males (80-89.9 mm CW).
Larger crabs are often found in areas of low salinity, suggesting a negative correlation of size with salinity. Laboratory studies evaluating the effects of salinity on growth of blue crabs, however, have yielded conflicting results.
Males become sexually mature at the 18 or 19th molt but may continue to grow and molt an additional 3-4 times thereafter. Female crabs were initially thought to rarely, if ever, molt again following their pubertal molt. However, mature females undergoing a second molt have been verified.
No data on maximum age of blue crabs is available from the Gulf of Mexico, although it has been estimated to reach a maximum age of 4 years in Florida and 7-8 years in Chesapeake Bay.
Autotomy (voluntary breaking of appendages) and regeneration are common in blue crabs. One survey found that 19-25% of blue crabs were either missing or regenerating a limb. Autotomy occurs when a the break devolops along a fracture plane located at the appendage’s distal base. A functional appendage is formed by regeneration following the next molt, although three molts may be required for 100% limb regeneration.