No. 06–Spring Delicacy, Part I

Spring is the bearer of many delights, not the least of which is the soft shell crab season. For a very short period, usually parts of April and May, local crabbers work long hours to, with luck, cash in on the mating habits of the Atlantic Blue Crab (Callinectes sapidus). That translates from the Latin as “beautiful swimmer”.

A soft shell crab is a crab that has just molted. The endoskeleton of a vertebrate grows with the animal. Crustaceans like crabs and shrimp have exoskeletons which do not grow and must be shed for growth to continue.

A blue crab will molt at least 24 times, to include two larval stages, in a full life span. The frequency of molts decreases as the size of the increases. Growth/molt frequency is also affected by the amount of food consumed. All cold blooded species grow at a rate commensurate with food intake. It is possible to have two animals of the same size but of different ages because of a difference in feeding success.

Molting is a dangerous process for crustaceans and I always know when that time is approaching for my charges at the Estuarium. Normally voracious crabs will refuse food and thereby signal a molt two days thereafter.

The molting process starts with the release of hormones that soften mussel tissue and begin the formation of a layer of skin which gradually detaches from the surrounding exoskeleton.

The molt commences as a seam that runs laterally around the body begins to separate from rear to front. The body swells with water forcing the carapace (top of shell) slowly upward as with small pulses of movement and energy the struggle of extrication begins. The large hormone softened mussels of the claws are slowly extruded through joint openings. Respiration ceases as the carapace rises cutting off the flow of water over the soon to be abandoned old set of gills.

The crab is totally defenseless at this point and facing the twin perils of being eaten by a predator and suffocation within it’s own shell. A crab will usually seek cover before molting to guard against the first and chances of succumbing to the latter are small but they do exist. On display at the Estuarium is a stone crab who came to such a fate after four years and seven previously successful molts in captivity.

Once a crab has completed a molt it continues to swell to it’s next size and after two days the new exoskeleton will be completely hardened and a really hungry crab will be able to get on with it’s life.

For those who like to catch and eat boiled crabs, keep this in mind. A bigger shell after molting does not equate to a bigger crab inside. It’s like buying shoes for kids. You buy them with room to grow. Same with the crab. You may have had the experience of cracking into a large crab to find little to eat inside. That was no doubt a crab that had recently molted. If you catch a crab with bright intense colors like it just came off the showroom floor, it probably did. You want what are called “rusties”. They’ve got brown algae growing on them and maybe a barnacle or two, that’s a full crab.

With the next issue you will learn the mating habits of the blue crab and why all the soft shell crabs you eat are females. This and future columns may be found on the Estuarium’s web site at www.lowcountryestuarium.org. By the way, don’t miss the Soft Shell Crab Festival in Port Royal, usually the third Saturday the afternoon of April.