No. 18–Molting

Some time back, the Estuarium’s board of directors adopted the motto, “Molt or Die”. It was a lesson learned from observing our crustacean friends who must shed their outgrown and confining exoskeletons in order to grow. In nature, as in the oft uncertain world of nonprofits, molting can be a dicey process fraught with challenge.

The molting process (ecdysis) in crustaceans is triggered by the release of hormones. Over an approximately two week period, a layer of cells separates from the interior surface of the exoskeleton forming a soft skin layer. In the process the old exoskeleton is cannibalized for the minerals and trace elements that will give the subsequent exoskeleton its strength.

The physical act of molting becomes visibly discernable as the animals muscle tissue swells with water. The crab literally busts out of the old shell. It begins at the rear with the carapace (upper section) rising and separating laterally from the rest of the body.

A crab is defenseless as it struggles to free its legs and claws. With luck it finds suitable cover to conceal itself from predators and is able to extricate its extremities before the continued swelling of its tissues make that impossible. Entrapment in the old exoskeleton brings the mortal threat of death by asphyxiation. The stone crab pictured on this page met such a fate.

Crabs respire by drawing in water through vents located above where the claw arms attach to the body. The water flows over the gills and exits through the mouth. That life sustaining flow ceases with fatal results if the crab becomes wedged with the carapace raised and pinching off the outflow. If it possesses the strength to do so, a crab will sacrifice entrapping appendages and will rip free from death’s grasp. Missing limbs will be regenerated on the succeeding molt.

The Estuarium’s resident mantis shrimp recently survived a particularly difficult molt to emerge moderately disabled. Respiration was not a problem as it is with crabs due to physiological differences in the species. In this case exhaustion and starvation were twin threats as it struggled for more than a week to free itself from old exoskeletal remnants covering its claw arms and mouth parts. It survives with one claw arm gone and the other strangely misshapen. I look forward to its next molt to see how the issue of the misshapen claw arm is resolved.

It should be noted that while researching this column I discovered a couple of previously held assumptions of mine which have appeared in previous columns were incorrect. I had mistakenly thought, through hearsay and observation, that female blue crabs spawned but once before death. The fact is that they mate but once and that the implanted spermatophore may give rise to multiple spawns.

I had also been under the impression that there were but two larval stages for the blue crab, zoea and megalop. I now understand that the zoea has seven expressed larval stages and the megalop is post larval. Such mistakes are the lot of the self-taught amateur naturalist. As in all pursuits, one must molt confining misconceptions or die intellectually.

To end on a whimsical note, I found that Microsoft Word Spell Check did not recognize the word ecdysis. It recommended instead “ecdysiast” which is a euphemism for striptease dancer. Get the connection?