The molt of a spider crab
Be warned. Now and then when walking along a sandy beach around New York Harbor or along a nearby Atlantic Ocean beach, you may come across a crime scene. So shocking, it’s like nothing you have ever seen. It’s a death of biblical proportions, the sight of many carcasses strewn along a beach.
The tide goes out with an omen - piles of dead bodies, perhaps in the thousands, stretched out across the edge of a beach. Worst of all, there will be no warning when it will happen. It just does, though it often takes place in the warmer months of the year.
As gory and grisly as it sounds, this is no ordinary crime scene. The victims are not human, but crustacean. The crabs are not even lifeless, just evidence of a crusty critter shedding its old shell.
These are not dead bodies, but empty shells or molts. As many crabs grow, they molt. Blue crabs do it, horseshoe crabs as well, and even the creepy looking spider crab. They all molt, which is the process of shedding an external skeleton for the purpose of growth and maturity. Just as kids outgrow their clothing, crabs outgrow their shells.
Every crab seems to molt at certain times of the year. In general, blue crabs tend to molt during full moon periods from May through October. Horseshoe crabs typically molt several times during their first year. After its third or fourth year, horseshoe crabs will shed its exoskeleton annually, frequently during August or September, until it reaches maturity.
Spider crabs seem to molt only once a year, usually in the summer or fall, and they appear to do it all at once. Spider crabs are not harvested in local waters, so when these crabs have their coordinated molting period, a select beach will appear to have an overflowing amount of spider crabs. So much so that it will be alarming at first sight.
Often text messages and emails from friends and family will alert me to run down to a beach littered with dead crabs. They fear the water must be polluted with toxic sludge or other deadly chemicals, why else would there be so many dead crabs on the beach? Surely this must be a canary in a coalmine.
My response is usually the same. I ask someone to pick up a crab. Does it feel light and airy like an empty exoskeleton? Next I ask if there are any foul odors or flies? If this were truly a pile of dead crabs on a beach then the scene would smell badly with lots of hungry flies, gulls and other scavengers hanging around. Trust me, if gulls aren't interested, there is nothing to eat.
Why do crabs molt all at the same time? In general this is an evolutionary tool to help a species survive. It takes a lot of energy to molt. A crab’s soft shell makes them vulnerable to predation by large fish and even other crabs. Soft shell crabs can also be very slow and unresponsive after a molt, increasing a crab’s helplessness. So a simultaneous molt of thousands or millions may overwhelm a predator, too many crabs to eat all at once, and increase the chance of survival for the overall species.
It seems the best opportunity we get to see what animals live in our nearby aquatic wilderness are from the remains that are found jumbled and muddled on the beach surface. Although some of the time marine remains are of concern for citizens, an animal stranded by an ebbing tide for instance, molted relics of spider crabs and other crusty crustaceans are nothing to be worried about. In fact, quite the opposite, the empty shell of a crab is a sign of growth and hope as it matures in the busy and bustling waters of New York Harbor.