Hundreds of horseshoe crab shells are washing up on Mass. beaches. Officials say it's normal.

"After concerned residents in Falmouth, Weymouth, and Nahant reported an unusual number of dead horseshoe crabs on local beaches, the Massachusetts Division of Marine Fisheries confirmed the crabs weren't dead. "It's OK," the state agency wrote on Facebook, "the horseshoe crabs are just molting!""

"Due to the molting, their exoskeletons are washing up on the beaches, according to the Division of Marine Fisheries. Peak horseshoe crab molting season is the late summer and early fall, so beachgoers should expect to see the exoskeletons cast off by horseshoe crabs as they grow. "To tell the difference between a dead crab and a molt, look for a slit along the front rim of the shell. This is where the crab split and exited its old shell," the state agency wrote on Facebook."

An unusual number of horseshoe crab shells appearing on Massachusetts beaches resulted from molting, not mortality. Molted exoskeletons commonly wash ashore during peak molting season in late summer and early fall. Observers can distinguish molts from dead crabs by a slit along the front rim of the shell where the animal exited. Horseshoe crabs molt multiple times during their first three years, then roughly annually until about nine to eleven years of age. Individuals can live to around twenty years. Adult horseshoe crabs are most often visible on sandy beaches when mating in May and June, while molts appear more frequently later in the year.

Read at Boston.com