Why So Many Dead Horseshoe Crabs Around NY Harbor in August?

BY Joe Reynolds
NY Harbor Nature Blog

No need to worry! Those are not dead crabs. Pollution or disease did not kill them.
 
Generally speaking, what you are finding on bay beaches around New York Harbor during late summer are not dead crabs, but molts or castings. When a young horseshoe crab grows, it sheds or molts its outer skeleton (or shell), similar to a blue-claw crab.
 
When a juvenile crab is done molting, the old shell is basically washed up onto the beach with the tide. The result is an artifact that looks exactly like a tiny horseshoe crab for people to stumble upon. It's evidence of what is living in nearby waters.
 
According to U.S. Fish and Wildlife:
“Since horseshoe crabs have a hard shell, they must molt to grow. Horseshoe crabs will molt at least six times in their first year of life and about 18 times before they reach sexual maturity. Females are generally larger than males and may molt more than males to reach the larger size. Once crabs are sexually mature, which takes at least nine years, they won’t shed their shells again. When the male crab completes his final molt, his first set of claws becomes modified into a boxer-glove shape that he uses to clasp onto a female for spawning. Adult crabs may live another eight to 10 years, making the total lifespan of a horseshoe crab as long 20 years.”
 
The horseshoe molts several times during its first year and may reach a width of about 1/2". After its third or fourth year it sheds its skin annually, mostly near a full moon in August around New York Harbor.
 
How can you easily tell if a crab shell is a molt or if a crab is dead? If the shell is light in color and lightweight in feel, then it's probably a molt. If the shell is dark brown in color and has heavy feeling, it is probably a dead crab. 

A dead crab on the left and a molt on the left. The dead horseshoe crab has two large cracks or gashes in its outer-skeleton. 

If you do find what appear to be a long line of lifeless crabs washed up on the beach, contact state wildlife organizations as soon as possible. Loads of dead crabs on a beach cmight be an indicator of a serious water quality issue.  

In New York State, please contact the New York Department of Conservation at http://www.dec.ny.gov/about/259.html 

In New Jersey, please contact the Department of Environmental Protection at http://www.nj.gov/dep/warndep.htm

Different size molts found on a beach around New York Harbor in August 2017.