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Why so Many "Dead" Horseshoe Crabs on the Beach?

By Joe Reynolds
Without a doubt the number one nature question concerned people ask me during September - what's up with all the dead crabs on the beach? A disturbing sight to see dozens or sometimes hundreds of unmoving creatures. 

Thankfully, the answer has nothing to do with a loss of life. Quite the opposite, it's all about a continuation of life.
Most of the "dead" Horseshoe Crabs that people see on beaches this time of year around Lower New York Bay, including Raritan Bay and Sandy Hook Bay, are probably not dead at all, but actually empty shells. The shells are molts. Horseshoe crabs grow by molting.  
As a Horseshoe Crab matures and increases in size, it will shed its old exoskeleton (outer shell or skeleton) and form a new, bigger one, leaving its old shell behind on the bottom of the bay. The animals increase in size by 25-30% with each molt.
(A molt from a young Horseshoe Crab)
It doesn't take long for the old exoskeleton, including tail, legs, abdomen, gill coverings, eye coverings, and everything else, to wash ashore for curious people to stumble upon. Although some crabs might be dead and in the mix within a wrack line, most of the time people are coming across a molt of a juvenile. Horseshoe Crabs need to shed at least once a year as they grow to adulthood. 
For most folks, it's a difficult task to tell an empty molt from a lifeless crab. A molt is often one intact piece of shell appearing just like a complete crab. Some of the molts may even fill up with sand or water to make the empty shell weigh about the same as a whole crab. It's surprisingly easy to mistake a molt for a dead Horseshoe Crab.
With careful observation, though, it can become easy to find the truth. Pick up the shell and look for an opening or split along the front ridge of the shell. If the seam is broken, then in all likelihood it's a molt. Unlike a true crab, like a Blue-claw Crab, which will back out of its old exoskeleton, a Horseshoe Crab will thrust forward to divide the upper shell with the bottom half. 
Also, the color of the shell is going to be pale and in some cases nearly translucent. In contrast, a dead crab is going be dark in color and have a strong saltwater-stench with many flies or gulls being attracted around the poor creature.
While there is still a lot of mystery about the juvenile life stage of Horseshoe Crabs, scientists are pretty sure that many young animals tend to stay near where they hatched for their first two years and then gradually move to deeper waters of the bay as they mature. Scientists are also pretty confident that many crabs of the same age molt at the same time of the year. According to wildlife biologist, Robert E. Loveland in the book, Limulus in the Limelight: A species 350 million years in the making and in Peril?, "there is evidence that a particular size class of crabs will all undergo simultaneous molting." Former Horseshoe Crabs shells often wash up on beaches at the same time and at the same size.

(Molts from female HSC)
August seems to be the time when many young female Horseshoe Crabs molt in Lower New York Bay, with old shells typically washing up towards the end of August and early September. You can tell if the molt is from a female crab by looking underneath at the appendages. The two front appendages for male Horseshoe Crabs look like boxing gloves. They use these limbs to hold on to the female when they mate. The two front appendages on females just look like regular claws.
Young females will also molt more than young males. Female Horseshoe Crabs grow around 30 percent larger than males in order to carry the eggs when they reach adulthood. Females on average molt 17 times before reaching adulthood and their final molt. As far as scientists know, adult Horseshoe Crabs, both males and females, do not molt.
 Horseshoe Crabs are characteristic species of the estuary. The old shells are frequent reminders as they wash up on beaches, either as whole shells, or as disarticulated pieces, that the bay is not dead. It an underwater nursery area for young Horseshoe Crabs.
If you are so inclined, feel free to take home a husked shell of a Horseshoe Crab for decoration. Clean the empty shell by soaking it in freshwater for a day or two and then let it dry in the sun. A Horseshoe Crab's molt is a great reminder of the often unseen life that exists unexpectedly downstream from New York City.
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 could 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