The Ultimate Guide to Horseshoe Crabs at Delaware Bay

The Ultimate Guide to Horseshoe Crabs at Delaware Bay

The Ultimate Guide to Horseshoe Crabs at Delaware Bay

The spawning of the horseshoe crabs at Delaware Bay is an extraordinary spectacle to behold.  

Horseshoe Crab Mating

As the high tides sweep in, males cling to the back of the female onto the shoreline of Delaware Bay. 

  • The female digs into the sand and releases eggs every few feet which are then fertilized by the male.
  • After the mating is complete, the horseshoe crabs at Delaware Bay return to the water, and the waves wash sand overtop of the eggs.

There is a yearly spawning report that accepts volunteers to help count the numbers of horseshoe crabs that come ashore each year.

Diminishing Horseshoe Crab Population

  • The horseshoe crabs at Delaware Bay are largely decreasing in population due to development and poor water quality, according to marine biologist, Dr. Clare McBane, from New Hampshire Fish and Game.
  • The National Wildlife Federation further credits the population decline to habitat loss and human predation for use in conch and eel bait.

Since the horseshoe crab population is diminishing, so is the population of the Red Knots shore birds, who really on the crab eggs for food.

These shore birds frequent the Delaware Bay area to feed on the horseshoe crab eggs every spring.

Red Knots

The rufa subspecies of Red Knots were listed as a threatened federal candidate species under the Endangered Species Act since 2006.

  • This status means the bird is likely to become endangered, facing extinction, in the foreseeable future.
  • The Red Knots fell from 100,000 to 25,000 along the Delaware Bay, as reported by SmithsonianMag in 2009.
  • Fast forward to the aftermath of Hurricane Sandy in 2012, and a restoration project took form which has resulted in a stabilized, maybe even slightly increased population in New Jersey.

The Fish and Wildlife Service links the decline to food availability and habitat loss due to rising sea levels and climate change.

Horseshoe Crab Anatomy

The appearance of horseshoe crabs at Delaware Bay are rather unnerving, as they are much larger than my feet, and the vast majority are bigger than my head.

Body Parts

  • They have ten eyes along their bodies!
  • Additionally, their tails have light sensing organs that synchronize the brain with light and dark cycles.
  • Interestingly, the horseshoe crab is the ONLY living Chelicerate to have the lateral eyes be arthropod compound eyes.

Chelicerata is a subphylum of Arthropoda which includes horseshoe crabs, arachnids (spiders and scorpions), and sea spiders.

Under the Shell

  • The horseshoe crab is also the single type of chelicerate that has a gut designed to handle solid food.
  • The gut has an esophagus, proventriculus (comprised of a crop to store food and gizzard to break up the food), and a midgut that digests the food.
  • Wastes go through the hindgut (rectum), and undigestible food such as fish bones are regurgitated.


Horseshoe Crab Blood

  • The crabs have unique blood cells that clot in the presence of bacterial endotoxins.
  • For this reason, blood is extracted from these crabs and used in human medicine to detect harmful endotoxins on needles, drugs and heart valves.
  • This can save people from toxic effects ranging from a fever to a hemorrhagic stroke.


Thank you for reading my ultimate guide on horseshoe crabs at Delaware Bay. For my personal experience with these incredibly fascinating creatures click here.

Digiprove sealCopyright secured by Digiprove © 2015 Jessica Claudio, DVM




    • Absolutely, you are so right! That must be one of the reasons they have been around so long. 🙂


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