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Horseshoe Crab Survey: Calvert County and Beyond

Help us document the distribution and abundance of horseshoe crabs during their annual mating season along the shores of the Chesapeake Bay in Calvert County and beyond! 

Sign up for the Calvert Stewards Opportunity

Use iNaturalist to photograph and map your observations in a few easy steps.

You can also help verify other people's observations to improve our data quality, come to a scheduled horseshoe crab survey, or even hold your own event. You can also explore on your own.

Spread the word—it's a big area! The area includes the entire Chesapeake Bay Shoreline in Maryland, Virginia and Delaware. We are really interested in Calvert County sightings, but any observations along the Chesapeake are important.


There are many questions about this “living fossils.”

  • Where are horseshoe crabs coming ashore and laying eggs?
  • Will they be found mating during the daytime?
  • Are they in the water if they aren’t on the shore?
  • Does the moon need to be full for them to mate?
  • How many horseshoe crabs are found on any one survey?

How You can Help

Become a Calvert Steward and learn how to:
    • Join the iNaturalist project
    • Explore the Chesapeake Bay shoreline to find horseshoe crabs--this could be on public beaches or in your community.
    • Map locations online via iNaturalist
    • Photograph them at the different times of the day.
    • Identify the moon and tide characteristics of each population.

    Upcoming Surveys

    • No upcoming events

    Download iNaturalist App

    Android app on Google Play   iPhone app in the Apple App Store

    Atlantic Horseshoe Crab

    Horseshoe Crabs have been nick named “living fossils:”

    Their scientific name, Limulus polyphemus, is from the Greek one-eyed monster, are one of 4 species worldwide.

    They occur along east coast from Maine to Florida and Yucatan Peninsula.  

    Horseshoe crabs today don’t look a whole lot different from the fossils of their ancestors – hence the nickname – living fossils. They have been relatively unchanged for about 350 million years

    Did You Know...

    • Horseshoe crabs have an exoskeleton that is divided into 3 sections, one of which is a scary looking tail. 
    • Do they sting with their tail? No. The spike-shaped tail, or telson, functions as a tool for digging in sand and a lever if the animal finds itself upside down.
    • They have ten eyes: 2 compound eyes good in low light, 2 simple eyes sense UV light from moon, and eye spots under shell front edge and along tail. Horseshoe crabs have the largest rods and cones of any known animal that are about 100 times the size of humans.
    • They can survive out of water for 4 days if their book gills stay moist.
    • Their mouth is on the underside of the body and is good for gobbling up clams and worms.
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