DDCSP Field Notes: Horseshoe Crabs

Updated: Jul 16

Last weekend, I had the chance to travel to the Delaware Bay to experience the largest spawning of horseshoe crabs in the world. I took the opportunity to create a natural history journal entry and record field notes for the Doris Duke Conservation Scholars Program. Despite the program being remote this summer due to COVID-19, we are encouraged to do ecology outdoors.

Taking field notes on Slaughter Beach, DE

Horseshoe crabs are a type of arthropod and considered to be living fossils: they have not evolved for 450 million years. Contrary to their name, horseshoe crabs are more closely related to spiders, ticks, and scorpions than to crabs. Three species exist in the world, and the only one found in the Americas is the Atlantic horseshoe crab (Limulus polyphemus).

Spawning Atlantic horseshoe crabs

Every May, Horseshoe crabs begin spawning and their eggs provide food for migrating shorebirds. Particularly, Red Knots (Calidris canutus) migrating over 9,000 miles (14,000 kilometers) from the southern tip of South America to the Arctic are heavily dependent on Horseshoe crab eggs for food.

However, horseshoe crabs will not begin to spawn until the temperature of the water reaches 59 degrees Farenheight (15 degrees Celsius). This year, the peak spawning period in the Delaware bay occurred when I went in mid-June. Considering migrating shorebirds that feed on horseshoe crab eggs, there will probably be an increase in death rates among them, especially Red knots which 50% to 80% of the migrating population stop to feed in the bay.

I encourage everyone who can to go and experience this ecological spectacle. It is important to plan your visit since horseshoe crabs lay their eggs between mid-May and mid-June, at night during the full moon and new moon, and at high tide. Abby Venture is the Marketing Director for Delaware State Parks and shares blogs with detailed information about this, and a great resource to start with is her post How to Experience Delaware's Horseshoe Crab Phenomenon.

Viewing conditions

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