Limulus Love

Horseshoe Crabs mating

Horseshoe Crabs on Slaughter Beach, DE (click to enlarge)

I borrowed the title of today’s blog from another blogger I ran across while reading about Horseshoe Crabs. Turns out there are lots of people fascinated by these ancient creatures, and The Beach Chair Scientist ( is one of them, so check her out for more information on these fascinating creatures and other topics of the sea. The blog title refers to part of the scientific name of Horseshoe Crabs, Limulus polyphemus, and the fact that this is the time of year when Horseshoe Crabs migrate to beaches for mating and egg-laying. The Delaware Bay region is believed to be home to the largest population of this species of Horseshoe Crab (there are 3 other species in the Pacific) and is therefore the place to be in late spring if you want to witness Limulus Love.

Horseshoe Crab

Horseshoe Crab male (click to enlarge)

Horseshoe Crabs are ancient arthropods, dating back perhaps as much as 300 million years (way before the dinosaurs). They not true crabs but are more closely related to spider and scorpions. Their anatomy is fascinating and you should check out some of the other web sites for details (also They use their tail (telson) to right themselves when flipped over (although it is not always successful). Two large compound eyes are located on the front part of the shell (prosoma), with other light receptors scattered elsewhere over the body. The eyes are quite sensitive to low light and can help them find mates in the dark waters. Underneath are five pairs of legs, the first pair modified in males to be claspers for grabbing the edge of a female’s shell during mating. The last pair of legs are called pusher legs and have a leaf-like structure at the tip used for pushing and clearing away sediments as the crab burrows. There are two small chelicera in front of the walking legs that help guide food to the mouth, which is the bristly area between all the legs.

Horseshoe Crabs on Slaughter Beach low angle 3

Horseshoe Crabs on Slaughter Beach (click to enlarge)

Horseshoe Crabs shed their chitinous exoskeleton as they grow, increasing in size about 25% each time they shed. It takes 16-17 molts over a period of 9-11 years to reach sexual maturity, with females being much larger than males.

Horseshoe Crabs on Slaughter Beach 5

Horseshoe Crabs on Slaughter Beach at high tide (click to enlarge)

For most of the year, Horseshoe Crabs are out at sea, feeding on marine worms and softer shellfish such as Razor Clams. But every spring, adult Horseshoe Crabs migrate to shallow waters to breed. Annual census data highlights Delaware Bay as the most important spawning ground in the world for American Horseshoe Crabs. The 2011 survey estimated well over a million Horseshoe Crabs utilized these critical beach habitats. The peak of spawning activity usually occurs several days around the new and full moons of May and June and coincides with the high tides.

During breeding, the smaller males couple onto a unattended female as she digs a shallow nest at the tide line. There may be several males in attending each female since the ratio of males to females on the beaches is about 3 to 1. The female releases several thousand eggs while the male(s) release sperm and the eggs are fertilized externally. Each mature female lays up to 20 clutches totaling up to 90,000 eggs during the spawning season, of which fewer than 1% will make it to adulthood.

Horseshoe Crab egg cluster

Horseshoe Crab egg cluster photographed in aquarium at DuPont Nature Center (click to enlarge)

Horseshoe Crab eggs on fingers

Horseshoe Crab eggs (click to enlarge)

The eggs are laid in clumps, but I only saw one on the beach. The tidal action plus all the digging from mating crabs tends to bust up the clusters so the beach is covered by millions of loosely scattered greenish eggs about the size of a sand grain.

Horseshoe Crabs at low tide on Slaughter Beach 2

Horseshoe Crabs at low tide on Slaughter Beach (click to enlarge)

Back at Slaughter Beach the next morning it was low tide with a windy, gray sky. The stark scene had the look of devastation, with dark, motionless clumps scattered as far as the eye could see. But most of the crabs were still alive and with the next rising tide, would begin the ancient ritual again.

Shorebirds on Slaughter Beach feeding on Horseshoe Crab eggs 1

Shorebirds on Slaughter Beach feeding on Horseshoe Crab eggs (click to enlarge)

This superabundance of fat and protein rich granules sets the table for one of the greatest bird feasts in North America – the annual migration of hundreds of thousands of shorebirds through Delaware Bay. This area is a critical staging area as these birds make their way thousands of miles from their wintering grounds on the beaches of South America to their nesting grounds in the Arctic. The birds gorge on the Horseshoe Crab eggs as well as various worms and invertebrates in the tidal flats and beaches. In their two to three week stopover, they may double of triple their weight, which fuels them for the remainder of their long flight. It was a privilege to sit on the beach with almost no one else around, watching and listening to this grand event.

Researchers processing captured shorebirds

Researchers processing captured shorebirds (click to enlarge)

The migrants are made up of primarily four species: Semipalmated Sandpipers, Dunlins, Ruddy Turnstones, and Red Knots. The latter species is one facing steep declines in their population over the past decades. Researchers were on a distant beach the first afternoon of my visit as they have been for many years – capturing, weighing, and tagging Red Knots and other shorebirds to learn more about their needs in their perilous journey in the hopes of helping populations recover.

B95 statue

Statue of Red Knot B95 outside the DuPont Nature Center 9click to enlarge)

An unusual and fun statue greets visitors to the DuPont Nature Center in Mispillion Harbor Reserve. It is a tribute to the most famous Red Knot, known as B95, for the bright orange leg band with that code attached by an Argentinian researcher in 1995. During my visit, news broke that B95 had been spotted once again in Delaware Bay. Scientists estimate this Red Knot to be 20 years old (it was already an adult when banded), making it the oldest Red Knot known. And because its annual journey from the tip of South America to the Arctic spans about 9,000 miles one way, in its lifetime, this amazing bird has flown a distance equal to going to the moon and halfway back. That feat has earned B95 the nickname, Moonbird, and it is the subject of an award-winning children’s book by the same name. During its lifetime, B95 has seen the population of his kind crash from over 100,000 birds to about 15,000 today. There are great concerns over the decline in Horseshoe Crab eggs due to over-harvesting of the adults crabs for bait and available habitat for both crabs and birds due to development. Red Knots were scarce during my visit and I spotted only a handful amongst the thousands of other shorebirds.

And while the natural connections of Horseshoe Crabs are amazing, they also have important connections to humans. They have been used for food and fertilizer in earlier cultures, and are now harvested as bait for the eel and conch fisheries (regulations now restrict this harvest to ensure enough egg-laying adults to maintain the shorebird connection). Scientists have learned a lot about the human eye from studying the electrical impulses in the compound eyes of Horseshoe Crabs. And chitin from Horseshoe Crabs is used in the chitin coating of surgical sutures and wound dressings for burn victims. But the main human connection now comes form the creatures’ blue blood (their blood contains a copper-based respiratory pigment and turns blueish when exposed to air). An extract of the Horseshoe Crab’s blood is used by the pharmaceutical and medical device industries to ensure that their products (any intravenous drugs, vaccines, and medical devices), are free of bacterial contamination. So, if you or anyone you know has ever been hospitalized, you owe a lot to Horseshoe Crabs.

Horseshoe Crabs on Slaughter Beach 2

Horseshoe Crabs on Slaughter Beach at sunset (click to enlarge)

The experience at Slaughter Beach is something that I will never forget – the sights and sounds of thousands of Horseshoe Crabs clambering over each other at the tide line at sunset followed by tens of thousands of birds feeding on the eggs the next morning is one of the great wildlife spectacles in America. I recommend a stop at the DuPont Nature Center ( if you are in that area. It has excellent displays on the incredible story of Horseshoe Crabs and the staff are very knowledgeable. You will come away appreciating these “living fossils” as much as I now do. Although it is a long drive from NC, I am considering offering a trip to this area in the future to witness this natural wonder. Let me know if you are interested.

A gallery of sights on the Horseshoe Crab beaches of Delaware…

Horseshoe Crab burying in sand 1

Horseshoe Crab burying in sand (click to enlarge)

Horseshoe Crabs on Slaughter Beach with overturned crab

Horseshoe Crabs on Slaughter Beach with overturned crab (click to enlarge)

Slipper Shells on Horseshoe Crab

Slipper Shells on Horseshoe Crab (click to enlarge)

Short-billed Dowitcher 2

Short-billed Dowitcher (click to enlarge)

Horseshoe Crab overwash slow motion

Horseshoe Crab over-wash (click to enlarge)

Horseshoe Crabs stylized

Horseshoe Crabs stylized (click to enlarge)

Black-bellied Plover on Slaughter Beach 2

Black-bellied Plover gleaning food from a Horseshoe Crab shell (click to enlarge)

Shorebirds in flight at Slaughter Beach, DE

Shorebirds in flight at Slaughter Beach, DE (click to enlarge)

5 thoughts on “Limulus Love

  1. Great pictures! Learning lots from your blog.
    Last year I visited Litchfield beach in SC (just a little South of Huntington) in the beginning of May. On a morning walk at that beach I saw what you described. Low tide and (I thought) hundreds of dead horseshoe crabs. Felt so bad then, especially because there were so many. Was wondering what happened and why they washed ashore all together. Now I think I know.

  2. Thank you for your beautiful post and ethereal images (ok, well – the giant red knot was just plain cool!). Also, much appreciated for sharing Beach Chair Scientist with your followers. Cheers!

  3. Is there any chance I would be able to use your photos of the Limulus eggs for a project? I am working on an informational tablet that will give people more information on species in our local area and horseshoe crabs are one of them. I would love to use this photo for that purpose, if you would allow it. I would, of course, add your name and credit to the photo. I am having trouble finding any good photos of the eggs- and these are perfect. Well done. If you would consider, please let me know.

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