I saw my first horseshoe crabs of the season. They start arriving in Seagrass around this time every year. Here are some horseshoe crab facts: The facts are AI generated.
Horseshoe crabs are one of the most fascinating parts of coastal life in lower Delaware, especially around the Delaware Bay shoreline and the Inland Bays region. Despite the name, they are not true crabs — they are more closely related to spiders and scorpions, and they have existed for more than 400 million years.
Why Delaware is Famous for Them
The beaches along Delaware Bay are one of the largest horseshoe crab spawning areas in the world. Every spring, usually from May into June, thousands come ashore during high tides around the full and new moons to lay eggs. Popular viewing locations include:
• Prime Hook National Wildlife Refuge
• Slaughter Beach
• Broadkill Beach
• Cape Henlopen State Park
In the Bethany Beach and Dagsboro area, horseshoe crabs can occasionally be seen along quieter bayshorelines and inland bays, although the largest concentrations occur on the Delaware Bay side.
What They Look Like
Horseshoe crabs have:
• A hard horseshoe-shaped shell
• Multiple walking legs underneath
• A long pointed tail called a telson
The tail is not a stinger. It is used to help the animal flip itself upright if overturned.
Why They Matter Ecologically
Horseshoe crab eggs are a critical food source for migrating shorebirds, especially the Red Knot. These birds stop in Delaware during migration and depend on the nutrient-rich eggs before continuing their long journey to the Arctic.This relationship makes Delaware internationally important for bird migration and coastal conservation.
Their Famous Blue Blood
Horseshoe crabs have blue blood because it uses copper instead of iron to transport oxygen.Their blood contains a substance called Limulus Amebocyte Lysate (LAL), which is used in medicine to testvaccines, IV drugs, and medical devices for bacterial contamination.
Biomedical collection is carefully regulated, and many crabs are returned alive after blood extraction.
Are They Dangerous?
Horseshoe crabs are harmless to humans.
They:
• Do not bite
• Do not sting
• Are not aggressive
If you find one upside down on the beach, you can gently flip it over by holding the shell — not the tail.
Best Time to See Them
The best time to see horseshoe crabs in Delaware is:
• Late May through June
• Around sunset and high tide
• Near the full moon or new moon
Many Delaware nature organizations offer guided horseshoe crab walks during spawning season.
Interesting Facts
• Females are much larger than males.
• One female can lay tens of thousands of eggs.
• Smaller males often attach themselves to females during spawning.
• Horseshoe crabs survived multiple mass extinctions over hundreds of millions of years.
Lower Delaware’s horseshoe crab migration is considered one of the great natural events along the Atlantic coast and is a favorite subject for nature photographers, birders, and visitors alike.
Horseshoe Crab Mating Habits
Horseshoe crab mating behavior is one of the most unusual and ancient wildlife events on the East Coast. In Delaware, especially along Delaware Bay, this activity becomes highly visible every spring during spawning season.
When Mating Happens
Mating season usually occurs:
• May through June
• During high tides
• Around full and new moons
• Often near sunset or at night
The timing helps maximize the chances that eggs remain moist and protected in the sand.
How They Find Mates
Male horseshoe crabs wait offshore for females moving toward spawning beaches. The males are much smaller than females and have special hook-shaped front claws designed for gripping the female’s shell. Once attached, a male may remain connected for days or even weeks.
The Attached Male Behavior
The attached male is called the “amplexus” male. He rides on the back of the female as she moves onto the beach.
As the female digs shallow nests and deposits eggs in the sand:
• The attached male fertilizes the eggs externally.
• Additional “satellite males” may crowd nearby trying to fertilize eggs as well.
This creates a highly competitive mating system.
Egg Laying
A female horseshoe crab may:
• Lay about 4,000 eggs per nest• Create multiple nests in one night
• Produce tens of thousands of eggs during a season
The eggs are small, greenish, and buried a few inches below the sand surface.
Why Beaches Become Crowded
During peak spawning periods, beaches can become densely covered with horseshoe crabs.
This behavior is synchronized by:
• Moon cycles
• Tides
• Water temperature
• Day length
Survival Strategy
Horseshoe crabs rely on a “many eggs” reproductive strategy:
• Huge numbers of eggs are produced
• Birds, fish, and waves destroy many of them
• Enough survive to continue the species
Only a small percentage of young horseshoe crabs survive to adulthood.
Young Horseshoe Crabs
After hatching:
• Larvae emerge from the sand
• Juveniles live in shallow protected waters for years
• They molt repeatedly as they grow
It can take nearly a decade for horseshoe crabs to reach reproductive maturity.
Interesting Detail
Female horseshoe crabs do not choose mates through courtship. Reproduction is driven more by timing, tides, and a male’s ability to remain attached during spawning.
There is no pair bonding or parental care after the eggs are laid.The horseshoe crab spawning migration in Delaware is considered one of the most important marine wildlife events on the Atlantic coast and plays a major role in supporting migratory shorebirds.